High calcium intake and vigorous exercise is the key to optimal increases in bone density

Prepubescence is a critical time for building bone mass in children. Strong bones are important for obvious reasons in growth and development. It is widely accepted that calcium intake and physical activity are two ways one can positively impact bone density.

But consuming the FDA recommended 800 milligrams of calcium per day, roughly the equivalent of three 8 oz. glasses of milk, or solely engaging in physical activity isn¡¯t enough to help kids in this age group retain the maximum amount of bone mass.

Researchers at the University of Wales and the University of Exeter, United Kingdom, suggest that the combination of high calcium intake and vigorous exercise is the key to optimal increases in bone density.

Few studies have examined the interactive effects of calcium intake and activity, especially ones that differentiate between the types of activity that most positively affect bone density. But the study ¡°Interactive effects of habitual physical activity and calcium intake on bone density in boys and girls¡± does just that. The authors of the study are Ann V. Rowlands and Sarah M. Powell, of the University of Wales School of Sport, Health and Exercise, Roger G. Eston of the Children¡¯s Health and Exercise Research Centre, University of Exeter, and David K. Ingledew of the University of Wales School of Psychology. The results of their study appear in the October 2004 edition of the Journal of Applied Physiology from the American Physiological Society.

Using food diaries and accelerometers, the researchers were able to track the behavior of 38 girls and 38 boys between the ages of 8 and 11. They observed that the synergistic effect of high levels of calcium intake and vigorous (as opposed to moderate or low-intensity) activity were the most effective in building and retaining bone mass. Their evidence indicates that children should engage in 25 to 40 minutes of vigorous activity and intake at least 700-800 milligrams of dietary calcium per day.

For this study, vigorous activity could include actions like running, playing tag, jumping rope, etc. ¡°You don't have to jump rope for half an hour. That would be considered very hard. In reality we are talking about the sort of energy you would expect kids to expend when playing around outside with a ball or rope,¡± said Dr. Eston. ¡°A brisk walk at about 4 mph would do the trick. We are talking about raising the resting energy levels by about 6-8 times. The idea is to 'accumulate' this sort of energy expenditure over the day to total about half an hour,¡± he added.

The purpose of this study was to assess the interactive effects of habitual physical activity (total and vigorous intensity) and calcium intake on bone mineral content (BMC) in prepubertal boys and girls.

Seventy-six children, 8 to 11 years old, wore accelerometers for up to 7 days to assess activity. Calcium intake was estimated by a 4-day weighted food diary. BMC and areal density (bone mineral density) were measured at the total body, proximal femur, and femoral neck by using dual-energy X-ray absorptiometry. Moderated regression analyses were used to assess the contributions of physical activity (total and vigorous) and calcium intake to BMC, residualized for bone area and body mass.

Interactive effects of vigorous activity (¡Ý6 metabolic equivalents) and calcium intake were found at the total body in boys (b = 2.90 x 10-3) and in girls (b = 6.58 x 10-3) and at the proximal femur (b = 9.87 x 10-5) and femoral neck (b = 2.29 x 10-5; where b is the regression coefficient from final equation) in boys only.

Residualized BMC was high only if both vigorous activity and calcium intake were high. There were no interactive effects of total (as opposed to vigorous) activity and calcium intake. This study provides evidence for synergistic action of habitual vigorous activity and calcium intake on bone mass in children. Recommendations for optimizing bone mass should reflect this synergism.