Using a first-order model to describe ruminal in situ digestion implies that the rate of digestion is affected only by the quantity of potentially digestible substrate remaining. Other factors, like the microbial efficacy for digesting substrate, are assumed to be constant. However, microbes are not only the cause but also the result of digestion, being one of the digestion end-products. Two sigmoidal models (a logistic and a Gompertz-like model) were derived from a general digestion function in which the rate of digestion equals the product of the quantity of potentially digestible substrate remaining and a non-constant fractional rate of digestion (microbial efficacy function). The models were compared with a first-order model with a discrete lag time. The logistic model specifically accounted for the conversion of substrate mass to microbial mass, but did not describe microbial migration between the substrate and the ruminal fluid. In contrast, the Gompertz-like model assumed that the change in microbial efficacy was only time-dependent. There was little difference between models in estimates of scale parameters, but the asymptotic microbial efficacy was consistently higher for the logistic model than for the other models. Estimates of discrete lag time in the first-order model were biased towards obtaining values identical to the independent variable. Scale estimators appeared to be more robust than kinetic estimators. Lack-of-fit was present for most model-data set combinations. The similar patterns of residuals between models suggested that a four-parameter model may be insufficient to describe the data. It was concluded that if a four-parameter model is to be used, the model with a discrete lag time would be the least biologically appropriate.

The β-agonist ractopamine (RAC) promotes protein deposition with little effect on fat deposition in the pig. To assess whether the lack of effect on fat deposition was due to changes in response to homeostatic signals, eight crossbred gilts (73 kg body weight (BW)) with venous catheters were used to examine plasma metabolite and hormone concentrations before and after intravenous injections of insulin and the β2-agonist fenoterol during dietary RAC (0 or 20 mg/kg) treatment. Pigs received intravenous challenges of insulin (1μg/kg BW) on days 3, 9 and 23 and fenoterol (2 μg/kg BW) on days 4, 10 and 24 of treatment. RAC was then withdrawn from the diet and insulin and fenoterol challenges were repeated 6 and 7 d later respectively. Blood samples for the determination of metabolite and hormone concentrations were taken at -30, -20, -10, -1, 2·5, 5, 10, 20, 30, 45, 60 and 120 min relative to the challenges. Dietary RAC decreased basal plasma insulin concentrations but had no effect on plasma glucose or non-esterified fatty acids (NEFA). Hypoglycaemic responses to insulin were not affected by RAC while the anti-lipolytic effects of insulin tended to be augmented. Dietary RAC decreased the lipolytic response to fenoterol, this being evident after 4 d treatment. Hypoglycaemic response to fenoterol was not changed whereas the hyperinsulinaemic response to fenterol was attenuated by dietary RAC. Previous treatment with RAC did not influence basal hormone and metabolite concentrations or responses to homeostatic signals during the withdrawal period. While these results suggest little change in glucose metabolism, the de-sensitization of adipose tissue β-adrenergic receptors is consistent with the observations that dietary RAC has little effect on the rate of fat deposition in the growing pig.

The polyamines putrescine, spermidine and spermine are essential for cell renewal and, therefore, are needed to keep the body healthy. It was previously believed that polyamines are synthesized by every cell in the body when required. However, in the present paper evidence is provided to show that, as in the case of the essential amino acids, the diet can supply sufficient amounts of polyamines to support cell renewal and growth. Systematic analysis of different foods was carried out and from the data obtained, the average daily polyamine consumption of British adults was calculated to be in the range 350–500 μmol/person per d. The major sources of putrescine were fruit, cheese and non-green vegetables. All foods contributed similar amounts of spermidine to the diet, although levels were generally higher in green vegetables. Meat was the richest source of spermine. However, only a part of the polyamines supplied by the diet is available for use by the body. Based on experiments with rats it was established that polyamines were readily taken up from the gut lumen, probably by passive diffusion, and were partly metabolized during the process of absorption. More than 80% of the putrescine was converted to other polyamines and non-polyamine metabolites, mostly to amino acids. The enzyme responsible for controlling the bioavailability of putrescine was diamine oxidase (EC 1.4.3.6). For spermidine and spermine, however, about 70–80% of the intragastrically intubated dose remained in the original form. Considering the limitations on bioavailability (metabolism and conversion), the amounts of polyamines supplied by the average daily diet in Britain should satisfy metabolic requirements.

The effect of long-term dietary protein excess and deficit on whole-body protein-N turnover (WBPNT) was examined in lambs nourished by intragastric infusions of nutrients. Ten sheep were given 500 mg N/kg metabolic weight (W0.75) per d from casein for 2 weeks and then either 50 (L), 500 (M) or 1500 (H) mg N/kgW0.75 per d for 6 weeks. Volatile fatty acids were infused at 500 kJ/kgW0.75 per d. Daily WBPNT was measured by continuous intravenous infusion of [l-13C]leucine 3 d before, and on days 2, 21 and 42 after the alteration in protein intake. Whole-body protein-N synthesis (WBPNS) was calculated as the difference between WBPNT and the protein-N losses as urinary NH3 and urea. Whole-body protein-N degradation (WBPNS) was then estimated from WBPNS minus protein gain determined from N balance. Fractional rates of WBPNS and WBPND were calculated against fleece-free body N content. WBPNS rates at the L, M and H intakes were respectively 35·1, 41·5 amd 6·37 g/d (P < 0.001) on average over the 6 weeks and WBPND rates were 39·5, 41·1 and 56·8 g/d (P < 0.001). The fractional rates of WBPNS were 5·01, 6·37 and 7·73% per d (P < 0.001) while those of WBPND were 5·64, 6·29 and 6·81% per d (P < 0.005) respectively. On days 2, 21 and 42, WBPNS rates at intake H were 54·0, 61·8 and 75·4 g/d (P = 0·03) respectively, and WBPND rates were 43·2, 56·4 and 70·9 g/d (P = 0.03); at intake L the amounts were 38·2, 34·2 and 32·8 g/d for WBPNS (P = 0.003) and for WBPND were 43·4, 38·0 and 36·9 g/d (P = 0·016) respectively. There were no significant (P > 0·05) differences in fractional rates of WBPNS and WBPND with time at either the L or H intake. We concluded that absolute protein turnover was affected both by dietary protein intake and by body condition while the fractional rate of turnover was predominantly influenced by intake.

The ability of β-carotene to protect against oxidative stress in vitro was assessed. Primary cultures of chicken embryo fibroblasts (CEF) were oxidatively stressed by exposure to paraquat (PQ). Activities of the antioxidant enzymes superoxide dismutase (SOD; EC 1.15.1.1), catalase (CAT; EC 1.11.1.6) and glutathione peroxidase (GSH-Px; EC 1.11.1.9) were measured as indices of oxidative stress. CEF incubated with 0·25 mM-PQ for 18 h exhibited increased SOD and CAT activities and decreased GSH-Px activity compared with the control (P < 0·001). Incorporation of added β-carotene (0·1 μM) into 0·25 mM-PQ-treated CEF returned SOD activity to that seen in non-PQ-treated cells. β-Carotene (0·1 μM) reduced the CAT activity from that seen in PQ-treated cells and returned the GSH-Px activity to its control value thus protecting the cells against PQ-induced oxidative stress. However, at higher concentrations of β-carotene (10 μM), SOD and CAT activities increased significantly (P < 0·001) relative to non-PQ-treated cells and GSH-Px activity decreased relative to its control value. Similar trends were observed when CEF grown in β-carotene-enricbed media (0·1–10μM) were oxidatively stressed by exposure to 0·25 mM-PQ for 18 h.β-Carotene: Chicken embryo fibroblasts: Oxidative stress Antioxidant enzymes

Orlistat, a selective inhibitor of gastrointestinal lipases, was used to investigate triacylglycerol absorption. Using mice and a variety of emulsified dietary lipids we found that the absorption of radiolabelled tripalmitin (containing the fatty acid 16:0), but not of triolein (18: 1n−9) or tri-γ-linolenin (18:3n−6), was incomplete from meals rich in esterified palmitate. Further, the absorption of radiolabelled tri-γ-linolenin, from both saturated and unsaturated dietary triacylglycerols, was 1·3- to 2-fold more potently inhibited by orlistat than that of triolein and tripalmitin. These radiolabelled triacylglycerols, which have the same fatty acid in all three positions, may not always be accurate markers of the absorption of dietary triacylglycerols. Orlistat was more effective at inhibiting the absorption of radiolabelled triacylglycerols with which it was codissolved than those added separately, which indicates that equilibration between lipid phases in the stomach may not always be complete. The saturation of the dietary lipid had little or no effect on the potency of orlistat. Orlistat provides a novel approach for studying the role of triacylglycerol hydrolysis in the overall process of triacylglycerol absorption.

The mechanism underlying the reduced Cu status in rats fed on a high-Sn diet was investigated. Male rats aged 4 weeks were fed ad lib. on purified diets containing either 1 or 100 mg Sn/kg and demineralized water for a period of 4 weeks. The high-Sn diet had no effect on feed intake, body-weight gain or weight of liver and kidney but significantly reduced Cu concentrations in plasma, liver and kidney. Biliary Cu excretion was decreased significantly in rats fed on the high-Sn diet. Apparent Cu absorption (Cu intake−faecal Cu) was not affected by the high-Sn diet, but the estimate of true Cu absorption (Cu intake−(faecal Cu−biliary Cu)) was significantly reduced. We conclude that high Sn intake reduces Cu status in rats through inhibition of Cu absorption. The decreased biliary Cu excretion observed on the high-Sn diet is a result of the reduced Cu absorption.

The effect of dietary supplementation with ascorbic acid or cholecalciferol on Fe utilization was studied using the metabolic balance technique, in rats in which 50% of the distal small intestine was removed, or in which the mid small intestine was transected and reanastomosed (controls). Three different diets were used. The first (basal diet) contained (g/kg dry wt): protein (casein+50 mg D, L-methionine/g) 120 and fat (medium-chain triacylglycerols, olive oil and sunflower oil, in equal parts) 40. The other diets were obtained by adding ascorbic acid (150 mg/kg diet) or cholecalciferol (0·4 mg/kg diet) to the basal diet. Apparent digestibility coefficient (ADC) and Fe retention were significantly lower in resected animals than in their respective control groups (transected rats). However, the addition of ascorbic acid or cholecalciferol to the basal diet increased the ADC and Fe retention in both transected and resected rats. Five weeks after surgery, resection also resulted in a reduced concentration of Fe in the sternum, but did not reduce the concentration of haemoglobin or serum Fe total Fe-binding capacity or the concentration of Fe in liver, testes, femur or muscle (longissimus dorsi). Supplementation with ascorbic acid increased serum Fe concentration, while the concentration of Fe in muscle was reduced by supplementation with both ascorbic acid and cholecalciferol. Neither supplementation had any effect on the Fe concentration in other tissues, on haemoglobin concentration or plasma total Fe-binding capacity. Thus, supplementation with ascorbic acid or with cholecalciferol increased Fe absorption and reduced the concentration of Fe in muscle.

Vitamin D status of Asian mothers in Cardiff was investigated during early pregnancy and at the time of the birth of their babies, using serum parathyroid hormone (PTH). Median values in Asian (n 32) and Caucasian (n 63) mothers in early pregnancy were 1·56 and 0·81 pmol/1 respectively. PTH levels from a separate sample of nineteen Asian and twenty-five Caucasian mothers at the time of birth were 3·0 and 2·20 pmol/1 respectively. Altogether twelve Asian and two Caucasian women had elevated PTH. All Asian women who had high PTH values also had a very low serum 25-hydroxycholecalciferol level (25OHD). All samples were taken from women with no significant medical history and normal obstetric history. These findings suggest that subclinical vitamin D deficiency is still a cause for concern in Asian women. More active measures need to be taken to implement current recommendations to improve their vitamin D intake in pregnancy.

Leucocytes from subjects from 0 to 80 years old were separated into mononuclear and granulocyte fractions and the retinoids and retinoid-binding fractions were examined. Both leucocyte ractions were found to contain retinol, retinoic acid and an additional retinoid; retinoic acid Comprised 40% of retinoids in some samples. The protein fractions containing retinoids included a 200 k Da protein and several 14–18 k Da proteins. Plasma concentrations of retinol changed little with increasing age. In contrast, leucocyte concentrations of retinoids and retinoid-binding proteins increased quadratically with age. However, in granulocytes from young children retinoids were almost undetectable.

Various tannin-complexing agentsw have been used to study the potential adverse effects of tannis on rumen metabolism. Using a method based on turbidity formation, the binding of various tannin-complexing agents (polyvinyl polypyrrolidone (PVPP), polyethylene glycol (PEG) of molecular weights 2000 to 35000, and polyvinyl pyrrolidone (PVP) of molecular weight 10000, 40000 and 360000) to tannins (tannic acid, purified tannins from quebracho (Aspidosperma quebracho) and leaves of trees and shrubs (Acioa barteri, Dichostachys cinerea, Guiera senegalensis, Piliostigma reticulatum)) was investigated at different pH values. The binding of all the tannins with PVPP was highest at pH 3–4 and lowest at pH 7. For all the pH range (3–7) studied, the binding of PEG was higher than that of PVP. For all the tannins except tannic acid, the binding to PVP was the same from pH 4·7 to 7. Similar results were observed for the PEG of molecular weight 6000 or higher for all the tannins except quebracho tannins for which the binding increased as the pH increased from 3 to 7. The binding with PEG 2000 decreased to a greater extent as the pH reached near neutral and for PEG 4000 this decrease was slightly lower. Addition of these tannin-complexing agents to the in vitro gas system resulted in higher gas production from tannin-rich feeds (increase varied from 0 to 135%). The PEG were the most effective followed by PVP and PVPP. The PEG 35000 was least effective. The efficiency of other PEG was similar. The PEG 6000 was preferred to PEG 2000 or 4000 as its binding to tannins was higher at near neutral pH values. The gas production increased with an increase in the amount of PEG 6000 up to 0·6 g/40 ml rumen-fluid-containing medium containing 0·5 g tannin-rich feed, beyond which no increase was observed. The percentage increase in gas value at 24 h fermentation correlated significantly with tannin values, the highest correlation (r 0·95) being with protein precipitation capacity of tannins. The increase in gas production was associated with higher production of short-chain fatty acids with little change in their molar proportions, suggesting an increase in organic matter digestibility by inclusion of the PEG in tannin-rich feeds. However, apparent and true digestibilities were lower on addition of the PEG, due to the presence of PEG-tannin complexes in the residues. The use of this bioassay (percentage increase in gas production in the presence of PEG 6000) along with other tannin assays would provide a better insight into the nutritional significance of tannins.