Protein-Energy Ratios

Human nitrogen metabolism is highly sensitive to altered dietary intakes of both protein and energy caused by substrate availability, as well as by hormone mediation, which can influence the status of body nitrogen and energy metabolism. Other areas of concern relate to protein-energy malnutrition (PEM). Severe deficiencies of protein and/or food energy, especially in association with infections, can lead to PEM. However, imbalance between protein and energy levels, as for example with excess food energy together with low protein diets, can also produce metabolic abnormalities, such as the kwashiorkor form of PEM. The role of infection in the pathogenesis of PEM is mediated, at least in part, by hormonal changes, which can directly affect both protein and energy metabolism in previously well nourished indi-viduals. The protein-energy ratio in formulas is of special importance in catch-up growth following infection (Dewey et al. 1996) and also for the needs of preterm infants (Tsang et al. 1993).

Body protein or nitrogen balance in the healthy mature human is maintained within relatively narrow limits, indicating a regulation that is achieved by varying the rate of N excretion in relation to changes in N intake and the individual’s metabolic state. H. N. Munro (1951, 1964b), D. H. Calloway and H. Spector (1954) and Calloway (1975) have demonstrated that nitrogen balance is influenced by the intake of both protein and energy, even when that of energy is more than adequate; in consequence, estimates of protein requirements, based on nitrogen balance, can be affected by the level of energy available. Changes in

Table IV. C.3.18. Mean values per capita for the availability of specific indispensable amino acids in developed and developing regions. Data for 1994

Amino acid

Per day (mg)

Per g protein (mg/g)

1Data for 61 countries.

2Data for 29 countries.

( ) = SD

Calculated from food balance sheet data (FAOSTAT 1996).

Food energy intake, below or above energy needs, affect nitrogen balance. The improvement in nitrogen balance caused by an increase in energy intake, however, can be attenuated if protein is inadequate; conversely, the beneficial effects of an increase in protein can be inhibited by an inadequate energy intake. Published nitrogen balance studies relating the influences of protein on energy and energy on protein requirements have been reevaluated by Pellett and Young (1992), who postulate that the effects on N balance are continuous when either protein or energy are increased and dispute the proposals of Munro and Calloway that the relationships exist as a series of plateaus. In summary, therefore, nitrogen balance must be seen as a result of both protein and energy intakes, and each must be defined before use can be made of the data for establishing protein requirements.