third class—the conditionally essential amino acids—is synthesized from other amino acids. However, this synthesis is confined to particular organs and may be limited by certain physiological factors such as age or disease state (Reeds and Becket, 1996). As knowledge increases and techniques improve, the distinction between essential and nonessential amino acids becomes less clear. Adding to this lack of clarity are observations such as the one by Stucky and Harper (1962), who found that if rats were fed a diet adequate in nitrogen but lacking in nonessential amino acids, the growth rate of the animals was significantly decreased.

Although consensus exists at present for the adult protein requirement this is not the case for the adult requirement of indispensable amino acids. Since the 1985 FAO/WHO/UNU report, Young and coworkers have presented data that contradict the findings of the report; based on these data, Young suggests that the adult requirement for total IAAs is 31 percent of the protein requirement, or about three times the FAO/WHO/UNU estimate (McLarney et al., 1996; Young, 1987, 1994; Young and El-Khoury, 1995a; Young and Marchini, 1990; Young et al., 1989; see also Chapter 10). This contention of the group at Massachusetts Institute of Technology (MIT) for higher indispensable amino acid needs has been countered by Millward and colleagues (Millward, 1994; Millward and Rivers, 1988, 1989; see also Chapter 9), who find significant methodological problems in the studies of Young and coworkers. This debate is important, because it influences whether or not protein quality is an issue to be considered