The combined ingestion of carbohydrate with a protein hydrolysate and amino acid mixture significantly increases de novo insulin production in patients with a long-term diagnosis of type 2 diabetes. The increased insulin response stimulates plasma glucose disposal and reduces postprandial glucose concentrations.

When using (13)C or (14)C tracers to study substrate metabolism, an acetate correction factor should be applied to correct for loss of label in the exchange pathways of the tricarboxylic acid cycle. We have shown recently that the [(13)C]acetate recovery factor has a high inter-individual variability and should therefore be determined in every subject. In the present study we examined the factors that might explain some of the variability between subjects in acetate recovery factor. Data were pooled from four different studies with identical protocols, in which the acetate recovery factor was measured, prior to an intervention, to correct plasma fatty acid oxidation rates. Acetate recovery was measured after 2 h of [1, 2-(13)C]acetate infusion at rest followed by 1 h of cycling exercise at 40-50% of maximal oxygen uptake. Inter-individual variance in acetate recovery was 12.0% at rest and 16.1% during exercise. Stepwise regression revealed that, at rest, 37.1% of the acetate recovery could be accounted for by basal metabolic rate adjusted for fat-free mass, percentage body fat and respiratory quotient (RQ). During exercise, 69.1% of the variance in acetate recovery could be accounted for by energy expenditure adjusted for fat-free mass, % body fat and RQ. In conclusion, we show that the acetate recovery factor has a high inter-individual variability, both at rest and during exercise, which can partly be accounted for by metabolic rate, RQ and % body fat. These data indicate that the acetate recovery factor needs to be determined in every subject, under similar conditions as used for the tracer-derived determination of substrate oxidation. Failure to do this might result in large under- or over-estimation of plasma substrate oxidation, and hence to artificial differences between groups.

1. During infusion of [5-15N]glutamine in patients with gastrointestinal cancer we unexpectedly observed a gradual decrease in time of the appearance rate (Ra) of glutamine in plasma. Here we investigate whether the failure to achieve a plateau isotopic enrichment in plasma is, among other factors, due to incomplete equilibration of the glutamine tracer with the large intramuscular free glutamine pool.2. Plasma and intramuscular glutamine enrichment were measured during 6-11 h infusions of L-[5-15N]glutamine and L-[1-13C]glutamine in post-absorptive patients admitted to hospital for elective abdominal surgery. L-[1-13C]Leucine and L-[ring-2H5]phenylalanine were infused to measure the proportion of glutamine appearing in plasma directly due to its release from protein.3. The glutamine tracer entered muscle, but the rise in intramuscular glutamine enrichment was small, presumably as a result of the enormous size of the intramuscular glutamine pool and the limited speed of entry of glutamine into muscle. In each patient the intramuscular glutamine enrichment was lower than that in plasma (P<0.001), and both increased with tracer infusion time (P<0.001), indicating incomplete equilibration of the glutamine tracer.4.A comparison of the results obtained by the two glutamine tracers indicated that recycling of the nitrogen label contributed to about 15% of the decrease in Ra.5. There was a gradual reduction in the glutamine release from proteolysis, which contributed to 16-21% of the decline in Ra.6. We conclude that slow equilibration of the glutamine tracer with the large muscle glutamine pool significantly contributes to the absence of isotopic steady state. Consequently, the appearance rate of glutamine in plasma measured during short tracer infusion periods (hours) considerably overestimates the whole-body glutamine flux.

Leucine has been suggested to have the potential to modulate muscle protein metabolism by increasing muscle protein synthesis. The objective of this study was to investigate the surplus value of the co-ingestion of free leucine with protein hydrolysate and carbohydrate following physical activity in elderly men. Eight elderly men (mean age 73^1 years) were randomly assigned to two cross-over treatments consuming either carbohydrate and protein hydrolysate (CHO þ PRO) or carbohydrate, protein hydrolysate with additional leucine (CHO þ PRO þ leu) after performing 30 min of standardized physical activity. Primed, continuous infusions with L-[ring-13 C 6 ]phenylalanine and L-[ring-2 H 2 ]tyrosine were applied, and blood and muscle samples were collected to assess whole-body protein turnover as well as protein fractional synthetic rate in the vastus lateralis muscle over a 6 h period. Whole-body protein breakdown and synthesis rates were not different between treatments. Phenylalanine oxidation rates were significantly lower in the CHO þ PRO þ leu v. CHO þ PRO treatment. As a result, whole-body protein balance was significantly greater in the CHO þ PRO þ leu compared to the CHO þ PRO treatment (23•8 (SEM 0•3) v. 23•2 (SEM 0•3) mmol/kg per h, respectively; P,0•05). Mixed muscle fractional synthetic rate averaged 0•081 (SEM 0•003) and 0•082 (SEM 0•006) %/h in the CHO þ PRO þ leu and CHO þ PRO treatment, respectively (NS). Co-ingestion of leucine with carbohydrate and protein following physical activity does not further elevate muscle protein fractional synthetic rate in elderly men when ample protein is ingested.