The influence of dietary form of [1-13C] lysine and [1-13C] leucine on their postprandial and postabsorptive oxidation

The purpose of this study was to determine the way different dietary forms of amino acids (aa) influence the level of their postprandial utilisation and to check whether as oxidation can detect/reflect the rate of protein turnover in postabsorptive phase in humans. Each substrate (90 mg of L-[1-C-13]-lysine and 60 mg of L-[1-C-13]-leucine) was individually ingested by 7 adult persons (two or three times) in three different ways: as a aqueous solution in post-absorptive state or served with the test meal (post-prandial) in either free form (mixed with eggs) or bound to egg white protein. Decarboxylation of amino acids was monitored using a [(CO2)-C-13]-breath test for 6 hours. The CO2-production of the subjects was calculated using Schofield equations for basal metabolic rate. The rate of [C-13]-recovery and its cumulative value were calculated and expressed as % of the dose. A time lag in maximal oxidation of label between their different forms of administering was observed. For both substrates the label was the fastest recovered in case of its free form in fasted state, then for the as physically mixed with a meal and the slowest for the as bound into the protein. This was probably due to the delay in its absorption from the gut. This dynamic of oxidation caused the differences in their magnitude, significant only for leucine. Leucine bound in egg white protein was ca. 12% better utilised than when served in a free form with the same meal, probably due to the larger input to the small free leucine pool, which can overcome the current protein synthesis capacity. In both forms of as in the meal, the cumulative values were lower for lysine than for leucine, but the difference was significant only in case of the free form (13.43 +/- 1.3 and 18.68 +/- 0.92, respectively). That could be the consequence of larger capacity of free amino acids pool for lysine than leucine. Both substrates (as an essential amino acids) administered in post absorptive phase reflect the oxidation of as from endogenous protein degradation which can be used to determine the rate of protein turnover. However leucine seems to fit better in this as it has simpler catabolism and is far more cost effective.