PHYSICAL-ACTIVITY, CALCIUM INTAKE, AND BONE-MINERAL CONTENT IN CHILDREN IN THE NETHERLANDS

Study objective - To examine the relation between physical activity, c alcium intake, and bone mineral content in children. Design - Populati on based, cross sectional study. Setting - Primary schools in Zoeterme er, The Netherlands. Participants - Altogether 1359 Dutch boys and gir ls, aged 7 to 11 years (response rate 88%). Measurements - Bone minera l content was measured by quantitative roentgen microdensitometry of t he midphalanx of the second digit at the diaphyseal and metaphyseal si te. Maximal exercise testing, according to the Bruce treadmill protoco l, was used to assess physical fitness. Habitual physical activity was assessed by use of a questionnaire on physical activities. Daily calc ium intake from dairy products was estimated by use of a semiquantitat ive food frequency questionnaire. Main results - Bone mineral content in boys was not linearly associated with physical fitness after adjust ments for differences in height, body weight, chronological age, and s keletal age. In girls a linear association was found at the metaphysea l site only. When extreme groups were compared, bone mineral content w as found to be higher in ''high fitness children'' (upper decile) than (''low fitness children'' (lowest decile), with statistical significa nce reached in boys only. When analyses were performed in subgroups of skeletal age, a clear linear relation between physical fitness and bo ne mineral content was seen in the mature subgroup in both boys and gi rls. No linear association was found between habitual physical activit y and bone mineral content, while the results in extreme groups (that is, upper versus lowest decile) and in subgroups of skeletal age were comparable to those on physical fitness in boys only. No association w as found between daily calcium intake and bone mineral content in this age group. Conclusions - This cross sectional study in children aged 7 to 11 years suggests that an increased bone mineral content is found only in those with a high level of physical activity. This associatio n is most pronounced in the more mature children. No evidence was foun d for an association between daily calcium intake and bone mineral con tent in childhood.