Protein and carbohydrates

Certain amino acids are effective secretagogues of insulin and have been found to synergistically increase the blood insulin response to a carbohydrate load when administered in combination (Floyd et al. 1966; Fajans et al. 1967). Of the 20 amino acids normally found in protein, the most effective insulin secretagogue is arginine (Fajans et al. 1967). When infused with carbohydrate, arginine has been found to increase the insulin response fivefold above that produced by the carbohydrate or arginine alone. However, we have found the use of amino acids to be impractical when added to a carbohydrate supplement because they produce many unwanted sideeffects such as mild borborygmus and diarrhoea. Protein meals and supplements also have been found to enhance the insulin response to a carbohydrate load and do not produce the unwanted side-effects of the amino acids (Rabinowitz et al. 1966; Pallota & Kennedy 1968; Spiller et al. 1987). For example, Spiller et al. (1987) demonstrated a nincreased blood insulin response and decreased blood glucose response with the addition of protein to a 58 g carbohydrate supplement. The insulin response was found to be directly proportional and the glucose response inversely proportional to the protein content of the
carbohydrate–protein supplement. No adverse side-effects were reported.

Endurance exercise and protein metabolism

The majority of the energy for endurance exercise is derived from the oxidation of lipid and CHO. As mentioned above, skeletal muscle has the metabolic capacity to oxidize certain amino acids for energy. While it may seem counterproductive to oxidize proteins during exercise since they serve either a structural or functional role, amino acid oxidation may also be required for exchange reactions in the tricarboxylic acid cycle, and this may increase their net utilization.

Early studies evaluated urea excretion as an indicator of protein oxidation (urea is a breakdown product formed in the liver following amino acid oxidation) and found that urinary urea excretion was higher following endurance exercise than at rest. This increase is missed if sweat is not collected because urea and other nitrogen compounds are contained in sweat.

For example, a person exercising in high ambient temperatures or humidity with a sweat rate of up to 2 l/h would be expected to have a high urea sweat loss that may contribute to a more negative nitrogen balance. Since urea excretion represents the full extent of amino acid oxidation, this method provides only indirect evidence for amino acid oxidation and, in some cases, does not correlate well with direct measures of amino acid oxidation.

By far, the amino acid leucine has been most often used to trace the effects of exercise on amino acid oxidation, and many studies have shown that endurance exercise increases leucine oxidation. An increase in lysine oxidation has also been observed during endurance exercise.

During endurance exercise, leucine oxidation demonstrates a positive correlation with exercise intensity.Leucine oxidation and plasma urea content also increase with exercise duration. Finally, leucine oxidation increases with glycogen depletion, which may partially explain the increase in leucine oxidation with exercise duration.

Following endurance exercise, there is a prompt return toward baseline leucine oxidation levels, although there appears to be a slight increase in leucine oxidation following eccentric exercise
that may persist for up to 10 days. This may partially explain why nitrogen balance is negative at the onset of unaccustomed endurance exercise, yet becomes more positive as the person adapts to the stress.

The increase in amino acid oxidation during endurance exercise may account for 1 to 6% of the total energy cost for an endurance exercise session at about 65% VO 2peak. If only a few of the nonessential amino acids are oxidized during endurance exercise, then the predicted effect on protein requirements may be minimal. Conversely, an increase in essential amino acid oxidation (e.g., leucine and lysine) may affect protein requirements since they can only come from dietary intake or protein breakdown.

Workouts, exercise and protein synthesis

Protein synthesis is initiated when a signal (e.g., nutrient, hormone, mechanical) to the cell is communicated to the DNA to induce gene expression (transcription), resulting in formation of messenger RNA (mRNA). The mRNA is translated into a protein through the process of translation by the ribosomes, which are free in the cytosol or bound to rough endoplasmic reticulum. The process of translation requiresa second form of RNA, called transfer RNA, and three distinct steps: initiation, elongation, and termination. Following translation, the nascent protein can be further modified through processes such as glycosylation or degradation (posttranslational modification). When the entire process of muscle protein synthesis is considered, there is ample evidence that this increases in a similar manner after both endurance and resistance exercise. Factors such as the intensity and duration of exercise also have profound effects on gene expression.

The sites of regulation and how this generalized protein synthetic response is fine-tuned to allow for phenotypical divergence are just becoming unravelled. We have used microarray technology and found that over 200 mRNA species are differentially expressed by only 3 h after endurance exercise, and only a minority of these same species are expressed in a similar fashion following resistance exercise. Others have found that phosphorylation of proteins such as p70S6k, 4E-BP1, eIF-2B, and AMPK is altered in response to different contraction patterns in skeletal muscle.

Collectively, the data show that there are changes at multiple levels (e.g., transcription and translation) within the protein synthetic pathway that simultaneously respond to exercise. It is also likely that the state of training will have a major role in determining the absolute and relative importance of transcriptional and translational control of certain proteins and how this relates to protein synthesis, ultimately modulating the phenotypic response to a given pattern of muscle contraction. Innovative approaches have revealed that muscle conserves the ability to acutely and directionally respond to divergent stimulii, even if its training history is at the opposite end of the metabolic demand spectrum.