4. Protein/energy ratios

Part of the rationale of this workshop on protein-energy interactions was to make recommendations for desirable protein/energy ratios in health and disease, and three of the four panels are constrained to provide such recommendations. It is nevertheless considered that protein/energy ratios (expressed as Pcal%) should also be discussed briefly here, especially as they concern the data presented in this chapter.

When the combined N-balance data were examined, it was noted that the correlation between Pcal% and NB was non significant (R²=0.007) and that therefore Pcal% by itself was of little predictive value. Nevertheless the combination of NI and Pcal% was marginally better in predicting NB than was the regression equation using NI and EI as the independent variables. The regression equation obtained was:

NB = 0.43 NI-4.69 Pcal-22.75

R2 = 0.57

suggesting that 57% of the variation in NB came from NI and Pcal%. The average and SD of the Pcal% (Pcal% = protein g/100 g x 400 divided by kcal/100 g) values in the data set (n = 361) was 6.1 ± 2.9 with a maximum of 30.2 and a minimum of 1.6. When the CALLOWAY and SPECTOR (1954) data were not included, the maximum value was only 11.4, and the mean and SD were reduced to 5.8 ± 2.4. When the data are classified into Pcal% groupings (eight groups from Pcal% 3 to > 9), as shown in Figure 13, a pattern of relationships does emerge in that, for this data set, the increase of Pcal% is mainly a result of increasing NI and there is a parallel increase in NB as Pcal% increases up to about Pcal% = 8. At higher Pcal% values NB is reduced (causing the lack of overall significance) as a result of lower energy intakes.

The mean Pcal% for the US diet, as calculated from USDA (1983) food availability data, is much higher than these values and averages about 16% (PELLETT and YOUNG, 1990). The range for requirement data (Table 5) is, however, only from 3.6 to 8.2% at moderate activity levels. Since energy allowances are averages and protein allowances are safe levels (means + 2SD), the direct ratios of allowances (Column A in Table 5) are misleading, and the ratios (Column B) calculated from average protein needs are more appropriate. It can thus be seen that the experimental data represent the low end of the range for Pcal% values. This is not surprising, when the rationale behind the experiments is considered, which was often to examine the effect of excess energy at low to normal protein intakes.

For a number of years, Pcal% has been suggested as a useful dietary indicator for protein sufficiency (MILLER and PAYNE, 1961; BEATON and SWISS, 1974), but its limitations have also been recognized (BEATON and SWISS 1974; PAYNE, 1975; FAO/WHO/UNU, 1985). The relevance of the simple ratio, even when correctly calculated (average protein: average energy), as a basis for assessing diets has been questioned since it does not take into account individual variability in the needs for energy and of the extent to which these are independent of variability in protein requirements. For the US diet, the Pcal% stays at about 16% (15.4 to 17.5) across a wide range of age, sex and income groups, despite a more than two-fold range in protein availability (PELLETT and YOUNG, 1990), and even for poor developing countries, average Pcal% values from food balance sheet data (YOUNG and PELLETT, 1991) are usually in excess of Pcal% = 11%. These ratios can be seen to be well in excess of the ratios calculated from requirement data (Table 5).

Figure 13. Relationship between N balance, energy and nitrogen intakes when classified by protein/energy ratios (Pcal%).

The major problem in assessing the relevance of Pcal% (or any other indicator of protein/energy ratios), is to evaluate the range over which individuals can adapt either energy intake to suit expenditure, or expenditure to suit intake, without detriment to health or growth (PAYNE, 1975). Two solutions to the problem have been suggested. The first is to use observed variability data for energy intakes in populations (BEATON and SWISS, 1974), while PAYNE (1975) has suggested use of experimental evidence for the minimum energy intake for the maintenance of body energy content. PAYNE (1975) suggests that both give quantitatively similar results.

Evaluation of the data set in terms of Pcal% is unable to address these considerations of adaptation. What, however, does emerge, is that both EI and NI seem to be individually effective in improving N balance, and no stepwise progression could be observed. In other words, the accepted concepts from both CALLOWAY and SPECTOR (1954) and MUNRO (1951; 1978) appear true only under much more extreme conditions of protein or energy limitation than is met within the analyzed studies.

Within the range examined, it appears that increasing either EI or NI or both can individually improve nitrogen balance even at the lowest levels of the other. This can make the use of Pcal%, or any other form of P/E ratio for assessing the value of diets, somewhat confusing. As can be seen, if it be true that increases in either protein or in energy intakes lead to improved N balance, then the former will increase the Pcal% while the latter will reduce it, yet both actions have a positive impact on NB. This is, of course, borne out by the lack of correlation between NB and Pcal% using the full data set.

A similarly confusing situation can be seen by examining Table 5, where at first sight it is surprising that the 'requirement' values for Pcal%, increase with age in apparent contradiction to the widely held view that the protein value of diet is of greater importance for the younger age groups than for the older. This occurs because the decrease in average energy needs per unit body weight with age is far steeper than the decrease in protein requirements. As is emphasized elsewhere in this volume, the use of a protein/energy ratio for assessment of dietary protein value requires a considerably greater degree of sophistication in its interpretation than the simple nature of the ratio would seem to imply.