Among environmental factors, physical activity and nutrition are key players for the acquisition of bone mass during growth. Growing bones are usually more responsive to mechanical loading than adult bones.(1) Physical activity increases bone mineral mass accumulation in children and adolescents. However, the impact seems to be stronger before than during or after the period of pubertal maturation.(2) Among nutrients that can specifically interact with bone metabolism, the influence of calcium has been extensively studied from infancy to the end of pubertal maturation.(3-5) Much less consideration has been given to protein intake, although this macronutrient is essential for bone accumulation during growth, as well as maintenance of the skeletal structural integrity throughout the adult life.(6)
The recommended dietary allowances (RDAs) for proteins in healthy children and adolescents have mainly been derived from nitrogen balance studies of short duration carried out in adults.(7-9) From these adult data, a factorial procedure was used so that values were assigned for maintenance requirements and an increment representing growth.(7-9) This latter estimate was increased for use efficiency and unevenness of daily growth rate, taking into account the inability of infants and children to store amino acids against intermittent needs.(7-9) The average requirements obtained by this factorial procedure were still raised by 2 SD (25%) in consideration of individual variability. Thus, in children 5-10 years of age, this derivation of daily protein allowances has been theoretically estimated to be ∼1.0 g/kg body weight/d.(7-9) Whether this level of protein intake is optimal for bone mineral mass accumulation in children is not known. In this report, we analyze in a homogenous cohort of healthy prepubertal boys the interaction between physical activity and protein compared with calcium intakes on bone mineral mass at several sites of the skeleton. This study strongly suggests that the positive effect of physical activity on bone mass acquisition before the onset of pubertal maturation is enhanced by protein intake higher than the usual recommended dietary allowance.