The majority of vitamin D required by humans is derived by ultraviolet (UV) radiation of the 7-dehydrocholesterol in the skin( Reference McCance and Widdowson 1 ). Vitamin D can also be obtained from the diet, and these sources of vitamin D are of particular importance here in the UK and Ireland as a number of factors, including age, skin pigmentation, clothing and the use of sun protection, and geographical location, negatively influence the skin's ability to synthesise the vitamin( Reference Hagenau, Vest and Gissel 2 ). Milk is a source of naturally occurring vitamin D (albeit providing a small contribution); the vitamin D content of whole, semi-skimmed and skimmed milk is currently listed as 0.03 μg, 0.01 μg and trace per 100 g, respectively( Reference McCance and Widdowson 1 ). Numerous studies have reported the effectiveness of dairy fortification as a strategy to increase dietary vitamin D intakes at the population level( Reference Calvo, Whiting and Barton 3 , Reference Kruger, Schollum and Kuhn-Sherlock 4 ). The aim of this study was to use population level dietary data to simulate how fortified cow's milk could contribute to total dietary vitamin D intake using a dietary modelling approach.
Data from the National Diet and Nutrition Survey (NDNS) rolling programme (2008–2011) were obtained from the UK Data Service( 5 ). A total of 12,239 recorded diary days (n3,073) were available for analysis. The vitamin D content of whole, semi-skimmed, skimmed and 1% fat milk was then manipulated to apply three incremental levels of fortification (1 μg, 1.5 μg and 2 μg per 100 ml), and the effect on total vitamin D intake was investigated in all milk consumers.
A total of 82.5% diary days reported an intake of cow's milk, with intakes of consumption ranging from 2–1,764 ml with a mean intake of 202 (SD 180) ml/d. While semi-skimmed milk proved the most popular type of milk among consumers (51% of diary days), the mean volume of whole milk consumed was higher, compared to that of semi-skimmed milk (249.7(SD 207) vs.187(SD 162.1 ml/d; P < 0.001). As a result, the simulated fortification of whole milk showed the greatest increase in total vitamin D intake.
* Vitamin D content of milk as listed in the McCance and Widdowson( Reference McCance and Widdowson 1 ).
Percentiles (25th, 75th).
a,b,c,d values within a row with different superscript letters represent significance (P < 0.001, Friedman Test and Wilcoxon Signed Rank Test).
Results show that fortification of UK cow's milk with vitamin D, at concentrations similar to that applied in the US/Canada (approximately 1 μg/100 ml), would significantly increase consumer's vitamin D intake. Such fortification would therefore be an effective strategy to increase vitamin D intakes, and subsequently improve the vitamin D status at the population level. While this study has focused solely on the effect of vitamin D fortification of cow's milk, more research is needed to investigate the impact of such fortification on the vitamin D content of other dairy products and their contributions to consumer's dietary vitamin D intakes.
