Although prior research has established that skeletal muscle and bone form highly integrated systems that undergo significant age-related changes, the relationships between muscle mass and trabecular versus cortical bone or trabecular microarchitecture have not been systematically investigated.
A recent study at Mayo Clinic, published in the Journal of Bone and Mineral Research, examined skeletal muscle mass and bone health across the life span and determined distinct differences in how muscle affects the two layers of bone in both men and women.
"Our study adds to the growing body of evidence supporting the highly integrated nature of skeletal muscle and bone," says lead author Nathan K. LeBrasseur, Ph.D., of the Department of Physical Medicine and Rehabilitation at Mayo Clinic in Rochester, Minn., "and it provides new insights into potential biomarkers that reflect the health of the musculoskeletal system."
Mayo researchers used data from a long-standing Mayo Clinic study of bone health involving 272 women and 317 men ages 20 to 97. They examined the association between appendicular skeletal muscle mass (ASM) relative to height squared (relative ASM) and a number of measures of bone health by conventional as well as high-resolution peripheral QCT at several sites. Looking beyond just bone marrow density, Dr. LeBrasseur's team measured the density and strength of cortical bone and trabecular bone.
"We saw that there were really nice correlations between muscle mass and different types of bone health and strength," says Dr. LeBrasseur. "Even if we adjust for age and physical activity levels, the fact that those relationships still persist at nonload-bearing as well as load-bearing sites shows that there's some clear relationship between how muscle and bone health are regulated throughout the life span."
The study found that muscle mass is associated with bone strength at particular places in the body. In women, relative ASM was associated with cortical thickness (CtTh) at load-bearing locations such as the femoral neck, lumbar spine and tibia (age- and physical activity-adjusted r = 0.19–0.32; all p < 0.01). Relative ASM was also associated with trabecular volumetric bone mineral density (vBMD) at the femoral neck and spine (all p < 0.05), and trabecular bone volume to tissue volume (BV/TV), number (TbN), thickness (TbTh), and separation (TbSp) at the radius (all p d 0.05), a nonload-bearing site.
In all men, relative ASM was associated with CtTh at all sites (age- and physical activity-adjusted r = 0.17–0.28; all p < 0.01). Associations between relative ASM and trabecular vBMD at the spine in men were lost after adjusting for age; however, relative ASM was associated with trabecular vBMD at the femoral neck and TbN and TbSp at the radius (all p < 0.01).
It's well established that men tend to have higher muscle mass through life span, achieve higher muscle mass relative to their height earlier in life and maintain this relatively higher skeletal muscle mass as they age. According to Dr. LeBrasseur, the Mayo team's research findings might indicate that as they age, women dip below a critical threshold of muscle mass that may then predispose them for poor bone health and increase their risk of fall-related fractures in their hips and wrists.
"The question that then needs to be asked is this: If you had an intervention that increased muscle mass in women to a point above this theoretical threshold, would they then be protected from age-related bone changes like osteoporosis and fall-related fractures?" says Dr. LeBrasseur.
The Mayo team also investigated a panel of established and novel circulating factors associated with bone health that may be indicative of relative ASM and found that serum insulin-like growth factor binding protein-2 (IGFBP-2) levels were the most robust negative predictors of relative ASM in both sexes.
"We found IGFBP-2, which has already been linked to osteoporotic fractures in men, is negatively associated with muscle mass in both sexes," Dr. LeBrasseur says. "This finding could potentially be used to identify people who have low muscle mass and are at a particular risk of falls and associated fractures."
LeBrasseur NK, et al. Skeletal muscle mass is associated with bone geometry and microstructure and serum insulin-like growth factor binding protein-2 levels in adult women and men. Journal of Bone and Mineral Research. 2012;27:2159.