The effect of level of protein intake in isoenergetic diets on energy utilization

Abstract

Energy intake is known to affect the efficiency of nitrogen (N) utilization. Less is known about the effect of protein intake on energy metabolism by the growing human. Initial observations were made on a single child consuming successive isoenergetic diets in which protein intake was varied in five levels between 1.0 and 2.0 g/kg per day. Rate of weight gain varied from 2.1 (low protein) to 8.4 (high protein) g/kg per day. The estimated percentage of N in the weight gained was relatively constant during all five dietary periods. Six children were subsequently studied while consuming diets in which protein provided 4, 5.3, 6.4 to 6.7, and 8% of energy intake during randomly assigned dietary periods. Mean rate of weight gain varied from 2.8 ± 2.4 (4% protein energy) to 6.7 ± 1.2 (6.4–6.7% protein energy) g/kg per day (P < 0.05). As protein intake increased from 6.4 to 6.7 to 8% of energy, no further increase in rate of weight gain appeared to occur. Assuming the cost of weight gain to be 5.6 kcal/g, a mean of 16.8 ± 7.9% of energy used for weight gain at the upper levels of protein intake was “wasted” at the lowest level of intake. Reanalysis of studies in which protein quality of wheat protein was improved by incremental supplements of lysine also demonstrated a similar effect of protein quality on weight gain. In addition, the estimated percentage of N in the weight gained did not differ significantly among dietary periods. The data suggest that energy utilization is remarkably sensitive to both the quantity and quality of protein in the diet. These findings have implications in a number of clinical settings in which there are attempts to improve weight gain by “pushing calories” without providing adequate protein. In addition the data raise questions concerning the current Food and Agriculture Organization/World Health Organization recommendations for protein and energy intakes by children in the 6-month to 3-year age groups.