The major implication of these animal data is that there are marked differences between the MD for maintenance and for growth. It is clear that in the growing rat and pig and the adult rat, leucine and lysine exhibit the biggest difference between growth and maintenance patterns, these two amine acids being most abundant for growth and among the least abundant for maintenance. The practical consequence of this, as pointed out by Hegsted (1973), is that the balance-intake curve is extremely shallow for leucine and lysine both in the sub-maintenance and growth range. This means that small differences in balance result in large differences in maintenance intakes so that measurement of a requirement value for maintenance is very difficult and depends on the exact criterion for adequacy. The several early reports of rats maintaining body weight for 6-month periods on very low lysine diets (e.g., zein [Osborne and Mendel, 1916] or even lysine-free diets [Bender, 1961]) are probably explained by coprophagy, given the clear evidence of a metabolic need for lysine in terms of the rapid onset of symptoms on a lysine-free diet in humans (Rose, 1957). However, no evidence exists for anything other than a low metabolic need for this amine acid.

A second type of study which is pertinant is the work of Yoshida (1983) who has done most to explore the concept that rate-limiting amine acids at maintenance differ from those that rate-limit growth. Having established that in