3. Protein quality

In absolute terms the requirement for protein in a non growing adult is the minimum amount needed to meet that body's metabolic demands for nitrogen, i.e. to secure N balance, in a particular situation. One aspect of the particular situation is the energy intake. It has long been recognized that, within limits, increasing or decreasing energy intake alters N balance by 1.5-2 mg/ kg. It was a criticism of Rose's original estimates of IAA requirements that the energy intake was unrealistically high. Therefore all comparative studies of protein quality in recent years have been made with subjects as far as possible in energy balance.** Obviously also protein quality can only be measured at low intakes. As Jackson (1995) has pointed out, a distinction has to be made between nitrogen requirements and protein requirements. A striking example of the need for this distinction is the breast-fed infant, whose food contains 30% of its nitrogen as non-protein N (see report on infant protein requirements). The NPN is usually included with protein in estimates of 'protein' requirements, which should more accurately be called nitrogen requirements, as they are measured by nitrogen balance.

** At MIT, for example, the nutritionist estimated the habitual energy intake from a dietary history and added 10%. Subjects were required to maintain their usual level of physical activity throughout the study and to monitor it by an activity diary. Weight changes were monitored and, if any trend was apparent, the energy intake was adjusted.

Jackson's analysis of the older literature shows that the requirement for IAAs is influenced by both the amount and the nature of the non-essential (NEN) component, effectiveness decreasing progressively as the NEN is provided by non-essential amino acids, glycine + glutamic acid, ammonium salts and urea. He has proposed that this gradation reflects the relative capacity of these components to provide substrates for the synthesis of IAAs by gut bacteria (see section 13). Whether or not this is the explanation remains to be seen, but the fact is clear, as shown, for example by the experiments of Kies & Fox (1978) among others. In conventional estimates of protein quality, NEN is provided by the NEAAs, which are readily exchangeable by transamination and to a lesser extent by deamination (glycine, serine). Thus in practice it is assumed that protein quality depends only on the amount and pattern of the IAAs.

Estimates of protein quality by N balance are in fact measures of the efficiency of utilization. If a protein could produce balance at an intake exactly equal to the obligatory loss, the efficiency of utilization would be 100%. In fact the efficiency, corrected for digestibility, is never better than about 80%, even with proteins such as those of egg or beef, which have an IAA pattern very close to that of tissue proteins. The Rome Report used a value of 70%. The reasons for this 'inefficiency' are not clear, and this is an important gap in our knowledge.

Millward et al (1989) have summarized the results of eight studies designed to measure the quality of different proteins by balance measurements at a series of different intakes (Figure 1). It happens that the first two in time of these studies, by Young et al (1973) on egg and by Inoue et al (1974) on wheat gluten, show fairly clear differences in biological value at low protein intakes. These early experiences encouraged Young and Scrimshaw to record the view that 'regardless of the method of measurement, our findings indicate that differences in the quality of dietary protein are important in the protein nutrition of adult man' (Young et al, 1975). The less clear-cut results of the later studies shown in Figure 1 do not mean that there are no differences in quality between different proteins; rather that they may be unimportant in practice with diets containing a mixture of proteins, such as those listed in Table 39 of the Rome Report. On the other hand, if the difference between cereals and animal protein is of practical importance, as claimed by Young & Pellett (1990) because of the difference in lysine content, why did it not show up in the balance studies? Rand et al (1981) calculated that to demonstrate a significant difference in biological value in adult humans would require a totally unrealistic number of subjects. Therefore, if Young's claim is sustained that the tracer balance is more sensitive than the nitrogen balance, it will be an important advance.

Figure 1 Measurements of biological value by the nitrogen balance method at different levels of protein intake.

The FAO/WHO reports of 1963 and 1973 gave much attention to the development of an IAA scoring pattern, with its linked concept of a limiting amino acid. The method can be applied to the protein of any diet whose amino acid composition is known, and the measurement is far simpler than the nitrogen balance. It is probably not of great importance whether the scoring pattern is taken to be that of milk, egg, beef, etc. whose relative contents of IAAs are similar to that of human tissue protein. However, the scoring pattern does not tell us anything about the absolute requirement for IAAs as a proportion of the total protein or N requirement. It is agreed that this proportion changes with age, being greater in the growing infant than in older children and adults. There is disagreement about whether the pattern changes with age, and is different for growth and maintenance (section 4). If, as Young maintains, it is not different, and is close to that of tissue protein, then to know the requirement for all IAAs it will be sufficient to know the requirement for one of them.

A third method of examining protein quality has recently been proposed by Millward et al (1991), as an outcome of his work on diurnal cycling: the slope of protein deposition vs intake (see section 5).