Bone mineral density is a measure the amount of mineral material per square cm of bone tissue, with lower levels being associated with brittle bones and increased risk of fractures. Peak bone mineral density occurs in most individuals around the ages of 30 years, and low peak bone mineral density measurements at this age are associated with an increased risk of fractures in later life because the skeleton generally loses density with age. Those most at risk of low bone mineral density are post-menopausal women. There is evidence that genetic factors are involved in the development of low bone mineral density, but there are also known to be a number of environmental factors that account for at least some of the risk. Twin studies indicate that genetic factors account for 50 to 80% of the individual differences in bone mineral density.
Because bone mineral density declines from around age 30, it is recommended that steps are taken by women in their third decade to increase bone mineral density. Weight bearing exercise is known to increase bone mineral density as the body adapts to the stresses placed upon the skeleton. Because weight bearing exercise would also increase lean body mass, it might be expected that lean body mass is positively associated with bone mineral density. Research1 has used duel-energy X-ray absorptiometry (DXA) to measure the lean body mass and bone mineral density of 30 to 40 year old women in the United States. The results showed that lean body mass was an independent predictor of hip bone mineral density. The researchers also measure a number of other parameters and found that greater lean body mass, African-American ethnicity and low serum triglycerides accounted for 28% of the variance in spine bone mineral density.
Weight training is known to increase both bone mineral density and lean body mass, which explains the association shown in this research. Increases in lean body mass can also increase resting metabolic rate, because muscle tissue has a high metabolic activity. This in turn can lead to a loss of body fat which may protect joints. Weight training is traditionally seen as a male orientated past time, but women in their third decade of life could benefit because of the benefits to bone mineral density. Basic compound exercises that incorporate movement around multiple joints, such as leg squats and dead lifts tend to be the most effective exercises for increasing the bone strength of the hips and spine. This is important because these two areas have been identified as being particularly prone to osteoporotic fractures as bone mineral density decreases with age.
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