Calcium, micronutrients and physical activity to maximize bone mass during growth

A large body of evidence relates nutrition in the early stages of life to health later in life. Osteoporosis, a disease affecting several million older people worldwide, is a condition that must be prevented from childhood. Bone grows longitudinally and cross-sectionally during childhood and adolescence, with the largest accumulation of skeletal tissue (50 percent of the total mass) taking place during the adolescent growth spurt. In the third decade of life, the growth and development of the skeleton is complete. The bone consolidates: there is an increase in bone density but no increase in height. After this time, bone mass decreases progressively and becomes increasingly fragile until a threshold is reached. At this stage, fractures are very likely, even as a result of minor injuries. Having a large skeletal mass in early adulthood postpones the time in life when this threshold is reached. A 20 percent variation in the skeletal mass (corresponding to approximately 1 SD [standard deviation] of skeletal mass of the population) leads to 50 to 100 percent variation in the risk of fractures in different sites of the vertebral column (Hui, Slemenda and Johnston, 1989). Both the formation and the subsequent maintenance of skeletal mass are determined by environmental, hormonal and genetic factors. Studies of twins have indicated that approximately 80 percent of the mass is decided by genetics (Slemenda et al., 1991), while the remaining 20 percent can be manipulated with environmental interventions such as modifications of diet and physical activity. Other studies have suggested that environmental factors such as physical activity and calcium intake can each contribute 40 percent of the variance in bone density (Kelly, Eisman and Sambrook, 1990). This large discrepancy suggests that there is a considerable overlap and interaction of the different factors, since the genetic background modulates nutrient absorption and utilization and also affects muscle mass, thus indirectly influencing the level of mechanical stimulation of the bone.