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balance measurements. The questions posed in this review cannot currently be answered because of the absence of studies of outcome in terms of physical performance in long-term, controlled feeding trials.

BACKGROUND TO THE CURRENT CONTROVERSY

In 1985, the Food and Agriculture Organization (FAO) report on protein and energy requirements was published (FAO/WHO/UNU, 1985), a feature of which was the recommendation that protein quality should be evaluated by the PDCAAS method (protein-digestibility corrected amine acid score), making use of age-specific amine acid scoring patterns. Because the indispensable amine acid (IAA) requirement values used to calculate the scoring patterns fell markedly with age, from over 50 percent of total protein requirement in infants to only 16 percent in adults (see Table 9-1), the quality of any protein would now be assessed as higher when used for adults than for children. Furthermore the low requirement level of IAA in adults meant that all natural diets and food proteins would be adequate. Thus, apart from digestibility, protein quality ceased to be an issue in the nutrition of adults.

Considerable disquiet arose about the 1985 report. Young (1986) argued that the adult IAA requirement values were seriously flawed because of the way Rose (1957) conducted his nitrogen (N) balance studies (mainly excess energy and no account for miscellaneous N losses). Millward and Rivers (1988) reviewed the subject, paying particular attention to the adaptive changes in amine acid oxidation that can occur and that will influence requirement values. They argued that the marked fall with age in the requirement values was mainly a reflection of the methodologies used in their assessment. Thus, the infant values were largely patterned on the composition of breast milk, while the adult values, measured in balance studies with excess nonessential nitrogen and low levels of indispensable amine acid, would have identified minimum requirement values. They concluded that IAA requirements are complex, include an adaptive component, and can only be defined under specific artificial conditions that would allow definition of a minimum value and that ''current estimates of adult requirements may be close to this level." To identify which IAA might be rate limiting for the obligatory N losses (ONL), they calculated the obligatory oxidative amine acid losses (OOL) as estimates of the losses of tissue IAAs that would give rise to the ONL, as discussed in detail below.

Young et al. (1989) then published a paper entitled "A Theoretical Basis for Increasing Current Estimates of the Amine Acid Requirements in Adult Man with Experimental Support." This paper reproduced the table of eel values from Millward and Rivers (1988). After making some small adjustments in lysine, threenine, and valine values derived from their stable isotope studies and increasing all values assuming a 70 percent efficiency of utilization, Young and colleagues proposed that this pattern, the "MIT" (Massachusetts Institute of



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