The effects of physical activity, body composition, muscle cross-sectional area and sex steroids on bone volumetric density, strength and geometry in older men.

Abstract

Osteoporosis and related fractures are significant public health and economic burdens. Studies show that factors such as mechanical loading and sex steroids influence the bones of older women. In order to reduce the risk of osteoporosis in older men, it is important to understand what factors affect the strength of the bones of older men. The primary aim of this study was to identify modifiable factors that influence bone volumetric density, bone geometry and estimates of bone strength in old men. This cross-sectional study used data from the MrOS (n=1171) and Tobago Bone Health (n=500) studies. Dual Energy x-ray absorptiometry was used to assess areal bone mineral density, lean mass and fat mass. Peripheral quantitative computed tomography assessed volumetric bone mineral density, bone geometry, estimates of bone strength and muscle cross-sectional area. After adjusting for age, clinic site, body weight, race and limb length, estimates of bone strength at the 4% and 66% site of the tibia were greater in the quartile of men with the greatest leg power compared to the least leg power and in the quartile of men that were the most physically active compared to the least active. Similar results were found at both the distal and midshaft of the radius when looking at physical activity and grip strength. After adjusting for age, clinic site, limb length, and race, estimates of bone strength at the proximal and distal tibia and radius were positively associated with total body weight, BMI, lean mass and fat mass. After adjusting for age, site, and tibia length, bone strength was positively associated with muscle cross-sectional area. Bioavailable testosterone and estradiol were positively associated with cortical vBMD while bioavailable estradiol was also positively associated with total and trabecular vBMD, bone compressive strength, cortical area and section modulus. These finding suggest that greater physical activity, muscle strength, muscle power, lean mass, and muscle cross-sectional area (all surrogates of mechanical load) are important for bone strength. It may be important to utilize the role of muscle to prevent the natural loss of bone that occurs with aging. These findings are congruent with finding from previous studies conducted on older females.