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Postprandial endothelial dysfunction is a key event mediating the effects of diet on early atherogenesis. The potential effects of protein intake have been overlooked in the past, although amino acids are precursors for homocysteine and nitric oxide (NO). Our objective was to study the effect of amino acids on postprandial vascular function, in relation to the utilization of meal arginine for NO production. In a crossover design, 9 men ingested 50 g of a complete amino acid mixture, trace-labeled with [13.sup.C]-glycine and [sup.15][N.sub.2]-arginine, without (meal A) or with (meal B) 3 g extra arginine. The postprandial utilization of meal arginine for NO production was determined from urinary [sup.15]N[O.sub.3]. We monitored endothelial function of the brachial artery, the stiffness of the common carotid artery, aortic pulse wave velocity and soluble markers related to endothelial function for 8 h. Meal A did not significantly increase plasma homocysteine and did not alter endothelial function markers. The amount of NO synthesized from meal arginine doubled after meal B (107.1 [+ or -] 16.5% increase vs. meal A, P < 0.01 ) but was very low (271 [+ or -] 84 ppm vs. 332 [+ or -] 73 ppm, P< 0.05, respectively). After meal B, flow-mediated and nitroglycerine-induced dilation decreased but common carotid artery compliance, pulse wave velocity, plasma soluble intercellular adhesion molecule-1, and von Willebrand factor, and urinary cGMP did not differ when compared with meal A. Together, the data indicate that, in healthy men, meal amino acids do not adversely affect endothelial function, and meal arginine only slightly enters the NO pathway. Unexpectedly, arginine in physiological amounts may acutely lessen smooth muscle cell reactivity to a high dynamic NO release.

The stable nitrogen ([sup.15]N) and carbon ([sup.13]C) isotopic composition of tissues reflects the isotopic pattern of food sources. We investigated whether the isotopic composition of human hair can be used as a biomarker to predict the dietary intake of animal-derived food. Hair samples were collected from subjects during a 1987-1988 German nutrition survey (VERA) in which dietary information was collected using a 7-d dietary record. Samples of 50 men and 50 women were randomly selected, in addition to 27 samples of subjects with a reported low meat intake. Isotope ratio MS was used to analyze hair bulk and amino acid-specific isotopic composition. Its relation with and feasibility for predicting animal protein intake were tested using regression analysis and cross-tabulation of observed and predicted dietary data and comparison of the individual values for the binary categories of high and low intake. [sup.15]N and [sup.13]C abundances strongly predicted relative animal protein and meat intake ([R.sup.2] = 0.31, P < 0.01 and [R.sup.2] = 0.20, P < 0.01, respectively). Distinct patterns of individual hair amino acid [sup.15]N and [sup.13]C abundances were observed. In contrast to bulk values, the isotopic abundances in individual amino acids did not show discriminating ability across sex and isotope-specific categories. We conclude that hair [sup.13]C values are as predictive for animal protein consumption as hair [sup.15]N values. Bulk isotopic abundance of hair can be used as a biomarker for animal protein intake to validate dietary assessment methods provided that the correlation between isotopic abundances and dietary protein intake is verified in dietary intervention studies. KEY WORDS: * animal or plant food protein * amino acids * stable isotopes * hair * biomarker

Soy proteins have been shown to result in lower postprandial nitrogen retention than milk proteins, but the mechanisms underlying these differences have not been elucidated. To investigate this question, we measured the postprandial kinetics of the appearance of individual [sup.15]N-amino acids in the serum of healthy adults after the ingestion of either [sup.15]N-soy (n = 8) or [sup.15]N-milk proteins (n = 8) in a mixed single meal (46 kJ/kg). The kinetics of total and dietary amino acids (AA) in the peripheral circulation were characterized by an earlier and higher peak after soy protein ingestion. Dietary AA levels peaked at 2.5 h in the soy group vs. 3.9 h in the milk group (P < 0.02). This time interval difference between groups was associated with a faster transfer of dietary N into urea in the soy group (peak at 3 vs. 4.75 h in the milk group, P < 0.005) and a higher level of incorporation into the serum protein pool from 3 to 8 h after the soy meal. The dietary AA pattern in the peripheral blood closely reflected the dietary protein AA pattern. Postprandial glucose, insulin, and glucagon levels and profiles did not differ between groups. Soy AA were digested more rapidly and were directed toward both deamination pathways and liver protein synthesis more than milk AA. We conclude that differences in the metabolic postprandial fates of soy and milk proteins are due mainly to differences in digestion kinetics; however, the AA composition of dietary proteins may also play a role. KEY WORDS: * dietary protein * amino acids * postprandial metabolism * GC-C-IRMS * humans

As the search for alternative sources of food to alleviate hunger continues, this study was undertaken to determine the biological value in growing rats (BV) of proteins of some lesser known tropical seeds gathered in Nigeria. Antinutritional factors (trypsin inhibitors, phytic acid, oxalate, tannin, alkaloids) and amino acid compositions were also determined, and protein digestibility-corrected amino acid score (PDCAAS) was calculated using the amino acid requirement pattern of the preschool child and individual seed-specific correction factors for crude protein. A rat growth and balance study was conducted to determine digestibility, nitrogen-, and energy balance by feeding as the only unsupplemented protein source milled and heat-treated seeds of Adansonia digitata (Bombacaceae) and Prosopis africana, Lonchocarpus sericeus, Enterolobium cyclocarpium, Sesbania pachycarpa and Pterocarpus osun (Leguminosae) in comparison to casein fortified with methionine (control). Diets containing P. africana and L. sericeus seeds caused poor feed intake and weight loss in rats and were excluded from the nitrogen-balance test. Among the seed samples, S. pachycarpa followed by A. digitata showed the most advantageous nutritional quality [amino acid composition, digestibility, BV and net protein utilization (NPU)]. True digestibility was 82.9 and 74.5 vs. 98.5, BV was 64.6 and 70.0 vs. 90.4, and NPU was 53.5 and 52.1 vs. 89.0 for S. pachycarpa and A. digitata vs. casein (control), respectively. In terms of PDCAAS, lysine was the first limiting amino acid for S. pachycarpa (88%) and for A. digitata (58%). The PDCAAS of all essential amino acids was below 100% for E. cyclocarpium (e.g., cysteine + methionine: 37%) and for P. africana (e.g., threonine: 46%, except valine and a very high content of cycteine and methionine). In conclusion, all seeds tested in the rat balance trial were of inferior quality compared to casein. Before these tropical seeds could be used as food components or feed supplements, safety studies and proper processing to remove antinutritional factors and possible toxic constituents were required. KEY WORDS: * rats * tropical crop seeds * protein quality * nitrogen balance * antinutritional factors * amino acid score

The maximum dietary protein intake that does not cause adverse effects in a healthy population is uncertain. We tested whether a high protein intake enhances oxidative stress. Adult rats were adapted to different casein-based diets containing either an adequate (13.8%; AP), medium (25.7%; MP), or high (51.3%; HP) level of crude protein; a fourth group received a HP diet but no RRR-[Alpha]-tocopherol acetate (HP-toc). After 15 wk of feeding, plasma protein carbonyl concentration, liver lipid peroxide levels [thiobarbituric acid-reacting substances (TBARS)], reduced glutathione (GSH) status and leucine kinetics ([1-[sup.13]C]leucine) were measured. Higher concentrations of protein carbonyls and TBARS were found in rats fed the AP and the HP-toc diets compared with those fed the MP and HP diets (P [is less than] 0.05). GSH concentrations in plasma did not differ but total blood GSH concentrations were significantly (P [is less than] 0.05) lower in rats fed the HP-toc diet compared with those fed the AP, MP and HP diets. Liver GSH concentrations were significantly (P [is less than] 0.01) lower in rats fed the AP diet compared with the other groups. Rates of postabsorptive leucine oxidation (LeuOX) and flux ([Q.sub.Leu]) were positively correlated with the dietary protein level (for AP, MP, and HP, respectively: LeuOX, 74.9 [+ or -] 28.5, 109 [+ or -] 35.2, 142.3 [+ or -] 38.4 [micro]mol/(kg [multiplied by] h); [Q.sub.Leu], 425 [+ or -] 102, 483 [+ or -] 82,505 [+ or -] 80 [micro]mol/(kg [multiplied by] h). Only HP-toc resulted in a significantly greater protein breakdown ([PB.sub.Leu]) and [Q.sub.Leu]. No difference was seen in nonoxidative leucine disposal. Long-term intake of high protein diets did not increase variables of oxidative stress, in contrast to our initial hypothesis. An unexpected finding was that adequate protein feeding (AP) may in fact induce oxidative stress. J. Nutr. 130: 2889-2896, 2000. KEY WORDS: * rats * high protein diets * oxidative stress * leucine flux * vitamin E deficiency

Metges, Cornelia C., Antoine E. El-Khoury, Lidewij Henneman, Klaus J. Petzke, Ian Grant, Shahinaze Bedri, Paulo P. Pereira, Alfred M. Ajami, Malcolm F. Fuller, and Vernon R. Young. Availability of intestinal microbial lysine for whole body lysine homeostasis in human subjects. Am. J. Physiol. 277 (Endocrinol. Metab. 40): E597-E607, 1999.--We have investigated whether there is a net contribution of lysine synthesized de novo by the gastrointestinal microflora to lysine homeostasis in six adults. On two separate occasions an adequate diet was given for a total of 11 days, and a 24-h (12-h fast, 12-h fed) tracer protocol was performed on the last day, in which lysine turnover, oxidation, and splanchnic uptake were measured on the basis of intravenous and oral administration of L-[1-[sup.13]C]lysine and L-[6,6-[sup.2][H.sub.2]lysine, respectively. [[sup.15][N.sub.2]]urea or [sup.15][NH.sub.4]Cl was ingested daily over the last 6 days to label microbial protein. In addition, seven ileostomates were studied with [sup.15][NH.sub.4]Cl. [[sup.15]N]lysine enrichment in fecal and ileal microbial protein, as precursor for microbial lysine absorption, and in plasma free lysine was measured by gas chromatography-combustion-isotope ratio mass spectrometry. Differences in plasma [[sup.13]C]-and [[sup.2][H.sub.2]] lysine enrichments during the 12-h fed period were observed between the two [sup.15N] tracer studies, although the reason is unclear, and possibly unrelated to the tracer form per se. In the normal adults, after [sup.15][NH.sub.4]Cl and [[sup.15][N.sub.2]]urea intake, respectively, lysine derived from fecal microbial protein accounted for 5 and 9% of the appearance rate of plasma lysine. With ileal microbial lysine enrichment, the contribution of microbial lysine to plasma lysine appearance was 44%. This amounts to a gross microbial lysine contribution to whole body plasma lysine turnover of between 11 and 130 mg [multiplied by] [kg.sup.-1] [multiplied by] [day.sup.-1], depending on the [[sup.15]N]lysine precursor used. However, insofar as microbial amino acid synthesis is accompanied by microbial breakdown of endogenous amino acids or their oxidation by intestinal tissues, this may not reflect a net increase in lysine absorption. Thus we cannot reliably estimate the quantitative contribution of microbial lysine to host lysine homeostasis with the present paradigm. However, the results confirm the significant presence of lysine of microbial origin in the plasma free lysine pool. lysine kinetics; amino acid requirement; tracer balance; lysine-15-nitrogen; gas chromatography-combustion-isotope ratio mass spectrometry