Introduction
Even though the importance of vitamin D is gaining more public attention, rickets still exists in the USA in dark-skinned infants who are exclusively fed on breast milk(Reference Rajakumar1). Rickets was first described in the literature in the mid-1600s, and cod-liver oil and sunshine exposure were recognised as the cures for rickets in the late 19th century. Vitamin D was then discovered in the early 20th century(Reference Rajakumar1). There are two types of physiologically important vitamin D: cholecalciferol (D3) and ergocalciferol (D2). D3 is synthesised in the skin from 7-dehydrocholesterol in cell membranes upon exposure to UVB (290–320 nm), while D2 is plant and yeast derived and produced exogenously by UV irradiation of ergosterol(Reference Wolpowitz and Gilchrest2, Reference Holick3). Vitamin D in the circulation is metabolised to 25-hydroxyvitamin D (25(OH)D) in the liver and further metabolised to the active metabolite, 1,25-dihydroxyvitamin D (1,25(OH)2D), in the kidney. The concentration of 1,25(OH)2D is highly regulated by a variety of factors including serum parathyroid hormone and P(Reference Wolpowitz and Gilchrest2, Reference Holick4).
The majority of circulating 25(OH)D and 1,25(OH)2D is bound to vitamin D binding protein (DBP) (80–90 %) and albumin (10–20 %), while a small fraction of both 25(OH)D (0·02–0·05 %) and 1,25(OH)2D (0·2–0·6 %) is free(Reference Zerwekh5). The vitamin D–DBP complex has been shown to be taken up by proximal tubules through the endocytic receptor megalin, after which DBP is proteolytically degraded, leaving the vitamin D metabolites for physiological action or metabolism(Reference Zerwekh5, Reference Nykjaer, Dragun and Walther6). Thus, measuring free vitamin D metabolites is not clinically indicated despite some efforts having been made to calculate the free plasma vitamin D metabolites based on measured total concentrations, DBP, and albumin concentrations(Reference Al-oanzi, Tuck and Raj7). The half-lives of vitamin D, 25(OH)D and 1,25(OH)2D are approximately 24 h, 3 weeks and 4 h respectively(Reference Zerwekh5). In addition, liver production of 25(OH)D is not significantly regulated and is primarily dependent on the availability of vitamin D(Reference Zerwekh5). Therefore measuring the total levels of serum 25(OH)D is considered the best estimate of vitamin D nutritional status.
Vitamin D nutritional status has been linked to many pathophysiological conditions. The present review will focus on ecological, case–control and cohort studies exploring the role of vitamin D in health and disease. Serum levels of 25(OH)D and the means of obtaining vitamin D will also be discussed. In addition, analytical techniques used in laboratory evaluation of vitamin D nutritional status will be critically reviewed.
Measuring vitamin D metabolites
There are a few commercial immunoassays available for measuring serum 25(OH)D, and liquid chromatography–MS (LC–MS) is considered the ‘gold standard’(Reference Zerwekh5). However, reporting both 25(OH)D2 and 25(OH)D3 may be confusing to clinicians without appropriate guidance(Reference Binkley, Drezner and Hollis8). Both 25(OH)D and 1,25(OH)2D were found to be stable at room temperature for at least 72 h in either whole blood or serum(Reference Lissner, Mason and Posen9). Exposure to UV light or freeze–thaw cycles (up to eleven times) of serum did not change the concentrations(Reference Lissner, Mason and Posen9). The stability of serum 25(OH)D was confirmed by DiaSorin RIA for up to four freeze–thaw cycles(Reference Antoniucci, Black and Sellmeyer10).
The international Vitamin D Quality Assessment Scheme demonstrated that most commercial 25(OH)D methods were capable of producing reliable results for those samples containing only 25(OH)D3. However, the results were operator-dependent and most methods had significant bias compared with HPLC methods for samples with a substantial proportion of 25(OH)D2(Reference Carter, Carter and Jones11). Evaluation of current RIA and chemiluminescent methods for serum 25(OH)D using patient samples showed substantial variability among the six methods (RIA including DiaSorin assays and chemiluminescent immunoassays) and the same methods used in the different laboratories, which may confound the diagnosis of hypovitaminosis D(Reference Binkley, Krueger and Cowgill12). Comparing to an HPLC method for serum 25(OH)D measurement, significant positive proportional bias was observed for DiaSorin and IDS RIA as well as the discontinued Nichols Advantage protein-binding assay in the range of 20–50 nmol/l in serum samples before D2 treatment(Reference Glendenning, Taranto and Noble13). All the immunoassays evaluated also underestimated serum 25(OH)D2 concentrations in samples after D2 treatment(Reference Glendenning, Taranto and Noble13). Using LC–MS as the standard, DiaSorin Liaison showed different but not clinically significant results (however, there were only eight patient samples treated with vitamin D2 included in the analysis) while the discontinued Nichols Advantage method showed significant difference(Reference Terry, Sandrock and Meikle14). A comprehensive evaluation of seven methods using 291 EDTA plasma samples (277 had no detectable 25(OH)D2 and fourteen had 25(OH)D2 between 5 and 8 nmol/l) showed that all methods except HPLC demonstrated considerable negative bias compared with LC–MS(Reference Roth, Schmidt-Gayk and Weber15). The deviation was more significant at levels of 25(OH)D>75 nmol/l than those < 75 nmol/l for most immunoassays. The methods evaluated were HPLC, IDS RIA, IDS enzyme immunoassay (competitive immunoassay), Advantage (protein-binding assay), Liaison 1 and 2 (competitive immunoassay) and Elecsys (competitive electrochemiluminescent assay)(Reference Roth, Schmidt-Gayk and Weber15).
Most HPLC methods require lengthy sample preparation including solid-phase extraction(Reference Lensmeyer, Wiebe and Binkley16) and liquid–liquid extraction(Reference Alvarez and De Mazancourt17–Reference Olkowski, Aranda-Osorio and McKinnon19), followed by a lengthy HPLC ranging from 10 to 30 min. It should be noted that potential late elution peaks may interfere with the analysis of the succeeding samples(Reference Lensmeyer, Wiebe and Binkley16). In addition, the C-3 epimer of 25(OH)D could be a significant interferant in infants < 1 year old, in which the epimer can be 8·7–61·1 % of the total 25(OH)D(Reference Lensmeyer, Wiebe and Binkley16, Reference Singh, Taylor and Reddy20).
The LC–MS is considered the ‘gold standard’ technology for 25(OH)D quantification(Reference Zerwekh5). Most LC–MS methods employ 2H-labelled 25(OH)D3 as the internal standard. 2H6-labelled 25(OH)D2, while recently available, has a molecular weight of 418·2. With the loss of a water molecule, a fragment of 401·2 m/z is formed, interfering with 25(OH)D3 quantification. To improve ionisation efficiency, some methods employ a derivatisation strategy using a Cookson-type reagent(Reference Higashi, Awada and Shimada21, Reference Higashi, Shibayama and Fuji22) or the Diels–Alder derivatisation(Reference Aronov, Hall and Dettmer23). For direct measurement without derivatisation, sample preparation can be cumbersome. In general, sample preparation includes protein precipitation followed by solid-phase extraction(Reference Vogeser, Kyriatsoulis and Huber24–Reference Chen, McCoy and Schleicher26) or liquid–liquid extraction(Reference Saenger, Laha and Bremner27–Reference Capote, Jimenez and Granados29). Turbulent flow technology is a robust and rapid online purification tool for high efficiency extraction(Reference Grant, Cameron and Mackenzie-McMurter30, Reference Zimmer, Pickard and Czembor31), and has been used for online sample cleaning in serum 25(OH)D quantification(Reference Singh, Taylor and Reddy20). This technology is efficient to reduce labour-intensive sample preparation and to improve reproducibility.
Serum levels of 25-hydroxyvitamin D
Vitamin D ‘deficiency’ refers to serum levels of 25(OH)D resulting in histologically evident bone diseases such as osteomalacia and rickets, while vitamin D ‘insufficiency’ refers to alterations in the parathyroid hormone concentration which if such persists over time may contribute to bone loss and fracture(Reference Wolpowitz and Gilchrest2). In general, rickets can occur for children with serum 25(OH)D less than 25 nmol/l (10 ng/ml) and osteoporosis is possible for adults with a serum 25(OH)D level at 80 nmol/l (32 ng/ml) or less(Reference Grant32, Reference Heaney33). Therefore, it is generally accepted that serum 25(OH)D levels of 25 and 75 nmol/l (30 ng/ml) are the cut-off values for deficiency and insufficiency, respectively(Reference Cherniack, Florez and Roos34, Reference Dawson-Hughes, Heaney and Holick35). Vitamin D insufficiency is recognised as an epidemic issue, especially in areas of higher latitudes or low sunlight and for individuals with darker skin(Reference Holick4). It has been estimated that 40–90 % of the elderly worldwide have vitamin D insufficiency(Reference Cherniack, Florez and Roos34). Even in southern parts of the USA, 45 % of black individuals and 11 % of white individuals aged 40–79 years had 25(OH)D ≤ 37·5 nmol/l(Reference Egan, Signorello and Munro36). In Canada, the majority (93 %) of children at ages 9, 13 and 16 years had insufficient vitamin D levels ( ≤ 75 nmol/l) and a significant fraction in each group had 25(OH)D ≤ 25 nmol/l, with higher percentages in the older groups(Reference Mark, Gray-Donald and Delvin37). The prevalence of vitamin D insufficiency (25(OH)D < 75 nmol/l) in adults with cystic fibrosis was 76 % in a retrospective study spanning 2 years at a cystic fibrosis centre in Atlanta (GA, USA)(Reference Wolfenden, Judd and Shah38). A 32 % higher risk for vitamin D insufficiency (25(OH)D < 75 nmol/l) was found for patients with chronic kidney disease in the Third National Health and Nutrition Examination Survey(Reference Mehrotra, Kermah and Budoff39).
Currently the most commonly agreed cut-off for vitamin D insufficiency is 25(OH)D < 75 nmol/l. This cut-off value was derived from studies using immunoassays or protein-binding assays which are significantly different from LC-based assays and vary significantly among these assays. Therefore, defining method-specific reference ranges or standardisation of 25(OH)D assays is important in evaluation of vitamin D nutritional status and patient management.
Effectiveness and safety of supplementation
Both dietary supplementation and sunshine exposure are effective in preventing vitamin D deficiency, though there is concern of skin cancer due to prolonged solar UV radiation exposure(Reference Wolpowitz and Gilchrest2, Reference Hedges and Scriven40). There is clear evidence that UV light exposure, consuming vitamin D-fortified food and/or vitamin D supplementation has a positive impact on serum 25(OH)D. Individuals can tolerate vitamin D at doses above the current dietary reference intake levels which are 200 IU (1 μg = 40 IU) for children and adults up to 50 years of age, 400 IU for 51–70 years of age, and 600 IU for adults aged 71 years or older(Reference Holick3, Reference Cranney, Weiler and O'Donnell41–43). Short-term (8 weeks) and long-term (1 year) efficacy and safety for 14 000 IU/week supplementation of D3 were evaluated in children and no study subject developed vitamin D intoxication while mean serum 25(OH)D increased from 110 to 135 nmol/l and from 38 to 90 nmol/l, respectively(Reference Maalouf, Nabulsi and Vieth44). At physiological inputs of both oral and cutaneous forms, there is a rapid conversion of vitamin D3 to 25(OH)D3 at low vitamin D3 concentrations and a much slower conversion rate at higher D3 concentrations(Reference Heaney, Armas and Shary45). Therefore, the increase of serum levels of 25(OH)D depends upon the serum vitamin D3 levels(Reference Heaney, Armas and Shary45). However, individual response to the same therapy (Calcichew D3 Forte containing 1250 mg calcium carbonate and 400 IU D3 per tablet(46)) was different; therefore, no single dose of vitamin D is appropriate for all(Reference Ryan47). In a 6-week randomised controlled study, supplementation with vitamin D2 2000 IU daily, vitamin D2 50 000 IU weekly, or vitamin D3 2000 IU daily yielded equivalent outcomes (median increased from 42·5 to 90 nmol/l) in the treatment of hypovitaminosis D (25(OH)D < 50 nmol/l) among young children(Reference Gordon, Williams and Feldman48). From a 6-month, prospective, randomised, double-blinded, double-dummy, placebo-controlled study of vitamin D3 supplementation, an optimal dose of 4600 IU daily is predicted to achieve serum 25(OH)D levels of 75–220 nmol/l(Reference Aloia, Patel and DiMaano49). In a different study involving sixty-seven men in Omaha (41·2°N latitude), to sustain the serum 25(OH)D levels obtained through summer, approximately 3800 IU vitamin D3 per d was needed(Reference Heaney, Davies and Chen50). It is generally accepted that serum levels of 25(OH)D < 250 nmol/l are safe and still significantly below the toxicity level(Reference Jones51). Vitamin D intoxication is observed when serum 25(OH)D is higher than 374 nmol/l(Reference Holick3).
Vitamin D2 and D3
Though both vitamin D2 and D3 have been used as supplementation, vitamin D2 was less efficient than vitamin D3 for increasing serum 25(OH)D with either a single dose (50 000 IU) followed for 1 month, a single high dose (300 000 IU) followed for 24 weeks, or a daily dose (about 4000 IU) for 14 d(Reference Armas, Hollis and Heaney52–Reference Leventis and Kiely55). Several mechanisms could contribute to this observation: 25(OH)D2 has a lower affinity for DBP which results in a shorter half-life than 25(OH)D3; also human liver enzymes may convert vitamin D3 to 25(OH)D at a more rapid rate than vitamin D2(Reference Houghton and Vieth54). However, a recent randomised, placebo-controlled, double-blinded study of healthy adults showed that 1000 IU vitamin D2 and D3 daily for 11 weeks had the same effectiveness in maintaining serum 25(OH)D(Reference Holick, Biancuzzo and Chen56).
Vitamin D and all-cause death
There is reasonable evidence from epidemiological and case–control studies that maintaining sufficient vitamin D is important for bone health, muscle strength, cancer, autoimmunity and CVD(Reference Wolpowitz and Gilchrest2, Reference Holick4, Reference Grant32). In a large prospective study involving 13 331 participants followed for a median of 8·7 years in the Third National Health and Nutrition Examination Survey (which is a nationwide probability sample of non-institutionalised civilian persons), the lowest quartile of the baseline 25(OH)D ( < 44·5 nmol/l) was associated with a 26 % increased rate of all-cause mortality compared with the highest quartile (>80 nmol/l)(Reference Melamed, Michos and Post57). The concentration range for reduced risk of mortality was 50–122·5 nmol/l, especially in women(Reference Melamed, Michos and Post57). CVD and cancer mortalities were higher but not statistically significant in the lowest quartile of baseline 25(OH)D levels(Reference Melamed, Michos and Post57). All-cause mortality but not CVD-cause mortality was higher in patients with 1,25(OH)2D less than 52 pmol/l in 226 patients with chronic kidney disease stages 3 and 4(Reference Inaguma, Nagaya and Hara58). Treatment with oral calcitriol was inversely associated with risk for mortality and combined death and dialysis initiation in pre-dialysis patients with chronic kidney disease for a median duration of 2·1 years of 258 subjects receiving calcitriol and 262 subjects without calcitriol(Reference Kovesdy, Ahmadzadeh and Anderson59). Though the study limitations included non-randomisation, observational design, lack of information on cause of death, the exclusive enrolment of men, and the small sample size, it emphasised the importance of vitamin D in the healthcare of this patient population and the immediate need for randomised prospective clinical trials(Reference Patel and Singh60).
Vitamin D and bones
Vitamin D status plays a very important role in bone health. Rickets may present in children with serum levels of 25(OH)D < 25 nmol/l and osteoporosis is possible in adults with serum 25(OH)D levels < 80 nmol/l(Reference Grant32). From a meta-analysis of double-blind randomised controlled trials of oral vitamin D supplementation (all used vitamin D3) in older individuals ( ≥ 60 years) for either hip fracture (n 9294 subjects in five trials) or non-vertebral fracture (9820 subjects in seven trials), a vitamin D dose of 700–800 IU/d reduced risk of hip fracture by 26 % and non-vertebral fracture by 23 %, while no significant benefit was observed for 400 IU/d(Reference Bischoff-Ferrari, Willett and Wong61). In a randomised double-blind controlled trial involving 2686 individuals, supplementation of 100 000 IU vitamin D3 once every 4 months resulted in significantly reduced fractures and mortality compared with the matching placebo-treated group(Reference Trivedi, Doll and Khaw62). The average serum 25(OH)D concentration in the supplementation group was 74·3 nmol/l v. 53·4 nmol/l in the placebo group(Reference Trivedi, Doll and Khaw62). Serum 25(OH)D was positively associated with bone mineral density at the hip and spine in 414 older men (mean age 74 years) at a clinic visit(Reference Saquib, von Muhlen and Garland63). However, a prospective study involving 60 689 women aged 40–74 years in central Sweden found no association between either baseline dietary Ca or vitamin D and fracture risk with an average follow-up of 11·1 years(Reference Michaelsson, Melhus and Bellocco64). In a case–control observational study within the European Prospective Investigation into Cancer and Nutrition-Oxford cohort, baseline plasma 25(OH)D levels (mean 80·4–83·7 nmol/l across the control and case groups) were not associated with fracture risk in 730 incident fracture cases and 1445 matched controls in 5 years after blood sample collection(Reference Roddam, Neale and Appleby65). In a 1-year randomised, double-blind, placebo-controlled trial involving 320 elderly women (age 77·2 (sd 4·6) years) whose serum 25(OH)D levels were less than 60 nmol/l, supplementation with 1000 IU vitamin D2 did not have additional benefits on bone structure, bone formation markers, or intestinal Ca absorption over an additional 1000 mg Ca per d, though the serum 25(OH)D was raised from 44·3 (sd 12·9) nmol/l to 59·8 (sd 13·8) nmol/l(Reference Zhu, Bruce and Austin66). In conclusion, it is important to have adequate vitamin D for bone health and the doses higher than the current recommendation of dietary vitamin D are needed.
Vitamin D and muscles
Serum vitamin D levels are related to muscle strength, size and non-specific musculoskeletal muscle pain(Reference Grant32). In a meta-analysis of five double-blind randomised controlled trials involving 1237 participants (mean age, 60 years), vitamin D supplementation reduced the corrected OR of falling by 22 % compared with those on Ca or placebo(Reference Bischoff-Ferrari, Dawson-Hughes and Willett67). In a retrospective cross-sectional study of haemodialysis patients receiving active vitamin D analogues for control of secondary hyperparathyroidism (n 49) v. those who were not (n 30), patients in the vitamin D group had a larger thigh-muscle cross-sectional area and were stronger across strength measures after controlling for age and sex(Reference Gordon, Sakkas and Doyle68). A low 25(OH)D level was associated with a high prevalence of falls in the previous year of blood draws in Japanese elderly women in a cross-sectional community-based survey involving 2957 subjects (950 men and 2007 women aged 65–92 years)(Reference Suzuki, Kwon and Kim69). In a retrospective study of 110 community-dwelling women with hip fractures, 96 % had 25(OH)D < 80 nmol/l and 38 % had ≤ 22·5 nmol/l(Reference LeBoff, Hawkes and Glowacki70). Those with 25(OH)D ≤ 22·5 nmol/l had poorer lower extremity function and higher falling rates compared with those with 25(OH)D>22·5 nmol/l(Reference LeBoff, Hawkes and Glowacki70). Interestingly, higher testosterone levels had a decreased OR of falling and the fall reduction was further enhanced by vitamin D and Ca supplementation in 199 men and 246 women aged 65+years living at home and followed for 3 years(Reference Bischoff-Ferrari, Orav and Dawson-Hughes71).
Vitamin D and autoimmune diseases
1,25(OH)2D3 regulates the growth and differentiation of multiple cell types displaying immunoregulatory and anti-inflammatory properties(Reference Adorini and Penna72). Cells involved in innate and adaptive immune responses including macrophages, dendritic cells, T and B cells can produce and respond to 1,25(OH)2D3, leading to an enhancement of innate immunity and multifaceted regulation of adaptive immunity(Reference Adorini and Penna72). Plenty of published ecological, case–control and cohort studies show the importance of vitamin D in a variety of autoimmune diseases(Reference Adorini and Penna72, Reference Ascherio and Munger73).
In a prospective, nested case–control study among more than 7 million US military personnel, the risk of multiple sclerosis significantly decreased (OR 0·59) with every 50 nmol/l increase of serum 25(OH)D(Reference Munger, Levin and Hollis74). A large case–control study based on death certificates by the National Cancer Institute found that the OR was 0·24 for the combined effect of the highest levels of residential and occupational sunlight exposure for multiple sclerosis while the OR was 1·38 for skin cancer(Reference Freedman, Dosemeci and Alavanja75). Vitamin D was also suggested for treatment and prevention of multiple sclerosis(Reference Brown76).
Vitamin D is also important in reducing type 1 diabetes. In a birth-cohort study with 10 366 children born in 1966 in Finland, vitamin D supplementation was associated with an 88 % reduction in type 1 diabetes incidence in 30 years of life(Reference Hypponen, Laara and Reunanen77). From a large multicentre trial covering many different European settings, vitamin D supplementation in infancy showed a protective effect for type 1 diabetes onset before the age of 15 years(Reference Dahlquist, Patterson and Soltesz78). Recently, age-standardised incidence rates of type 1 diabetes in fifty-one world regions in 1990–4 were shown to be significantly inversely associated with UVB irradiance adjusted for cloud cover(Reference Mohr, Garland and Gorham79).
Some epidemiological evidence shows that vitamin D status is associated with systemic lupus erythematosus, rheumatoid arthritis and other autoimmune diseases(Reference Adorini and Penna72, Reference Merlino, Curtis and Mikuls80, Reference Kamen, Cooper and Bouali81). However, more prospective controlled clinical research is needed in these areas.
Vitamin D and type 2 diabetes
Vitamin D deficiency impairs insulin secretion of pancreatic β-cells and increases insulin resistance in target tissues, both of which play critical roles in type 2 diabetes development(Reference Knekt, Laaksonen and Mattila82). In the Nurses' Health Study, 83 779 women with no history of diabetes, CVD or cancer were followed for 20 years(Reference Pittas, Dawson-Hughes and Li83). Risk of type 2 diabetes was reduced by total Ca intake or supplemental vitamin D, while a combined daily dose of>1200 mg Ca and>800 IU vitamin D was associated with a 33 % risk reduction of type 2 diabetes compared with those with Ca < 600 mg and < 400 IU vitamin D(Reference Pittas, Dawson-Hughes and Li83). Baseline serum 25(OH)D levels were inversely associated with the risk of type 2 diabetes in the Min-Finland Health Survey of 4097 eligible participants followed for 17 years(Reference Mattila, Knekt and Maennistoe84). The relative risk (RR) of the highest (mean 70·9 nmol/l) to the lowest (mean 22·4 nmol/l) serum 25(OH)D quartiles was 0·60(Reference Mattila, Knekt and Maennistoe84). In a combined analysis of two nested case–control studies with 412 cases and 986 controls in the Finnish Mobile Clinic Health Examination Survey (19 518 men and women aged ≥ 20 years) and the Mini-Finland Health Survey (8000 individuals aged ≥ 30 years) followed for 22 and 17 years, respectively, the relative odds of the highest (mean about 75 nmol/l) relative to the lowest (mean about 24 nmol/l) quartiles of baseline serum 25(OH)D was 0·28 for type 2 diabetes in men but not significant in women who had lower serum 25(OH)D (highest quartile mean about 63 nmol/l)(Reference Knekt, Laaksonen and Mattila82).
Vitamin D and cardiovascular diseases
The mechanisms of the protective role of vitamin D in CVD are proposed to be inhibition of vascular smooth muscle proliferation, suppression of vascular calcification, down-regulation of pro-inflammatory cytokines, up-regulation of anti-inflammatory cytokines, and the action of vitamin D as a negative endocrine regulator of the renin–angiotensin system(Reference Zittermann, Schleithoff and Koerfer85).
Baseline dietary vitamin D intake was found to be inversely associated with the risk of hypertension in 28 886 US women aged ≥ 45 years followed for 10 years in the Women's Health Study(Reference Wang, Manson and Buring86). In two prospective cohort studies including 613 men from the Health Professionals Follow-up Study and 1198 women from the Nurses' Health Study followed for 4–8 years, the RR of incident hypertension among men whose plasma 25(OH)D levels were < 37·5 nmol/l was 6·13 compared with those whose levels were ≥ 75 nmol/l, while in women the RR was 2·67(Reference Forman, Giovannucci and Holmes87). A prospective nested case–control study of 18 225 men in the Health Professionals Follow-up study followed for 10 years showed that men with 25(OH)D ≤ 37·5 nmol/l had a RR of 2·42 for myocardial infarction compared with those with 25(OH)D ≥ 75 nmol/l(Reference Giovannucci, Liu and Hollis88). Among 4839 participants of the National Health and Nutrition Examination Survey 2001–2004, the prevalence ratio of peripheral arterial disease after multivariable adjustment was 1·35 for each 25 nmol/l lower baseline serum 25(OH)D(Reference Melamed, Muntner and Michos89). In a prospective cohort study of 3258 consecutive patients (mean age 62 years) scheduled for coronary angiography followed for a median of 7·7 years, the multivariate-adjusted hazard ratios for patients in the lower two serum 25(OH)D quartiles were 2·08 and 1·53 for all-cause death and 2·22 and 1·82 for cardiovascular mortality, respectively, compared with the highest quartile(Reference Dobnig, Pilz and Scharnagl90). Similar but less significant relationships were also found for serum 1,25(OH)2D(Reference Dobnig, Pilz and Scharnagl90).
CVD is the leading cause of death (>70 %) in dialysis patients and some form of vitamin D intake is recommended in those patients(Reference Moe91). Vitamin D deficiency has been known to affect cardiac contractility, vascular tone, cardiac collagen content and tissue maturation, while treatment with vitamin D improves survival rates in the patients with end-stage renal disease(Reference Achinger and Ayus92). In a prospective cohort study (follow for 61 (sd 23) months) comparing the risk of death between users (n 162) and non-users (n 80) of oral 1,25(OH)2D3 in a cohort of end-stage renal disease undergoing haemodialysis, the vitamin D users showed a hazard ratio of 0·287 compared with non-users for death from CVD(Reference Shoji, Shinohara and Kimoto93). Baseline serum 25(OH)D was significantly associated with a reduction of fatal or non-fatal cardiovascular events in 230 peritoneal dialysis patients followed for 3 years or until death with every 1-unit increase in log-transformed serum 25(OH)D associated with a 44 % reduction(Reference Wang, Lam and Sanderson94). Low serum levels of 25(OH)D and 1,25(OH)2D were independent risk factors for fatal strokes during a median follow-up of 7·75 years in the Ludwigshafen Risk and Cardiovascular Health study with 3316 patients who were referred to coronary angiography(Reference Pilz, Dobnig and Fischer95).
However, highly elevated serum 25(OH)D levels ( ≥ 222·5 nmol/l) had an adjusted OR of 3·18 for IHD in a cross-sectional case–control study with 143 patients with either angiographic evidence of coronary artery disease or acute myocardial infarction and seventy controls(Reference Rajasree, Rajpal and Kartha96). In a randomised prospective study involving 36 282 postmenopausal women (aged 50–79 years) in the Women's Health Initiative study, supplementation of 500 mg calcium carbonate with 200 IU vitamin D3 twice daily neither increased nor decreased coronary or cerebrovascular risk in 7 years of follow-up(Reference Hsia, Heiss and Ren97), probably due to the inadequate intervention vitamin D dose or concurrent use of vitamin D and/or Ca in the controls(Reference Michos and Blumenthal98).
Overall, there is strong evidence that maintaining sufficient vitamin D nutritional status has a significantly favourable impact on cardiovascular and cerebrovascular diseases.
Vitamin D and cancer
1,25(OH)2D-mediated repression or activation of proto-oncogenes or tumour-suppression genes that are related to cell proliferation and differentiation has been observed in a variety of normal and tumour tissues, including the small and large intestines(Reference Lamprecht and Lipkin99). This genetic mechanism seems responsible for the anti-cancer properties observed for vitamin D.
From ecological, case–control and cohort studies, sunlight was shown to be inversely associated with mortality or incidence of prostate, breast, ovary and colon cancer(Reference van der Rhee, de Vries and Coebergh100–Reference Freedman, Dosemeci and McGlynn102). Serum levels and dietary vitamin D are associated with reduced risks of colorectal cancer and, less certainly, prostate cancer(Reference Kricker and Armstrong101). Solar UVB radiation was found to be associated with reduced risks of breast, colon, ovary, prostate and non-lymphoma cancer while an inverse correlation between mortality rates and UVB radiation was found for bladder, oesophageal, kidney, lung, pancreatic, rectal, stomach and corpus uteri cancer in a ecological study covering the entire USA with only a few states excluded(Reference Grant103). In a large cohort consisted of 416 134 skin cancer and 3 776 501 non-skin cancer as the first cancer extracted from thirteen cancer registries, risk for all second solid primary cancers except skin and lip cancers after skin melanoma were significantly lower for the sunny countries(Reference Tuohimaa, Pukkala and Scelo104). This relationship is more pronounced after non-melanoma skin cancer as the first cancer(Reference Tuohimaa, Pukkala and Scelo104). From a prospective cohort study (Health Professionals Follow-Up Study) followed for up to 14 years, serum 25(OH)D levels were predicted for 47 800 men via a multiple linear regression model including variables of dietary and supplementation vitamin D, skin pigmentation, adiposity, geographic residency and leisure-time physical activity(Reference Giovannucci, Liu and Rimm105). From multivariable models, an increase in predicted serum 25(OH)D of 25 nmol/l was associated with a 17 % reduction in cancer incidence and a 29 % reduction in total cancer mortality(Reference Giovannucci, Liu and Rimm105). In another prospective cohort study involving 363 renal transplant recipients followed for at least 3 years, pre-transplant serum 25(OH)D3 levels were inversely associated with cancer incidence after the transplantation(Reference Ducloux, Courivaud and Bamoulid106).
Colorectal cancer
There are many trials reporting an inverse relationship between vitamin D and colorectal or colon cancer. In 12 823 men and 14 922 women with diagnosis of colon cancer, the survival rate 18 months after diagnosis was dependent on the season of diagnosis, with higher calculated serum 25(OH)D3 levels at diagnosis offering better survival rates(Reference Moan, Porojnicu and Robsahm107). In a large cohort study of 34 702 postmenopausal women followed for 9 years, both Ca and vitamin D intakes were inversely associated with rectal cancer risk, though the trend for vitamin D was not significant(Reference Zheng, Anderson and Kushi108). It is worth mentioning that Ca and vitamin D intakes had additive protective effects for rectal cancer risk in the population(Reference Zheng, Anderson and Kushi108). In a population-based, case–control study of colorectal cancer in Wisconsin women (678 controls, 348 colon and 164 rectal cancers), high Ca intake was associated with reduced colon and rectal cancer risk(Reference Marcus and Newcomb109). Similar relationships were found with vitamin D intake but were less significant(Reference Marcus and Newcomb109). In a population-based case–control study of 352 colon and 217 rectal cancers with 512 controls, dietary vitamin D was inversely associated with colorectal cancer risk, while dietary Ca was not(Reference Pritchard, Baron and deVerdier110). One shortcoming of the study is that the supplementation of vitamin D and Ca was not ascertained(Reference Pritchard, Baron and deVerdier110). In a prospective study of 60 866 men and 66 883 women followed for up to 5 years, both Ca and vitamin D intakes were inversely associated with colorectal cancer risk(Reference McCullough, Robertson and Rodriguez111). From a large population-based study with 48 115 US women followed for 22 years, both Ca and vitamin D intakes were weakly inversely associated with distal colorectal adenoma risk, while vitamin D intake was strongly associated with reduced risk of distal colon adenoma(Reference Oh, Willett and Wu112). In a multicentre randomised clinical trial of 1905 participants designed for dietary effects on recurrence of colorectal adenoma, dietary and supplement data were collected in each of the 4 years(Reference Hartman, Albert and Snyder113). Total vitamin D intake was weakly inversely associated with adenoma recurrence, while Ca was not(Reference Hartman, Albert and Snyder113). Ca supplementation was found to reduce colorectal adenoma recurrence only when the serum 25(OH)D was>72·8 nmol/l (median level) and serum 25(OH)D levels were inversely associated with the risk only among the subjects having the Ca supplement in a multicentre, placebo-controlled randomised trial of Ca supplementation for the prevention of colorectal adenoma recurrence involving 803 patients(Reference Grau, Baron and Sandler114).
However, there are many trials reporting non-significant relationship between vitamin D and colorectal or colon cancer. Both vitamin D and Ca intakes were found inversely associated with colon cancer risk in a prospective study of 47 935 US male health professionals followed for 6 years, but the associations were not significant after adjusted for confounding variables(Reference Kearney, Giovannucci and Rimm115). From a multi-state cohort study of 1993 colon cancer cases and 2410 population-based controls in the USA, dietary Ca but not dietary vitamin D was inversely associated with colon cancer, while vitamin D supplementation was inversely associated with colon cancer risk(Reference Kampman, Slattery and Caan116). Similarly, from a population-based study involving 61 463 women in Sweden followed for an average of 11·3 years, dietary Ca, not vitamin D, was inversely associated with colorectal cancer risk(Reference Terry, Baron and Bergkvist117). Intakes of Ca and vitamin D were not associated with the risk of colorectal cancer in a large, prospective, female cohort from the US Women's Health Study with 39 876 women aged>45 years followed for an average of 10 years(Reference Lin, Zhang and Cook118). Ca, but not vitamin D intake, was inversely associated with the risk of both colorectal adenoma and cancer in another large prospective cohort study of 73 034 French women followed for up to 7 years(Reference Kesse, Boutron-Ruault and Norat119). Daily supplementation of Ca with vitamin D for 7 years had no effect on the incidence of colorectal cancer among postmenopausal women in a randomised, double-blind, placebo-controlled trial involving 36 282 postmenopausal women from forty Women's Health Initiative centres(Reference Wactawski-Wende, Kotchen and Anderson120).
To understand the conflicting results from these studies, a few factors should be considered: (1) subjects' baseline intake of Ca and vitamin D; (2) duration of the studies, considering the long latency (10–20 years) of colon cancer; (3) insufficient vitamin D intake in the intervention studies, considering that serum 25(OH)D>80 nmol/l is considered sufficient and most dietary studies had intake of only 400–500 IU/d(Reference Grant32, Reference Newmark and Heaney121). There are many confounders associated with dietary vitamin D studies including in vivo vitamin D synthesis upon exposure to UVB. Therefore, measuring serum levels of 25(OH)D is a more accurate way of assessing vitamin D status in a clinical study exploring the association of vitamin D with diseases.
In a case–control study with 473 primary distal colorectal adenoma cases and 507 controls, plasma 25(OH)D showed a linear trend (not statistically significant) toward decreasing risk of the adenoma(Reference Levine, Harper and Ervin122). A nested case–control study within a Finnish clinical trial cohort involving 146 cases (ninety-one colon, fifty-five rectal cancer) and 290 controls showed that baseline serum 25(OH)D, not 1,25(OH)2D, in cases was significantly lower by 11·6 % with an average of 3·5 years between sample collection and case diagnosis(Reference Tangrea, Helzlsouer and Pietinen123). Another case–control study involving 239 colorectal adenoma and 228 controls showed an inverse association between serum 25(OH)D and colorectal adenoma risk; the relationship was strengthened by Ca intake above the median(Reference Peters, McGlynn and Chatterjee124). A subset (179 colorectal cancer cases and 356 controls) of the Health Professionals Follow-up Study was followed for 8 years, and higher plasma 25(OH)D levels were significantly associated with decreased risk of colon cancer(Reference Wu, Feskanich and Fuchs125). When pooled with the Nurses' Health Study, higher plasma 25(OH)D levels were significantly inversely associated with both colorectal and colon cancers(Reference Wu, Feskanich and Fuchs125). A more recent case–control study in Japan involving 375 colorectal cancers with two controls for each case showed that the lowest quartile of plasma 25(OH)D was associated with an increased risk of rectal cancer in both men and women, though no significant correlation was observed between plasma 25(OH)D and colorectal cancer in the 11·5-year follow-up after blood collection(Reference Otani, Iwasaki and Sasazuki126). An analysis of eighteen prospective cohort or retrospective case–control studies showed that individuals with intake of ≥ 1000 IU/d or serum 25(OH)D ≥ 82·5 nmol/l had a 50 % lower incidence of colorectal cancer compared with reference values (100 IU/d or < 32·5 nmol/l)(Reference Gorham, Garland and Garland127). A meta-analysis of five nested case–control studies of serum 25(OH)D in association with colorectal cancer risk showed that a 50 % lower risk of colorectal cancer was associated with serum 25(OH)D ≥ 82·5 nmol/l compared with serum 25(OH)D ≤ 30 nmol/l(Reference Gorham, Garland and Garland128).
Breast cancer
Breast cancer is the most commonly diagnosed cancer in US women(Reference Cui and Rohan129). The vitamin D receptor is present in breast tissue and 1,25(OH)2D has anti-proliferative and pro-differentiation effects on breast cancer cells(Reference Rohan130). Vitamin D and Ca are metabolically interrelated and are suggested in playing a role in the development of breast cancer by some epidemiological studies(Reference Cui and Rohan129). In an analysis of the first National Health and Nutrition Examination Survey Epidemiologic Follow-up Study, among a cohort of 5009 white women followed for an average of 17·3 years, several measures of sunlight exposure and dietary vitamin D intake were associated with a reduced risk of breast cancer(Reference John, Schwartz and Dreon131). The highest risk reduction was observed for women who lived in US regions of high solar radiation while no risk reduction was observed for the women who lived in regions of low solar radiation(Reference John, Schwartz and Dreon131). In a population-based case–control study in Germany involving 278 premenopausal cases and 666 age-matched controls, vitamin D intake was significantly inversely associated with breast cancer risk(Reference Abbas, Linseisen and Chang-Claude132). During 16 years of follow-up of 88 691 women in the Nurses' Health Study, both dairy Ca (RR 0·69; >800 v. ≤ 200 mg/d) and total vitamin D intake (RR 0·72; >500 v. ≤ 150 IU/d) had inverse associations with breast cancer risk in premenopausal but not postmenopausal women(Reference Shin, Holmes and Hankinson133). From a large prospective cohort study of 34 321 postmenopausal women followed for 18 years in the Iowa Women's Health Study, women with vitamin D intake >800 IU/d had an adjusted risk for breast cancer of 0·89 (weak association) compared with those with vitamin D intake < 400 IU/d(Reference Robien, Cutler and Lazovich134). From a population-based case–control study in Canada involving 972 newly diagnosed invasive breast cancer and 1135 controls, reduced breast cancer risk was associated with increased sunlight exposure from age 10 to 19 years(Reference Knight, Lesosky and Barnett135). In addition, cod liver oil use and increased milk consumption were also associated with a reduced risk of breast cancer(Reference Knight, Lesosky and Barnett135). The associations were weaker for women aged 20–29 years and null for women aged 45–54 years(Reference Knight, Lesosky and Barnett135). The importance of adolescent exposure to vitamin D on breast cancer risk reduction in adulthood was not observed in either the Nurses' Health Study or the Nurses' Health Study II in which diet during high school was assessed by dietary questionnaire at adulthood(Reference Frazier, Li and Cho136, Reference Frazier, Ryan and Rockett137). However, dietary Ca but not vitamin D was found to be inversely associated with breast cancer risk in the Cancer Prevention Study II Nutrition Cohort of 68 567 postmenopausal women followed up to 9 years(Reference McCullough, Rodriguez and Diver138). Though these studies are important at initial identification of a vitamin D and breast cancer relationship, dietary vitamin D intake cannot be considered a complete assessment of vitamin D nutritional status(Reference Cui and Rohan129).
Among 790 breast cancer survivors in the Health, Eating, Activity, and Lifestyle Study, forty-nine (6·2 %) had serum 25(OH)D < 25 nmol/l and 548 (69·4 %) had serum 25(OH)D between 25 and 80 nmol/l(Reference Neuhouser, Sorensen and Hollis139). The overall mean serum 25(OH)D was 62 (sd 26) nmol/l, while African American survivors had 45·3 (sd 21·8) nmol/l and Hispanic survivors had 55·3 (sd 23) nmol/l(Reference Neuhouser, Sorensen and Hollis139). In a case–control study followed for about 6 years nested within the Nurses' Health Study involving 701 cases and 724 controls, cases had a significantly lower plasma 25(OH)D than controls, while mean 1,25(OH)2D levels were similar in the two cohorts(Reference Bertone-Johnson, Chen and Holick140). Serum 25(OH)D was significantly inversely associated with postmenopausal breast cancer risk in a population-based case–control study in Germany with 1394 cases and 1365 controls(Reference Abbas, Linseisen and Slanger141). Compared with the lowest category ( < 30 nmol/l serum 25(OH)D), OR in other categories for breast cancer were 0·57 (30–45 nmol/l), 0·49 (45–60 nmol/l), 0·43 (60–75 nmol/l) and 0·31 ( ≥ 75 nmol/l)(Reference Abbas, Linseisen and Slanger141). In a short-term (mean 3·9 years between blood draw and cancer diagnosis) prospective cohort case–control study in the Prostate, Lung, Colorectal, and Ovarian Cancer Screening Trial, neither serum 25(OH)D nor 1,25(OH)2D was associated with breast cancer risk in the postmenopausal women(Reference Freedman, Chang and Falk142). This negative finding may be due to the very short period of follow-up.
In conclusion, epidemiological evidence of protective effects of vitamin D on breast cancer risk is strong, though some conflicting data have been reported.
Lymphoma
Strong evidence from case–control studies exists for protection against non-Hodgkin lymphoma by sun exposure and vitamin D intake. In a case–control study involving 704 adults with non-Hodgkin lymphoma and 694 controls in Australia, sun exposure was inversely associated with the risk of non-Hodgkin lymphoma, especially in women and in childhood(Reference Hughes, Armstrong and Vajdic143). Sunbathing and sunburns at age 20 years, 5–10 years before the interview, and sun exposure during vacations abroad were inversely associated with the risks of non-Hodgkin lymphoma as well as Hodgkin lymphoma, though the association was weaker for Hodgkin lymphoma in a population-based case–control study with 3740 patients and 3187 controls in Denmark and Sweden(Reference Smedby, Hjalgrim and Melbye144). UV radiation exposure but not dietary vitamin D was associated with a reduced risk of non-Hodgkin lymphoma in a case–control study with 551 cases and 462 controls in the USA(Reference Hartge, Lim and Freedman145). UV radiation exposure was also found to reduce overall lymphoma risk in a population-based case–control study with 710 paired malignant lymphoma cases and controls in Germany(Reference Weihkopf, Becker and Nieters146). Total sun exposure was found inversely related to the risk of non-Hodgkin lymphoma in a population-based case–control study in the USA with 387 cases and 535 controls(Reference Soni, Hou and Gapstur147). In a pooled analysis including ten case–control studies covering 8243 cases and 9697 controls in the USA, Europe and Australia, the risk of non-Hodgkin lymphoma fell significantly with the composite measure of increasing recreational sun exposure (OR 0·76 for the highest category)(Reference Kricker, Armstrong and Hughes148). In a hospital-based case–control study with 190 cases and 484 controls, the risk of non-Hodgkin lymphoma was reduced by the intake of vitamin D, PUFA and linoleic acid(Reference Polesel, Talamini and Montella149).
Prostate cancer
There are perplexing data on the relationship between vitamin D and prostate cancer risk(Reference Grant32). Analysis of a cohort of 3414 white men, among whom 153 developed prostate cancer after up to 21 years of follow-up in the First National Health and Nutrition Examination Survey Epidemiologic Follow-up Study, residential sunlight exposure was associated with significant and substantial reductions in prostate cancer risk(Reference John, Dreon and Koo150). An inverse correlation between the UVB levels and prostate cancer incidence and mortality rates were observed for white men, while for black men only prostate cancer incidence was significantly inversely associated with UVB radiation in the continental USA(Reference Colli and Grant151). However, both higher latitude and July UVB radiation were associated with a higher risk of prostate cancer mortality rates in the USA in the periods of 1970–94 and 1950–69, indicating that both high and low levels of vitamin D impose risk for prostate cancer mortality(Reference Grant152).
Effects of dietary vitamin D on prostate cancer can be confounded by other ingredients in the food that can be either risk enhancers or reducers for prostate cancer(Reference Grant152). In a population-based case–control study in Sweden with 526 cases and 536 controls, dietary vitamin D intake was not associated with prostate risk while Ca intake was positively associated with prostate cancer risk(Reference Chan, Giovannucci and Andersson153). Dietary vitamin D intake was shown not to be associated with prostate cancer risk in another population-based, case–control study involving 605 incident cases and 592 controls in the USA(Reference Kristal, Cohen and Qu154). Dietary vitamin D was not significantly associated with prostate cancer risk, while Ca intake was a positive risk in a prospective study of 3612 men followed for up to 10 years in the First National Health and Nutrition Examination Survey Epidemiologic Follow-up Study(Reference Tseng, Breslow and Graubard155). Comparing 1294 men with incident prostate cancer with 1451 men admitted to hospital for acute non-neoplastic diseases in Italy, no material association of dietary Ca or vitamin D with prostate cancer risk was found(Reference Tavani, Bertuccio and Bosetti156). No association of either dietary vitamin D or Ca intake with prostate cancer risk was found for 82 483 men followed for a mean of 8 years in the Multiethnic Cohort Study(Reference Park, Murphy and Wilkens157). In a meta-analysis of 26 769 cases from forty-five observational studies, neither dietary vitamin D nor Ca demonstrated a significant association with prostate cancer risk(Reference Huncharek, Muscat and Kupelnick158). Most of the study subjects had very low vitamin D intake and therefore the true effects of vitamin D on prostate cancer might not be determined by these data.
Using serum or plasma levels of vitamin D metabolites as indicators of vitamin D nutritional status, many case–control studies showed no significant relationship between vitamin D and prostate cancer. In a nested case–control study including 232 cases and 414 age-matched controls in the 14 916 participants of the Physicians' Health Study followed for 10 years, no significant association between either 25(OH)D or 1,25(OH)2D and prostate cancer risk was observed(Reference Gann, Ma and Hennekens159). No significant association was found between prostate cancer risk and either 25(OH)D or 1,25(OH)2D in a nested case–control study in a cohort of 3737 Japanese American men followed for over 23 years(Reference Nomura, Stemmermann and Lee160). In a prospective case–control study involving 460 men who developed prostate cancer and an equal number of controls in the Health Professionals Follow-up study followed for up to 5 years, there was no inverse association between plasma 25(OH)D or 1,25(OH)2D and incident prostate cancer risk(Reference Platz, Leitzmann and Hollis161). No significant inverse correlation between 25(OH)D or 1,25(OH)2D with prostate cancer risk was observed in a nested case–control study involving eighty-three cases and 166 controls within the Nutrition Prevention of Cancer trial followed for up to 19 years(Reference Jacobs, Giuliano and Martinez162). No statistically significant trend was observed for overall prostate cancer risk with increasing season-standardised serum 25(OH)D in a nested case–control study with 749 cases and 781 controls within the Prostate, Lung, Colorectal, and Ovarian Cancer Screening Trial followed for up to 10 years(Reference Ahn, Peters and Albanes163). However, serum 25(OH)D levels greater than the lowest quintile were associated with an increased risk of aggressive disease(Reference Ahn, Peters and Albanes163).
Contrarily, there are studies showing a significant relationship between serum or plasma vitamin D status and prostate cancer risk. In a nested case–control study involving 149 prostate cancer cases and four controls for each case based on a 13-year follow-up of about 19 000 middle-aged men within the Helsinki Heart Study, prostate cancer risk was inversely associated with baseline serum 25(OH)D, with an OR of 1·7 for serum 25(OH)D levels below the median compared with those above the median(Reference Ahonen, Tenkanen and Teppo164). This relationship was more pronounced among the young men (aged < 52 years), with an OR of 3·5(Reference Ahonen, Tenkanen and Teppo164). In a longitudinal nested case–control study with 622 prostate cancer cases and 1451 controls on Nordic men using serum banks of about 200 000 samples followed for up to 24 years, both low ( ≤ 19 nmol/l) and high ( ≥ 80 nmol/l) serum 25(OH)D had association with higher prostate cancer risk(Reference Tuohimaa, Tenkanen and Ahonen165). The optimal concentration of serum 25(OH)D was 40–60 nmol/l(Reference Tuohimaa, Tenkanen and Ahonen165). Low serum 25(OH)D ( ≤ 40 nmol/l) significantly strengthened the relationship between the risk of prostate cancer and factors related to the metabolic syndrome while serum 25(OH)D was not significantly associated with prostate cancer risk in a longitudinal nested case–control study with 132 prostate cancer cases and 456 controls within a cohort of 18 939 Finnish middle-aged men followed for about 16 years in the Helsinki Heart Study(Reference Tuohimaa, Tenkanen and Syvala166). In a prospective case–control study with 1066 men with incident prostate cancer and 1618 controls among 14 916 men followed for 18 years within the Physicians' Health Study, men with serum levels of both 25(OH)D and 1,25(OH)2D below the medians had a significantly increased risk of aggressive prostate cancer(Reference Li, Stampfer and Hollis167). There was also significant interaction between circulating 25(OH)D and vitamin D receptor genotype for prostate cancer risk(Reference Li, Stampfer and Hollis167).
Overall, the relationship between prostate cancer risk and vitamin D nutritional status is conflicting and not conclusive. Genetic polymorphisms seem to play an important role. Further long-term comprehensive studies evaluating the effects of both serum levels of 25(OH)D and genetic variations in vitamin D receptor on prostate cancer risk are needed.
Ovarian cancers
The evidence on the relationship between vitamin D and ovarian cancer is contradictory and no definitive conclusion can be drawn from the data currently available.
Residential exposure to sunlight was significantly inversely correlated with mortality of ovarian cancer from data collected from 1984 to 1995 in twenty-four US states(Reference Freedman, Dosemeci and McGlynn102). In an analysis of UVB data for July 1992 and cancer mortality rates in the USA for 1970–94, solar UVB radiation was associated with a reduced risk of ovarian cancer(Reference Grant103). Fatal ovarian cancer in the 100 largest US cities in 1979–88 was inversely associated with mean annual intensity of local sunlight(Reference Lefkowitz and Garland168). UVB irradiance was also inversely associated with ovarian cancer risk based on age-adjusted incidence rates for 175 countries using the International Agency for Research on Cancer GLOBOCAN database(Reference Garland, Mohr and Gorham169). In a case–control study in Mexico City with eighty-four new cases of ovarian cancer and 629 controls, dietary vitamin D intake was significantly associated with reduced ovarian cancer risk(Reference Salazar-Martinez, Lazcano-Ponce and Lira-Lira170). However, other dietary studies yielded negative results. In a hospital-based case–control study with 1031 ovarian cancer patients and 2411 controls in Italy, dietary vitamin D was not significantly associated with epithelial ovarian cancer risk(Reference Bidoli, La Vecchia and Talamini171). A case–control study in Hawaii and Los Angeles with 558 patients and 607 controls did not show a significantly inverse association between dietary vitamin D and the risk of ovarian cancer(Reference Goodman, Wu and Tung172). No significant relationship was found for dietary vitamin D and ovarian cancer risk in a prospective cohort study among 31 925 subjects followed for an average of 8·3 years(Reference Koralek, Bertone-Johnson and Leitzmann173). No significant association between dietary vitamin D intake and ovarian cancer risk was found in a meta-analysis of twelve prospective cohort studies that consisted of 553 217 women, among whom 2132 had epithelial ovarian cancer(Reference Genkinger, Hunter and Spiegelman174). In a nested case–control study with 224 cases and 603 controls within the Nurses' Health Study, Nurses' Health Study II, and Women's Health Study, neither plasma 25(OH)D nor 1,25(OH)2D was significantly associated with ovarian cancer risk(Reference Tworoger, Lee and Buring175).
Other cancers
There are limited reports on the relationship between vitamin D nutrition status and the risk of other types of cancer. More prospective studies are needed.
In the Health Professionals Follow-up Study with 47 800 men followed for 14 years, an increment of plasma 25(OH)D of 25 nmol/l was associated with significant reduction of the following cancers: pancreatic cancer (RR 0·49); oesophageal cancer (RR 0·37); colorectal cancer (RR 0·63)(Reference Giovannucci, Liu and Rimm105). In an analysis of two prospective cohort studies of 46 771 men in the Health Professionals Follow-up Study followed for 14 years and 75 427 women in the Nurses' Health Study followed for 16 years, higher vitamin D intake was associated with a lower risk of pancreatic cancer(Reference Skinner, Michaud and Giovannucci176). The association was stronger in men than in women(Reference Skinner, Michaud and Giovannucci176). Contrarily, higher serum 25(OH)D was associated with an increased risk for pancreatic cancer in a prospective nested case–control study with 200 cases and 400 controls within the Alpha-Tocopherol, Beta-Carotene Cancer Prevention cohort of male Finnish smokers(Reference Stolzenberg-Solomon, Vieth and Azad177). However, caution should be taken in interpreting the results due to the special study population who smoked and obtained vitamin D primarily from fish, which may contain ingredients that increase the risk for pancreatic cancer(Reference Michaud178). Higher dietary vitamin D intake was associated with an increased risk for laryngeal cancer in a hospital-based case–control study with 527 cases and 1297 controls(Reference Bidoli, Bosetti and La Vecchia179). In a prospective case–control study with 979 cases and 1105 controls followed for 6 years in China, higher serum 25(OH)D was associated with a higher risk for oesophageal squamous cell carcinomas in men but not in women(Reference Chen, Dawsey and Qiao180). No association was found between serum 25(OH)D with either gastric cardia or non-cardia adenocarcinoma(Reference Chen, Dawsey and Qiao180). It is noticeable that the serum 25(OH)D level was low in the study population, with the 75th percentile at 48·7 nmol/l(Reference Chen, Dawsey and Qiao180). Among 100 multiple myeloma cases, 40 % had serum 25(OH)D ≤ 36 nmol/l, 35 % had serum 25(OH)D 36–75 nmol/l, and only 25 % had ≥ 75 nmol/l(Reference Badros, Goloubeva and Terpos181). Based on the age-adjusted incidence rates for 175 countries in a UN cancer database (GLOBOCAN), lower levels of UVB and higher intakes of energy from animal foods were independently associated with a higher risk for kidney cancer(Reference Mohr, Gorham and Garland182). No significant relationship was found between endometrial cancer and dietary vitamin D in a pooled analysis of three case–control studies(Reference McCullough, Bandera and Moore183). Intermittent sun exposure was significantly inversely associated with the risk of death in 260 melanoma patients within a population-based case–control study in Italy followed for up to 21 years(Reference Rosso, Sera and Segnan184).
Conclusions
There is strong evidence that maintaining sufficient vitamin D nutritional status is beneficial to bone health, muscle strength, cardiovascular and cerebrovascular diseases, autoimmune disease, type 2 diabetes and many types of cancer. The best biomarker for vitamin D nutritional status is serum level of total 25(OH)D. The epidemiological studies are heterogeneous in respect of study design, study population and technologies used for 25(OH)D quantification. Based on the data summarised in Table 1, serum 25(OH)D level of at least 50 nmol/l seems required for beneficial impact on general health, bone metabolism, muscle strength, autoimmune disease, type 2 diabetes, CVD and various cancers. The optimal serum 25(OH)D concentration may be over 75 or 80 nmol/l. However, 25(OH)D levels higher than 125 nmol/l or 222·5 nmol/l may present adverse impacts on general mortality and IHD, respectively. The best serum 25(OH)D level for prostate cancer prevention might be 40–60 nmol/l. Prospective intervention studies are needed to define the optimal levels of vitamin D nutritional status for a variety of diseases.
Table 1 Selected studies on relationship between blood 25-hydroxyvitamin D (25(OH)D) (nmol/l) and risk for various diseases
NA, not available.
* OR were statistically significant (P < 0·05).
The sufficient level of vitamin D nutritional status is currently considered to be serum 25(OH)D >75 nmol/l, which is derived from the clinical studies using immunoassays or protein-binding assays. These assays show significant disparities among themselves and significant bias compared with LC–MS, which is considered the ‘gold standard’. Therefore, to better serve patients and advance the understanding of the relationship between vitamin D nutritional status and health and disease, assay-specific reference ranges should be established, or all assays should be standardised to LC–MS with an appropriate reference range established.
Acknowledgements
The present research received no specific grant from any funding agency in the public, commercial or not-for-profit sectors.
The author declared no potential conflicts of interest.
