This symposium is concerned with the assessment of protein nutritionalstatus in man. The question we have to examine is whether measurements ofenzyme 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 thebody, the effect on activity of a biochemical lesion at the cellular level,and the adaptive changes that reflect an altered balance of metabolicpathways.

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

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

Before considering the various possible protein sources, we must make somesort 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 onprotein supplies and the degree of novelty that we should envisage. Theestimate depends on the number of people to be fed and the amount of proteinthat each needs. Both quantities are uncertain: the first because it must beguessed, the second because it is hotly disputed.