Increased levels of physical activity (PA) have been linked to higher peak bone mass and increased bone area. The aim of the study was to identify the association between lifetime and current PA with estimated Bone Mineral Density (BMD) in a population of older, socio-economically disadvantaged South Africans of mixed racial ancestry.
Methods: Estimated BMD and T-scores were measured using calcaneal ultrasound (SAHARATM) in 47 men and 105 women, mean age, 65 +/-7 years. Lifetime PA was assessed using a questionnaire comprised of three activity domains (household, occupational, leisure) during five age epochs (14- 21; 22-34; 35-49; 50-64; > 65 years). Current PA was assessed using the Yale Physical Activity Survey. Peak bone strain units for each activity were estimated, based on impact loading. Confounding factors such as BMI, smoking patterns and nutritional status were also quantified.
Results: Men and women had similar mean estimated calcaneal BMD, 0.454+/-0.01 and 0.453+/-0.1g/m2, respectively. The proportion of subjects presenting with apparent osteopenia and osteoporosis was similar in men and women (52% vs. 53% and 7% vs. 6%, respectively). Occupational PA between 14 and 21 years, for men (r=0.35; p=0.034) and 22-34 years for women (r=0.24; p=0.033) was significantly correlated to estimated BMD. There were no other significant associations between any of the PA domains and estimated BMD. Leisure time PA only accounted for 0.8% of total energy expenditure for both the men and women. Individuals who were occupationally active between ages 14 and 34 years, were more likely to remain active throughout life. Smokers had lower estimated BMD and T-scores than non-smokers (p<0.01). Reported alcohol and calcium intake was not correlated to any of the ultrasound parameters.
Conclusion: Estimated BMD was weakly, but significantly correlated to occupational PA during the years of peak-bone mass accretion, which may be protective against accelerated bone loss in later life. From a public health perspective, these data highlight the importance of quantifying lifetime PA in all domains, including occupation.