This symposium is concerned with the assessment of protein nutritional status in man. The question we have to examine is whether measurements of enzyme activity are, or may be in the future, of any value for this purpose. There are several concepts which may be useful in considering this problem, such as the nature of enzymes and the regulation of their amount in the body, the effect on activity of a biochemical lesion at the cellular level, and the adaptive changes that reflect an altered balance of metabolic pathways.

In normal young men, in whom the calorie requirements are fully met, an intake of about 50 g protein per day promotes nitrogen balance, and little additional storage results even from much larger protein intakes (Galloway & Spector, 1954). Chittenden maintained that an adult who adopted the intake of 120 g per day suggested by Voit, or who advised others to do so, was ‘encouraging individual and racial suicide’ (Cathcart, 1912), and the animal experiments of Ross show that by all objective criteria, except that of body-weight, the rats whose dietary intake is restricted fare better than those allowed unlimited quantities of a diet of good quality (Ross, 1961; Ross & Bras, 1965).

Analysis of the effects of amino acid starvation in reticulocytes is comparatively simple compared with similar analysis in other tissues of whole organisms. This is mainly because of the absence of RNA synthesis in reticulocytes, but also because the bulk of the protein being synthesized is haemoglobin, a protein whose structure is completely known. The absence of RNA synthesis eliminates complications that would otherwise arise through RNA-mediated control mechanisms which in turn might mask the effects of amino acid starvation on the protein synthetic machinery in the cells (Munro, 1969). Consequently reticulocytes have been used to study the effect of amino acid starvation on the actual process of protein synthesis and assembly.

Before considering the various possible protein sources, we must make some sort of estimate of the possible protein need by—say—the end of the century. That estimate controls the amount of effort we should put into work on protein supplies and the degree of novelty that we should envisage. The estimate depends on the number of people to be fed and the amount of protein that each needs. Both quantities are uncertain: the first because it must be guessed, the second because it is hotly disputed.