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.