|Protein-Energy Interactions (IDECG, 1991, 437 p.)|
|The metabolic basis of amino acid requirements|
The consequence of specific human needs for IAAs should be variable utilisation of dietary protein sources according to their IAA content. However, in contrast to studies in laboratory animals, in which it is easy to demonstrate differences in the biological value of proteins in relation to their amino acid content and chemical score, in humans this is extraordinarily difficult (MILLWARD) et al., 1989).
While differences between protein sources have been reported in N-balance studies in young adults (e.g., biological values of 0.27 for wheat compared with 0.51 for beef; YOUNG et al., 1975), when the calculated bv of several proteins, measured in separate studies, is examined together, the differences between wheat gluten, and other proteins and mixed diets are much less apparent. This is because of the lack of reproducibility between studies with the same protein.
Within individual studies, inter-individual variability is very marked, with biological values often associated with CVs of 15-20% (e.g., YOUNG et al., 1973), and even 50% (YOUNG et al., 1975). The combination of within-study varability and poor reproducibility between trials means that statistical analysis of the data is well nigh impossible. RAND et al., (1981) calculated the size of the experimental groups necessary to provide significant differences in biological value between proteins with the variability observed in the balance trials done at MIT. They showed that, unless biological values differ by the order of 50%, significant differences cannot be demonstrated without unrealistic numbers of subjects (e.g., 21 subjects needed to discriminate between proteins which differ in their bv by 15% with a beta error of 50%). To reduce the error to a more acceptable 10%, 54 subjects would be needed, and such trials are not feasible.
So it is not always easy in human studies to demonstrate differences between proteins. However, when we can, as with wheat gluten for example, we have to be clear what such results mean. With wheat gluten we assume from animal studies that it is lysine-deficient, but we can only prove that with lysine supplementation studies.
In fact, lysine supplementation studies have not in general demonstrated marked improvements of wheat gluten utilisation (VAGHEFI et al., 1974), and if we are to be truly rigorous in our critique, then we have little unequivocal evidence that wheat gluten is lysine-limited for humans.
The most comprehensive study in the literature is the MIT study (SCRIMSHAW and YOUNG, 1973). In response to lysine supplementation of wheat gluten-based diets, fed at two levels of protein and two levels of energy, there were small reductions (2.9-7.7%) in urea excretion which, although significant on a paired basis, were remarkably small responses if lysine content does limit wheat gluten utilisation. Indeed, the responses were so small that they can be explained as a consequence of the experimental design. In the nitrogen balance studies, lysine supplementation was evaluated in individuals after they had been fed the unsupplemented diet. Since nitrogen balance improves with time due to adaptation (RAND et al., 1985), the small improvement of nitrogen balance with the lysine supplementation may have been such an improvement with time. In my view, the assumption that wheat gluten is lysine-limited in human diets is unproven. Specific examination of the nitrogen balance response to lysine supplementation of wheat-based diets in young children failed to show any response (REDDY, 1971). Furthermore, examinations of nitrogen balance responses in several studies of children fed mixed vegetarian diets (TORUN, YOUNG and RAND, 1981), did not indicate that dietary protein quality was a determinant of nitrogen balance. However, as already indicated in the comments relating to studies such as those shown in Figure 1, part of the problem relates to our limited ability to understand nitrogen balance.