Effect of ascorbic acid dose taken with a meal on nitrosoproline excretion in subjects ingesting nitrate and proline

We determined the dose of ascorbic acid (ASC) given to subjects with a standard 400‐calorie meal that inhibited N‐nitrosoproline (NPRO) formation when we gave 400 mg of nitrate one hour before and 500 mg of L‐proline with the standard meal. Volunteers consumed their normal US diets but restricted their intakes of nitrate, proline, NPRO, and ASC. NPRO and N‐nitrososarcosine (NSAR) were determined in the 18‐hour urines by methylation followed by gas chromatography‐thermal energy analysis. Mean NPRO yields were 10.7, 41.9, 33.2, 22.3, and 23.1 nmol for groups of 9–25 subjects taking proline alone, proline + nitrate, and proline + nitrate + 120, 240, and 480 mg of ASC, respectively. There was a significant trend to lower NPRO yields as the ASC dose was raised. These results correspond to inhibitions by ASC of 28%, 62%, and 60%, respectively. Pairwise comparison showed that each group taking ASC formed significantly less NPRO than the group given only proline + nitrate. Mean NSAR yields were 9.0 nmol when proline alone was taken and 16.9–24.0 nmol when proline + nitrate + ASC was taken, with no trend to increase as the ASC dose was raised. However, NPRO and NSAR yields in individual urines were correlated with each other. We concluded that 120 mg of ASC taken with each meal (360 mg/day) would significantly reduce in vivo nitrosamine formation, similar to tests by Leaf and co‐workers (Carcinogenesis 8, 791–795, 1987) in which the reactants were taken between meals. The inhibitory dose of ASC may be <120 mg/meal when doses of nitrate and proline are not taken.     

Teas and tea components as inhibitors of carcinogen formation in model systems and man.

BACKGROUND. Epidemiological evidence points to a cancer protective role of green-yellow-orange vegetables and fruits. The involvement of teas as a protective factor in carcinogenesis has not received the attention it seems to merit. To gain relevant information, attempts were made to stimulate in vitro those conditions to which human groups are actually exposed. METHODS. The inhibitory effects of infusions of Chinese, Japanese, and Ceylonese teas were examined by adding them to a nitrosation mixture consisting of 0.8 mg sodium nitrite and 340 mg equivalent of a widely consumed salt-preserved fish (Pak Wik) and estimating the frequency of mutants in TA 1535 strain of Salmonella typhimurium. RESULTS. The tea samples exhibited a strong inhibitory effect at concentrations that are actually ingested by man. A comparable inhibition was obtained by several tea phenolics. A second series of experiments dealt with the formation of nitrosoproline (NPRO) which can be strongly inhibited in vitro by the tea infusions and tea phenolics. The effects of the tea infusions and caffeic acid on the endogeneous formation of NPRO in man were examined by having volunteers ingest 300 mg sodium nitrate and 30 min later 300 mg proline, collecting urine samples over a 24-hr period, and estimating the excreted NPRO. The tested teas, at doses regularly consumed, again exerted a strong inhibitory effect on endogeneous NPRO formation in humans. Comparable inhibitory effects were obtained by ingesting caffeic acid, chlorogenic acid, or ferulic acid with the nitrosation mixture. CONCLUSIONS. These results indicate that the simultaneous intake of teas with food products that are being nitrosated within the stomach of human subjects should exert a protective, beneficial effect.     

中华猕猴桃汁阻断胃癌高发区人群的内源性N-亚硝基化合物合成

本文以空腹胃液中挥发性亚硝胺(VNA)和24小时尿N-亚硝基脯氨酸(NPRO)排出量为指标,观察中华猕猴桃汁(简称桃汁)阻断胃癌高发区人群内源性N-亚硝基化合物(NOC)合成的阻断作用。受试者服用30ml桃汁后,空腹胃液中VNA总量由平均2.08±1.06μg/L显著下降至0.42±0.43μg/L(P<0.01),阻断率为79.8％。在NPRO试验中,受试验者摄入300mg脯氨酸后,尿NPRO排出量由本底的3.3±1.2μg/d显著升高至9.4±4.7μg/d(P<0.001)。与脯氨酸同时服用30ml桃汁后,NPRO排出量显著降低至本底水平(2.9±1.9μg/d,P<0.001)。上述阻断作用在胃粘膜病变程度不同的三组人群间未见显著性差别(P>0.1)。本研究证实,中华猕猴桃汁能有效地阻断胃癌高危人群内源性NOC合成。     

茶叶中影响N-亚硝化反应有效成分的初步探讨

首先通过对茶叶中可能影响N-亚硝化反应的物质(抗坏血酸、茶多酚、茶儿茶素,黄酮)进行测定,并与阻断能力进行简单相关和多元(相关、回归、主成分)分析,结果提示茶叶中以茶多酚作用为最大。而后,进行证实试验。结果:黄山毛峰(绿茶)水提取液2ml的阻断率为88％,2ml水提取液所含抗坏血酸的阻断率为28％,而水提取液中茶多酚阻断率为74％;相应量的茶儿茶素阻断率为75％,茶多酚及茶儿茶素对N-亚硝化反应的作用与茶叶水提取液相似,即高水平抑制,低水平促进。由此可以认为,茶叶中影响N-亚硝化反应的主要成分为茶多酚,尤为茶儿茶素。     

The bioavailability and postprandial utilisation of sweet lupin (Lupinus albus)-flour protein is similar to that of purified soyabean protein in human subjects: a study using intrinsically 15N-labelled proteins

Sweet lupin (Lupinus albus), a protein-rich legume devoid of anti-nutritional factors, is considered to have a high potential for protein nutrition in man. Results concerning the nutritional value of lupin protein are, however, conflicting in animals and very scarce in human subjects. Furthermore, where fibre-rich protein sources are concerned, the long-term nutritional results are often obscured, particularly since fibre-promoted colonic fermentation may bias the energy supply and redistribute N flux. We therefore studied, during the postprandial phase, the bioavailability and utilisation of lupin-flour protein in nine healthy men who had ingested a mixed meal containing intrinsically 15N-labelled lupin flour as the protein source (Expt 1). The real ileal digestibility (RID) and ileal endogenous N losses (IENL) were assessed using a perfusion technique at the terminal ileum, and the N content and 15N enrichment of ileal samples. Lupin flour exhibited a high RID of 91 (SD 3)% and low IENL (5-4 (SD 1.3) mmol N/h). Postprandial dietary deamination was also assessed from body dietary urea and urinary dietary N excretion, and compared with results in nine healthy men following an iso-energetic meal containing a 15N-soyabean-protein isolate with a similar RID, as a control (Expt 2). Postprandial dietary deamination was similar after lupin and soyabean meals (17 (SD 2) and 18 (SD 4)% ingested N respectively). We therefore conclude that lupin protein is highly bioavailable, even if included in fibre-rich flour, and that it can be used with the same efficiency as soyabean protein to achieve postprandial protein gain in healthy human subjects.     

Guar gum does not impair the absorption and utilization of dietary nitrogen but affects early endogenous urea kinetics in humans

BACKGROUND: Viscous gums enhance viscosity in the upper gastrointestinal lumen, quickly disturbing motility and promoting fluid secretion. OBJECTIVE: We sought to determine whether guar gum could acutely affect the absorption and utilization of dietary nitrogen and whether these luminal effects could also perturb the kinetics of urea. DESIGN: We studied the short-term effect of adding 1% of highly viscous guar gum to a (15)N-labeled protein meal (30 g soy protein isolate in 500 mL water) during the postprandial phase in humans. The effects on bioavailability were studied by using the [(13)C]glycine breath test (to assess gastric emptying) and (15)N enrichment in plasma amino acids (for systemic amino acid bioavailability). The kinetics of dietary and endogenous urea were assessed in plasma and urine. RESULTS: Guar gum modulated the gastric emptying kinetics of the liquid phase of the meal slightly (P < 0.05), but had no significant effect on either the systemic appearance of dietary amino acids or plasma and urinary dietary urea kinetics. Without significantly affecting plasma urea concentrations, guar gum reduced by approximately 40% the urinary excretion of endogenous urea for the first 2-h period after the meal (P < 0.01), although endogenous urinary excretion was similar at later stages. CONCLUSIONS: Guar gum did not significantly affect the bioavailability or utilization of dietary protein. We showed an early effect of guar gum on endogenous urea kinetics, which most probably arose from very early, short-term stimulation of the intestinal disposal of endogenous urea, at the expense of its urinary excretion.     

Effect of oral contraceptives on ascorbic acid status of young women consuming a constant diet

The effects of various combined oral contraceptives (OCs) on plasma, leukocyte, and urinary levels of ascorbic acid were studied in 10 well-nourished women. Based on dietary records, prestudy ascorbic acid intake ranged from 100 to 500 mg/day. By the end of the 1st day of treatment, no marked alterations in plasma or leukocyte ascorbic acid levels, or in tissue ascorbic acid absorption, were observed. After the subjects had lived in a metabolic unit for 8 or 12 days, in which they received a constant diet containing about 155 mg/day ascorbic acid, there was still no marked differences between the treatment group and controls in plasma, leukocyte, and urinary ascorbic acid. The results are contrary to previously reported findings which suggest that OC use adversely affects ascorbic acid status.     

Effect of dietary ascorbic acid and vitamin E on metabolic changes in rats and guinea pigs exposed to PCB

The effect of the addition of dietary ascorbic acid and/or vitamin E (all-rac-alpha-tocopheryl acetate) in rats and guinea pigs exposed to PCB (polychlorinated biphenyls) was studied. Rats were fed diets containing one of three levels of vitamin E (30, 500 or 1000 mg/kg diet) with or without PCB (200 mg/kg diet). Guinea pigs were fed diets containing PCB (40 mg/kg diet) with 200 or 1000 mg ascorbic acid/kg diet and/or 70 or 2000 mg vitamin E/kg diet. For rats fed PCB, ascorbic acid in urine was 40-fold higher and in liver, 2-fold higher than for rats fed no PCB, and thiobarbituric acid-reactive substances (TBA-RS, indicators of lipid peroxidation) in liver was 1.5-fold higher. In rats fed PCB, high dietary vitamin E significantly lowered the urinary ascorbic acid and TBA-RS. Liver ascorbic acid was lowered by high dietary vitamin E only in control rats. In guinea pigs, feeding PCB caused severe growth retardation and the liver TBA-RS was 1.8-fold higher than in guinea pigs not fed PCB. Feeding high levels of both ascorbic acid and vitamin E was more effective in reversing the growth depression and in lowering TBA-RS level (due to PCB) than feeding the vitamins separately. Ascorbic acid metabolism in rats was affected by high dietary vitamin E. The possibility of a higher requirement for ascorbic acid and vitamin E in guinea pigs exposed to PCB was indicated. Interaction of ascorbic acid and vitamin E in animals exposed to PCB was suggested.     

食物中总亚硝基化合物及其相关危险因素研究

目的　探索南方食管癌现场中亚硝基化合物综合危险因素。方法　在南方食管癌高发区南澳县和低发区陆丰县各随机选择12 0名35～6 4岁健康男性居民,收集2 4小时膳食和3天12小时夜尿。分别为空白尿样、5 0 0mg脯氨酸+2 0 0mg维生素C(VC)负荷尿样和5 0 0mg脯氨酸负荷尿样。测定总亚硝基化合物(TNOC)、挥发性亚硝胺、亚硝基氨基酸(NAAs)和还原型VC。用非条件logistic模型分析各类亚硝胺摄入水平和排泄水平、NAAs内源性合成能力、VC的营养状况及其对内源性合成的阻断能力等综合因素与食管癌死亡率的关系。结果　非条件logistic回归分析筛选出两个食管癌亚硝胺危险因素—膳食TNOC含量(OR为9 6. 13,95 %CI 1 92 .1～4 8 .115 )和服VC后尿NAAs浓度(OR为1 137,95 %CI为1 0. 0 1～1 2 .98)。结论　膳食TNOC高含量和VC对NOC内源性合成的低阻断能力是南方食管癌高死亡率的两个重要危险因素。     

Use of biological markers to validate self-reported dietary intake in a random sample of the European Prospective Investigation into Cancer United Kingdom Norfolk cohort

BACKGROUND: The validity of dietary assessment methods should be established before diet-disease associations are reported. OBJECTIVE: Our objective was to validate a 7-d food diary and a food-frequency questionnaire (FFQ) against independent biomarkers of intake in urine (nitrogen, potassium, and sodium) and blood (plasma ascorbic acid). DESIGN: A total of 146 healthy middle-aged men and women were recruited from the European Prospective Investigation into Cancer UK Norfolk cohort, a free-living cohort of approximately 25000 persons. Over a 9-mo period, urinary nitrogen, potassium, and sodium were estimated from 2-6 complete 24-h urine collections in 134 subjects and plasma ascorbic acid was estimated from 2-3 fasting blood samples in 118 subjects. Subjects completed 2 FFQs and two 7-d food diaries. RESULTS: In men and women combined, correlations between 24-h urinary nitrogen excretion and dietary intake from the 7-d food diary were high (r = 0.57-0.67) compared with those for the FFQ (r = 0.21-0.29). Similarly, correlations between urinary potassium and dietary potassium were higher for the 7-d food diary (r = 0.51-0.55) than for the FFQ (r = 0.32-0.34). There was no overall difference in correlations between plasma ascorbic acid and dietary vitamin C between the 7-d food diary (r = 0.40-0.52) and the FFQ (r = 0.44-0.45). CONCLUSIONS: These data indicate that, despite increased subject burden, the 7-d food diary provided a better estimate of nitrogen and potassium intakes than did the FFQ in this study population. However, with respect to plasma ascorbic acid, both the FFQ and 7-d food diary provided a similar ranking of subjects according to vitamin C intake.     

Nitrate contamination of drinking water: relationship with HPRT variant frequency in lymphocyte DNA and urinary excretion of N-nitrosamines.

We studied peripheral lymphocyte HPRT variant frequency and endogenous nitrosation in human populations exposed to various nitrate levels in their drinking water. Four test populations of women volunteers were compared. Low and medium tap water nitrate exposure groups (14 and 21 subjects) were using public water supplies with nitrate levels of 0.02 and 17.5 mg/l, respectively. Medium and high well water nitrate exposure groups (6 and 9 subjects) were using private water wells with mean nitrate levels of 25 and 135 mg/l, respectively. Higher nitrate intake by drinking water consumption resulted in a dose-dependent increase in 24-hr urinary nitrate excretion and in increased salivary nitrate and nitrite levels. The mean log variant frequency of peripheral lymphocytes was significantly higher in the medium well water exposure group than in the low and medium tap water exposure groups. An inverse correlation between peripheral lymphocyte labeling index and nitrate concentration of drinking water was observed. Analysis of N-nitrosamine in the urine of 22 subjects by gas chromatography-mass spectrometry revealed the presence of N-nitrosopyrrolidine in 18 subjects. Analysis of the mutagenicity of well water samples showed that a small number of the well water samples were mutagenic in the Ames Salmonella typhimurium test after concentration over XAD-2 resin. In conclusion, consumption of drinking water, especially well water, with high nitrate levels can imply a genotoxic risk for humans as indicated by increased HPRT variant frequencies and by endogenous formation of carcinogenic N-nitroso compounds from nitrate-derived nitrite.     

Endogenous flow of amino acids in the avian ileum as influenced by increasing dietary peptide concentrations

The aim of the present study was to establish whether feeding broiler chickens with diets containing increasing dietary peptide concentrations would cause increases in ileal endogenous amino acid flow. The flow of N and most amino acids increased quadratically (P < 0.05 to 0.001) with increasing dietary concentrations of peptides. The exceptions were the flow of threonine, serine, glycine, tyrosine and cystine, which increased linearly (P < 0.001) with dietary peptide levels. Another notable exception to the general trend was the flow of proline, which was significantly higher (P < 0.01) in birds fed the protein-free diet. The amino acid profile of endogenous protein, expressed as proportion of crude protein, indicated that the ratios of threonine, glutamic acid, proline, glycine, leucine, histidine, arginine and cystine were influenced (P < 0.05) with increasing dietary peptide concentrations. In general, compared with the protein-free diet, the ratios of threonine and arginine in endogenous protein were lower (P < 0.05) and those of glutamic acid, glycine and histidine were greater (P < 0.05) in diets with high concentrations of peptides. The ratio of proline was found to decrease (P < 0.05) with increasing dietary peptide concentrations. These changes in the amino acid profile of endogenous protein are probably reflective of changes in the output of one or more of the components of endogenous protein. Overall, the present results demonstrated that increasing dietary peptide concentrations increased the flow of endogenous amino acid flow at the terminal ileum of broiler chickens in a dose-dependent manner and also caused changes in the composition of endogenous protein. The observed changes in endogenous amino flow will influence the maintenance requirements for amino acids and also have implications for the calculation of true digestibility coefficient of feedstuffs.     

Estimation of nonheme-iron bioavailability from meal composition

BACKGROUND: Considerable data are available on the individual effects of dietary factors on nonheme-iron absorption, but their combined effect when they are present in the same meal is not known. OBJECTIVE: Our objective was to predict the bioavailability of iron from complex meals that are consumed commonly in the United States on the basis of the contents of factors that are known to promote or inhibit food iron absorption. DESIGN: Radioisotopic measurements of nonheme-iron absorption from 25 meals were made in 86 volunteer subjects by using extrinsic radioiron labeling. The meal contents of nonheme iron, calcium, ascorbic acid, polyphenols, and phytic acid were determined by biochemical analysis; energy and protein contents were estimated from food-composition tables. Animal tissue content was based on weight or was obtained from the manufacturer. RESULTS: After adjusting iron absorption for differences in iron status, the significant biochemical predictors of iron absorption as determined by multiple regression analysis were the contents of animal tissue (P = 0.0001), phytic acid (P = 0.0001), and ascorbic acid (P = 0. 0441). Collectively, these 3 variables accounted for 16.4% of the variation in absorption. On the basis of the multiple regression analysis, we developed the following equation to estimate iron absorption: Ln absorption, % (adjusted to serum ferritin concentration of 30 microg/L) = 1.9786 + (0.0123 x animal tissue in g) - (0.0034 x phytic acid in mg) + (0.0065 x ascorbic acid in mg). CONCLUSION: For the 25 meals evaluated, only the contents of animal tissue, phytic acid, and ascorbic acid were useful for estimating nonheme-iron absorption.     

Effect of ascorbic acid on the absorption of zinc and calcium in man

The effect of ascorbic acid on the absorption of zinc and calcium, was studied in healthy subjects by a radionuclide technique with 65Zn and 47Ca and whole body retention measurements. Addition of 100 mg of ascorbic acid to a water solution of 40 or 200 mumol zinc and 1 g of ascorbic acid to a high phytate meal with wholemeal bread had no effect on zinc absorption. Neither did 1 g of ascorbic acid added to 6.4 mmol of calcium influence the retention and calculated absorption of calcium. The results suggest that ascorbic acid does not affect the absorption of normal dietary levels of zinc and calcium.     

Effect of dietary level of protein on liver microsomal drug-metabolizing enzymes, urinary ascorbic acid and lipid metabolism in rats fed PCB-containing diets

Effects of dietary level of protein on liver microsomal drug-metabolizing enzyme system, lipids and urinary ascorbic acid metabolism were studied in the rats fed 0.1% polychlorinated biphenyl (PCB) containing diets. In the rats not receiving PCB, the activity of the enzyme system increased as the protein level was increased up to around 35%. In a short-term experiment (8-9 days), dietary PCB caused an increase in the activity of the enzyme system regardless of dietary level of protein. In a semi-long term experiment (29-30 days), however, 35-55% protein diets did not allow for induction to the same extent. As a result the highest activity was observed with 10-20% protein diets in this period. The increments in urinary ascorbic acid and in serum cholesterol were generally potentiated with high-protein diets. Maximum increments in urinary ascorbic acid and serum cholesterol were obtained with 20% protein and 35% protein, respectively, in the semi-long term experiment. In contrast to serum cholesterol, the accumulation of liver cholesterol and triglyceride due to PCB was markedly potentiated with low-protein diets.     

Caco-2 cells can be used to assess human iron bioavailability from a semipurified meal

A Caco-2 cell model with extrinsic radioiron was used to evaluate the effect of dietary factors on nonheme iron bioavailability from a semipurified meal. Study 1 was conducted to evaluate the effect of enhancers (ascorbic acid) and inhibitors (bran, phytate and tea) on iron bioavailability when added to semipurified meal containing egg albumen as a protein source. The effect of various proteins [bovine serum albumin (BSA), casein, beef and soy] on iron bioavailability was evaluated in Study 2 by substituting the above protein sources for egg albumen. Protein solubilization following in vitro digestion for individual test meals was not significantly different from the control. On the other hand, nonheme iron solubilization (0.8+/-0.0 to 5.9+/-0.3 vs. 4.9+/-0.8 mg/L) varied significantly. The total iron uptake for each meal was calculated based on the percentage of radioiron taken up and transported by Caco-2 cells and the amount of nonheme iron present in uptake solutions. Iron uptake ratios represent test/control values. With the exception of BSA and ascorbic acid, the effect of dietary factors was similar to that found in humans reported in the literature. A significant correlation (r = 0.97; P<0.0001) was found between the published human absorption data and the iron uptake by the Caco-2 cells. The results of our study indicate the usefulness of Caco-2 cells in assessing human iron absorption and the feasibility of this cell model in studying iron bioavailability from various food combinations, otherwise not easily performed in humans.     

Effects of dietary methionine, cystine and potassium sulfate on serum cholesterol and urinary ascorbic acid in rats fed PCB

Liver weight, liver and urinary ascorbic acid levels and serum cholesterol concentration were higher in rats fed polychlorinated biphenyls (PCB) than in controls. The influences of methionine, cystine and potassium sulfate on these metabolic responses were studied. Methionine or equivalent moles of cystine or potassium sulfate were added to a basal diet containing 10% soy protein isolate. The basal diet contained 0.3% of total sulfur-containing amino acids (S-AAs). When methionine was added to the basal diet, maximum gain in body weight was obtained with 0.5% of dietary S-AAs, while the highest values in serum cholesterol and urinary ascorbic acid were obtained with 0.8% of dietary S-AAs in rats fed PCB. Dietary addition of cystine had little effect on body weight gain. Nevertheless, in rat fed PCB, urinary ascorbic acid and serum cholesterol were significantly higher in rats fed the cystine-supplemented diet than in those fed the unsupplemented diet. Addition of potassium sulfate had no effect on body weight gain, urinary ascorbic acid or serum cholesterol. These results suggest that more S-AAs are required for the highest metabolic response to PCB than for maximum growth, and the higher requirement for S-AAs cannot be replaced by inorganic sulfate.     

Dietary taurine alters ascorbic acid metabolism in rats fed diets containing polychlorinated biphenyls

The effect of dietary taurine on ascorbic acid metabolism and hepatic drug-metabolizing enzymes was investigated in rats fed diets containing polychlorinated biphenyls (PCB) to determine whether taurine has an adaptive and protective function in xenobiotic-treated animals. Young male Wistar rats (60 g) were fed diets containing 0 or 0.2 g/kg diet PCB with or without 30 g/kg diet of taurine for 14 d. The rats fed the PCB-containing diets had greater liver weight, higher ascorbic acid concentrations in the liver and spleen and greater hepatic cytochrome P-450 contents than control rats that were not treated with PCB (P < 0.01). In PCB-fed rats, urinary ascorbic acid excretion was enhanced, and serum cholesterol concentration (especially HDL-cholesterol) was significantly elevated compared with those in control rats. Dietary taurine significantly potentiated the increases in the urinary excretion of ascorbic acid and the rise in the levels of cytochrome P-450 which were caused by PCB treatment. On the other hand, the supplementation of taurine to control diet did not alter these variables. Taurine may enhance the hepatic drug-metabolizing systems, leading to the stimulation of the ascorbic acid metabolism in rats fed diets containing PCB.     

Dietary fat and N-nitrosation in the rat

1. Groups of four conventional (CV) rats ate natural or purified diets either with or without 100 g fat/kg and drank 0.235 M-sodium nitrate. The fats tested were butterfat, coconut oil, olive oil, maize oil and safflower oil. 2. Decreased urinary excretion of N-nitrosoproline (NPRO) was observed in rats fed on fat-supplemented diets compared with those fed on low-fat diets, with butterfat having the greatest effect of the fats tested. 3. Reduced excretion of NPRO was not the result of inhibition of the intragastric N-nitrosation reaction or absorption of nitrosamine from the gastrointestinal tract. 4. The availability of nitrite in aqueous solution was decreased by the fat diets but the effect was similar in all the fats tested. 5. Nitrate reductase activity was present in the forestomach contents of CV rats at pH greater than 4 and was apparently inhibited by feeding a fat diet. No nitrate reductase activity was detected in stomach contents of germ-free rats. 6. Nitrate reductase activity in stomach and small intestinal tissue was not altered by feeding a fat diet. 7. It was concluded that nitrate reductase activity in stomach contents was of microbial origin and the decreased urinary excretion of NPRO on feeding the fat diets was mainly due to the inhibition of nitrate reductase activity in stomach contents.