To the authors’ knowledge, this is the first study to determine the effects of weight loss on absolute and relative (adjusted for weight) BMD in overweight AA and EA premenopausal women immediately after weight loss and one year following weight loss. Although AA women had higher bone densities than EA women, both groups changed bone density similarly across time. In addition, although the subjects tended to lose BMD during weight loss, they actually increased weight-adjusted bone density with weight loss, with the increase in weight-adjusted bone density persisting even one year following the weight loss.
Similar to previous studies (4
) absolute hip and spine BMD decreased with weight loss. This was expected since there was a decrease of mechanical loading on the bones. However, contrary to previous studies, when comparing hip and spine BMD relative to body weight, BMD increased as body weight declined at all sites measured, suggesting that weight loss could decrease risks for fractures. Therefore, the results of this study suggest that it is safe and beneficial for overweight premenopausal women to lose weight since it improves BMD relative to body weight.
It is important to note that all BMD sites relative to body weight after the one year weight maintenance phase (with the exception of L1) were significantly higher than baseline measurements. This suggests that dieting and/or exercising for as little as six months has positive long-term effects on relative BMD. This conclusion should encourage overweight individuals who previously have not been on any type of exercise plan and who may be wary about starting any such plan. If overweight individuals learn that an exercise program as short as six months in duration will have lasting, positive effects on their BMD, this knowledge may encourage them to not only begin an exercise program, but perhaps continue participation in such a program. Future studies are needed to determine how much weight loss is needed to onset a positive BMD effect. Also, it would be beneficial to know at what point weight loss is no longer effective on relative BMD.
It is unclear why BMD Z-scores remained significantly higher than baseline scores after a one year follow-up. It is important to note that during the one year follow-up, subjects attended biweekly behavioral interventions which informed them about the importance of diet and exercise. It is possible that by attending these sessions, subjects improved their eating habits which may have had a positive effect on their BMD. For example, consuming more calcium and vitamin D has positive effects on BMD (24
). It is also possible that subjects may have been more physically active during the weight maintenance phase as compared to pre-enrollment activity levels. This increased activity may explain the higher BMD values because of increased mechanical loading on the bones (25
). Furthermore, previous studies have found BMD to be in proportion to lean mass rather than fat mass (10
). Results from the present study showed that subjects had a lower percent body fat after the one year follow-up when compared to baseline measures. Therefore, since lean mass would be a higher fraction in subjects at one year follow-up when compared to their baseline measurements, it makes sense that BMD would be increased relative to body weight.
While absolute bone loss did occur in total hip and hip shaft as evidenced by a significant drop in BMC, it is notable that absolute bone loss did not occur in any of the other sites of interest. However, it is puzzling that our results indicated no time by exercise group interactions in bone geometry. Beck et al. found that effects of exercise are more evident in bone structural geometry than in BMD (26
). Beck’s finding supports the proposition that mechanical loading induced by exercise mainly affects the outer bone surface, thereby more likely influencing BMC, regional area, and section modulus (26
). Although bone loss was not occurring among our subjects, we were unable to detect a significant positive bone geometry effect in our study. However, the geometry estimate is a crude one and mechanically meaningful changes can be small. Our results suggest that exercise had little effect on bone geometry during weight loss which raises the possibility that rapid weight loss (≥ 12kg) may reduce exercise benefits. More research is warranted to determine the impact of weight loss and exercise on bone geometry.
Similar to previous findings, AA’s BMD was significantly higher than EA’s at all sites (14
). A review by Aloia explored why AAs have higher BMD values when compared to EAs. Aloia noted first that AAs have longer periods of bone formation during adolescence than EAs, and second that AA girls seem to absorb more calcium during adolescence than EA girls since calcium absorption and renal calcium conservation is greater in AAs (16
). Aloia also suggested the increased BMD in AAs could result from a tendency of AAs having higher body weights than EAs, which would naturally cause a heightened BMD. In the present study we cannot assume AAs’ higher BMD is caused by increased body weights as both AAs and EAs had similar body weights and BMIs. Also, total body lean mass in both races were not significantly different.
When comparing time by race effects of absolute BMD in this study, no significant difference existed between AAs and EAs (except for L3). Since a significant difference did exist between races when comparing changes of L3 across time (P=0.046) more research is needed to determine if AA and EA’s BMD respond differently during weight loss. EAs’ L3 BMD decreased over time (baseline mean = 1.345± 0.022 g/cm2, after weight loss mean =1.329± 0.022 g/cm2, one year follow-up mean = 1.323 ± 0.022 g/cm2), whereas AAs L3 BMD decreased during the weight loss program, but then increased back to baseline levels during the one year weight maintenance phase (baseline mean= 1.425± 0.020 g/cm2, after weight loss mean =1.413± 0.020 g/cm2, one year mean= 1.425± 0.020 g/cm2). Although L3 was the only site showing a significant race by time interaction, the mean trends of the other spine BMD sites were similar. This suggests that, unlike EAs, AAs may regain absolute BMD a year following weight loss despite similar body fat percentage gains.
It is interesting to observe that BMD sites were higher in AAs, whereas regional area sites of the spine were higher in EAs. Although AAs had higher BMDs, while EAs had higher region areas of the spine, neither group encountered bone loss since BMC remained relatively constant (29
). If this finding is replicated in future studies, we will have reason to believe that neither race is at greater risk for fracture.
Strengths of the study were using a longitudinal design and controlling for diet and exercise. A limitation was the lack of information on the micro-architecture of bone in subjects, which could also play a role in predicting bone fractures (30
In conclusion, weight loss induced by dieting, whether with or without exercise, increases BMD at the hip and spine relative to body weight. AAs have higher absolute BMD compared to EAs, and also may retain absolute BMD better than EAs during weight loss, but EAs have higher regional areas at the spine. More importantly, a weight loss program, as short as six months, has a significant long term effect on BMD Z-scores.