It may appear a simple task to identify and quantify the maintenance MD for each amine acid in terms of the various metabolic pathways involved. However, difficulty arises in distinguishing between obligatory intrinsic, functionally important demands and those that serve a purpose, but can and do vary according to circumstances, that is, an adaptive component It is this adaptive component that brings complexity to the maintenance MD.

The ONL at 54 mg N or 0.34 g protein/kg/day, is only 50 percent of current estimates of the protein requirement (0.6 g/kg/d), and the nature of this additional need (the difference between 0.34 and 0.6 g protein) has in the past been difficult to account for. Usually, it has been attributed to an inefficiency of utilization, although why proteins such as those in milk, eggs, or meat were not utilized more efficiently was always puzzling. It is much easier to understand the inefficiency of utilization as representing an adaptive component of MD.

When subjects are fed a protein-free diet, their urinary N losses initially reflect their normal dietary protein intake and then falls over 7 to 14 days to reach a low stable output level (see FAO/WHO, 1973). That is, an additional loss of body N occurs on a daily basis for some time before equilibrium is reached at the lower level. This additional daily N loss demonstrates the existence of this adaptive component of the MD. Traditionally, this has been defined as ''the labile protein reserves," which imply that metabolically it was a pool of protein that varied in size with the dietary protein intake. In the rat, liver and visceral protein content does vary directly with dietary protein intake (see Munro, 1964) in support of the labile protein reserve concept. However, no such