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healthy adults given either 14 or 38 mg leucine per kg per day for 6 days prior to the measurement of 13C=leucine kinetics (El-Khoury et al., 1994b).

In summary, it is proposed that a 6-day period of dietary "lead-in" permits an appropriate adjustment, or adaptation, to variable intakes of specific IAAs. This hypothesis should now be further validated through metabolic studies of varying duration and, preferably, in population groups in different geographic regions of the world.

Diurnal Cycling and Amine Acid Requirements

Millward and coworkers (1996) have conducted interesting and important studies on the relationships between the postabsorptive N losses and prandial N gains and how the magnitude of this diurnal cycle of protein metabolism is affected by the habitual and prevailing intake dietary protein. Millward also points out in Chapter 9 that the key question is the extent to which this diurnal cycle". . . influences the IAA composition of the adaptive metabolic demand" or, in other words, "the amounts and amine acid pattern of the maintenance requirement."

We accept, as concluded by Millward, that it does not necessarily follow that all of the IAAs liberated via tissue proteolysis are quantitatively oxidized. Therefore, a conservation of lysine (and perhaps threonine) may occur in the free amine acid pools, whereas there may be little conservation of other amine acids, such as leucine.

Regarding this metabolic issue, discussed above, evidence suggests some apparent conservation of lysine during the postabsorptive phase of amine acid metabolism. Millward uses the data of Bergstrom et al. (1990) to make his case for a significant contribution made by the free pool of lysine in muscle to the dietary retention of amine acids as body protein. Unfortunately, specific data on muscle free amine acid concentrations for the postabsorptive state are not given by Bergstrom et al. (1990). These data would be the most useful for evaluating the possibility of a postabsorptive conservation of lysine at different lysine intakes and in reference to estimations of the lysine requirement based on 13C-tracer techniques.

We have, however, considered this postabsorptive retention of lysine in relation to the fasted:fed ratio of lysine oxidation at a generous intake and whether this is far lower than that for leucine. If so, this would indicate that a significantly greater proportion of the lysine released from protein breakdown during the fasting state is retained within the flee lysine pool compared with that for leucine. Such a retention might occur, particularly in skeletal muscle and perhaps in other tissues and organs; the free lysine pool of muscle is relatively large and it responds to ingestion of protein-free and protein-containing meals (Bergstrom et al., 1990). Hence, there is a theoretical capacity to accommodate, or store, some free lysine that is liberated via proteolysis during the fasting

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