M.A. McNurlan

The role of degradation in the acute control of protein balance in adult man: failure of feeding to stimulate protein synthesis as assessed by L-[1-13C] leucine infusion

The effect of feeding on whole-body protein turnover was measured in six healthy volunteers using the essential amino acid, L-[1-13C]leucine, as a tracer for protein metabolism. Varied lengths of periods of feeding and isotope infusion produced different apparent responses to feeding. When parameters of protein turnover were estimated from 8-hour infusions, the change from post-absorptive in the first four hours to mixed feeding during the final four hours was found to produce positive leucine balance by decreasing degradation from 89.5 +/- 5.0 to 31.7 +/- 7.3 mumol leucine/kg/h (P less than .001), with no apparent change in synthesis. By contrast, when tracer was infused for 24 hours with 12 hours of feeding followed by 12 hours of fasting, the estimate of protein synthesis during feeding was 35% higher than during fasting (P less than .01). However, when tracer infusion during the 12-hour feeding/12-hour fasting protocol was limited to the last four hours of each nutritional period, the estimates of fed and fasted protein synthesis showed no significant difference, 71.3 +/- 6.5 and 66.2 +/- 5.6 respectively, while the calculated rate of protein degradation was 43% lower during feeding (P less than .002). As relatively higher levels of enrichment in plasma leucine were detected in comparable nutritional states following longer infusions, the possibility of significant recycling of label was investigated. Residual tracer was still detectable in both breath and plasma 12 hours after cessation of a 12-hour tracer infusion, supporting the conclusion that significant errors in estimates of protein turnover due to recycling of label arise with prolonged infusions.(ABSTRACT TRUNCATED AT 250 WORDS)

Temporal responses of protein synthesis in human skeletal muscle to feeding.

In attempting to evaluate alterations in metabolic responses to dietary nutrients that occur in pathological conditions in man, it is first necessary to understand normal metabolic responses. The present study set out to determine the temporal responses of protein synthesis in the skeletal muscle of healthy subjects to the consumption of food. Sequential measurements of protein synthesis in quadriceps muscle were made in eight subjects by injection of 0.05 g L-[1-13C]leucine/kg body-weight. The rate of protein synthesis after an overnight fast (i.e. in the post-absorptive state) was 2.2% muscle protein. After 1 h of eating, protein synthesis was unaltered (2.2%/d), but after 10 h of consuming small hourly meals the rate had risen to 2.9%/d, with a variation in response among individuals. The response of muscle to 10 h of feeding was also investigated in subjects who underwent only one measurement each, either after 10 h of eating small meals or after the same time-period when no food was given. Protein synthesis rates were only slightly elevated in the group of fed individuals (2.3%/d, n 6) compared with the fasted group (2.1%/d, n 6). Taken together the two studies suggest that in healthy adults muscle protein synthesis does not respond quickly to the influx of dietary nutrients and that even after 10 h of feeding any stimulation of protein synthesis is small.

Organ-specific measurements of protein turnover in man

Methods that were originally developed for animal studies have now been modified for measuring tissue protein turnover in man with stable isotopes. Of the three techniques which have been discussed, only the arterio-venous difference method has the capacity to measure both the synthesis and degradation. This technique is, however, difficult to perform and has a number of potential sources of error, one of the most important being the assessment of precursor labelling. Constant infusion of a labelled amino acid with measurement of the incorporation into protein of a biopsy is a simpler and more precise technique for measuring synthesis in patients as well as volunteers, but the most appropriate means of assessing the precursor labelling still remains to be worked out. The flooding-dose procedure aims to minimize the difficulty of assessing precursor labelling and there is no evidence that the unphysiological dose of labelled amino acid given influences the synthesis rate which is measured. It is rapid to perform and is very well suited to measurements in patients, in whom a wide range of tissues can be studied. These advances in techniques have been facilitated particularly by improvements in mass spectrometry, which have allowed the use of stable in place of radioactive isotopes, and have enabled measurements of isotopic enrichments to be made in small samples of tissue. These techniques for assessing the dynamics of protein metabolism in individual tissues are now being used to answer nutritionally and clinically important questions in human volunteers and patients.

Protein synthesis in regenerating rat liver during malnutrition

To examine the effect of malnutrition on liver protein metabolism and synthesis during liver regeneration, 104 rats were allocated to semi-starvation or ordinary food intake for 1 week. Half of each group was sham operated and the other half was partially hepatectomized. Specimens were taken from the liver at the time of liver resection and from animals killed 24, 48 and 72 h after the primary operation. Liver samples were analysed for DNA and protein, and in the 48-h groups RNA and protein synthesis were also analysed. Protein synthesis was measured by the flooding method using L[4-3H] phenylalanine. The liver weight during regeneration increased very rapidly in the well-nourished animals, but when expressed as percent of body weight or as proportional increases, the difference between well-nourished and malnourished animals disappeared. The fractional rate of protein synthesis was not changed in sham-operated malnourished or well-nourished animals. During regeneration, protein synthesis in well-nourished animals was elevated compared to sham-operated controls, but a lesser stimulation was seen in malnourished rats. It was concluded that the mechanism of liver regeneration depends on nutritional state, involving an increase in protein synthesis in well-nourished animals, but relying more on a decrease in protein degradation or cessation of secretory protein synthesis in malnourished animals.

Effect of a short-term infusion of glutamine on muscle protein metabolism postoperatively

The acute effect of a short-term postoperative infusion of glucose supplemented with glutamine (0.285 g/kg body weight), on muscle protein metabolism, was studied by analyses of free amino acid concentrations and determinations of protein synthesis. A glutamine-glucose infusion was given for 5.5 h to 6 patients 2-3 days after elective surgery for colon cancer. The free glutamine concentration was 5.72 +/- 0.96 mmol/kg wet weight (ww) before and 6.14 +/- 1.10 mmol/kg ww 4 h after the glutamine infusion. The rate of protein synthesis was 1.26 +/- 0.15%/24 h before the infusion and 1.12 +/- 0.16%/24 h during its latter part. The percentage of polyribosomes was 42.2 +/- 3.4% before and 40.9 +/- 1.3% after the infusion. The results showed no difference in these biochemical parameters, indicating that a short-term infusion of glutamine given postoperatively is insufficient to affect protein metabolism in human skeletal muscle.

Factors controlling the disposition of primary nutrients

In the steady-state an organism maintains a dynamic balance between the dietary intake and metabolism of each of the primary nutrients, carbohydrate (CHO), fat and protein. However, such a steady-state, while generally true in adults in the long term, is never achieved during short periods (i.e. within 1 d) because of the discontinuous nature of food intake. The fact that we consume our food as meals, with postabsorptive periods in between, means that short-term storage in glycogen, adipose stores and tissue protein is necessary. Thus, the capacity to control the partition of nutrients after meals between immediate metabolism and storage, and then the mobilization of the stores during fasting, is essential for maintaining balance over the longer term.

Carbohydrate Oxidation: Do We Eat Ourselves or Our Food?

In post-absorptive man, energy is derived solely from oxidation of body stores, mainly by oxidation of fat rather than glycogen.1 Eating changes this pattern so that carbohydrate (CHO) oxidation predominates.1 If during feeding energy intake exceeds energy expenditure, the energy needs of the whole body can in theory be met entirely from the diet. However, it is not clear whether the CHO utilised in the fed state does come directly from the absorbed diet, or whether some continues to be removed from body stores.