Antoine E. El-Khoury

The 24-h whole body leucine and urea kinetics at normal and high protein intakes with exercise in healthy adults

In healthy adult men adapted to a diet/exercise regimen for 6 days, the effects of small, frequent meals supplying daily protein intakes of 1 (n = 8) or 2.5 g . kg-1 . day-1 (n = 6) on leucine oxidation, urea production, and whole body protein synthesis (PS) and degradation (PD) have been compared with the use of a 24-h continuous intravenous [13C]leucine and [15N,15N]urea infusion protocol. Two 90-min periods of exercise (approximately 50% maximal O2 consumption) were included during the fasting and the fed periods of the 24-h day. Subjects were determined to be at approximate energy, nitrogen, and leucine balances on both diets. Increased protein intake raised the urea production rate; the absolute rate of urea hydrolysis was the same on both diets. When the first-pass splanchnic uptake of leucine was taken to be 25% of intake, PS was stimulated by feeding (after an overnight fast) at both protein intake levels (P < 0.05 and P < 0.01), whereas PD declined significantly (P < 0.01) at both protein levels. Protein gain at a high protein intake appears to be the result of both a stimulation of PS and a marked decline in PD, whereas at a less generous intake, the gain appears to be a result of a fall in PD with a less evident change in PS. Exercise moderately decreased PS during and/or immediately after exercise at each protein level, and there was a postexercise-induced increase (P < 0.01) in PD, which was more dramatic when feeding was at the higher protein intake level.In healthy adult men adapted to a diet/exercise regimen for 6 days, the effects of small, frequent meals supplying daily protein intakes of 1 ( n = 8) or 2.5 g ⋅ kg-1 ⋅ day-1( n = 6) on leucine oxidation, urea production, and whole body protein synthesis (PS) and degradation (PD) have been compared with the use of a 24-h continuous intravenous [13C]leucine and [15N,15N]urea infusion protocol. Two 90-min periods of exercise (∼50% maximal O2 consumption) were included during the fasting and the fed periods of the 24-h day. Subjects were determined to be at approximate energy, nitrogen, and leucine balances on both diets. Increased protein intake raised the urea production rate; the absolute rate of urea hydrolysis was the same on both diets. When the first-pass splanchnic uptake of leucine was taken to be 25% of intake, PS was stimulated by feeding (after an overnight fast) at both protein intake levels ( P < 0.05 and P < 0.01), whereas PD declined significantly ( P < 0.01) at both protein levels. Protein gain at a high protein intake appears to be the result of both a stimulation of PS and a marked decline in PD, whereas at a less generous intake, the gain appears to be a result of a fall in PD with a less evident change in PS. Exercise moderately decreased PS during and/or immediately after exercise at each protein level, and there was a postexercise-induced increase ( P < 0.01) in PD, which was more dramatic when feeding was at the higher protein intake level.

Availability of intestinal microbial lysine for whole body lysine homeostasis in human subjects

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-(13)C]lysine and L-[6,6-(2)H(2)]lysine, respectively. [(15)N(2)]urea or (15)NH(4)Cl was ingested daily over the last 6 days to label microbial protein. In addition, seven ileostomates were studied with (15)NH(4)Cl. [(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 [(13)C]- and [(2)H(2)]lysine enrichments during the 12-h fed period were observed between the two (15)N tracer studies, although the reason is unclear, and possibly unrelated to the tracer form per se. In the normal adults, after (15)NH(4)Cl and [(15)N(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. kg(-1). day(-1), depending on the [(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.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 ofl-[1-13C]lysine andl-[6,6-2H2]lysine, respectively. [15N2]urea or15NH4Cl was ingested daily over the last 6 days to label microbial protein. In addition, seven ileostomates were studied with15NH4Cl. [15N]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 [13C]- and [2H2]lysine enrichments during the 12-h fed period were observed between the two15N tracer studies, although the reason is unclear, and possibly unrelated to the tracer form per se. In the normal adults, after15NH4Cl and [15N2]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 ⋅ kg-1 ⋅ day-1, depending on the [15N]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.

Leucine kinetics in reference to the effect of the feeding mode as three discrete meals

In a recent study, we observed that the 24-hour leucine oxidation measured when three equal meals providing a generous intake of leucine (approximately 90 mg x kg(-1) x d(-1)) are eaten during the day is 16% lower (P < .01) than that for the same diet given as 10 hourly, equal meals. We hypothesized that the pattern of meal intake at a lower level of dietary leucine would affect the 24-hour rate of leucine oxidation and possibly improve the retention of dietary leucine. A total of 11 healthy adults participated in this investigation. The daily leucine intake was 182 micromol x kg(-1) x d(-1) (38 mg x kg(-1) x d(-1)) given with an L-amino acid diet. All subjects received three discrete meals daily for 6 days prior to a 24-hour intravenous (IV) tracer infusion of L-[1-13C]-leucine on day 7 (study 1). Four of these subjects participated in two additional studies of similar design. Study 2 involved giving [1-13C]-leucine as a constant IV infusion together with tracer added to the amino acid mixture at each meal time. In study 3, subjects received the three meals with added [1-13C]-leucine tracer while [2H3]-leucine was given as a constant IV infusion. Total leucine oxidation in studies 1 and 2 was 238+/-66 and 231+/-85 micromol x kg(-1) x d(-1), respectively. Leucine balance was positive, amounting to 18% of the total (diet + tracer) intake. The estimated mean nitrogen balance was +8 mg x kg(-1) x d(-1). Leucine oxidation was higher (P < .01) for breakfast than for the lunch meal. This difference was associated with lower insulin and higher plasma leucine concentrations at breakfast versus lunch periods. The results from study 3 suggest that the higher rate of leucine oxidation observed at breakfast as compared with lunch is not due to a difference in the immediate splanchnic fate of absorbed leucine from each meal. In comparison to our previous small frequent-meal studies, the pattern of meal feeding influences overall leucine utilization at both generous and limiting leucine intakes. Hence, it is possible that the pattern of meal feeding may affect estimations of amino acid requirements using the tracer-balance approach. Longer-term dietary studies will be needed to establish whether and the extent to which this is so.