Some interesting findings from studies on twins
What controls your BMD? A question I asked when I found out I had osteoporosis, and one I hear from men who visit the Website, is, "What could I have done to prevent my osteoporosis?" We want to know if we should have eaten more foods with calcium or vitamin D, should we have exercised more, why did we get it and someone else didn't? There is a recent study that might help explain this problem in Ann Med 2002;34(6):434-43, by Videman T and others. Note that calcium intake had only a 1% influence on the femoral neck and none on the spine, while exercise had only a 1% influence on the spine. Age and familial aggregation explained 73% of the variance in BMD in both the femoral neck and the lumbar spine. There was no apparent explanation for the other 20% or so of of bone density detected. What the authors had hoped to find in this study of identical twins was behavioral targets for preventive management strategies for adult men. That is, what is the relative importance of occupational and leisure time physical loading, smoking, alcohol consumption, and dietary calcium beyond the effect of shared family and genetic influences. Using identical twins this can be determined by evaluating BMD and comparing how each twin differed in relation to the variables previously mentioned. If, e.g., one twin ate 1,500 mg calcium day and another ate only 300 mg/day, and calcium intake is important to BMD, we would expect the twin ingesting the most calcium to have the greatest BMD. The authors didn't find a very strong influence on BMD from any of the variables tested--genetics and family influences were the major factors.
Extreme contrasts in twin pairs. Based upon interview data, the authors found four subgroups of pairs that allowed comparisons based upon "extreme" contrasts of smoking, endurance and game sports, resistance training, and calcium intake. The smoking group had 17 twin pairs, the endurance and game sport group had 21 pairs, the resistance training group had 10 pairs, and the calcium group had 19 pairs. One would expect these individuals with extreme variation in the variables tested would give the most valuable results. For instance, in the resistance training, the difference was 2200 versus 160 hours lifetime training. In the calcium group, one twin reported calcium intake at least 450 mg/day above his co-twin, whose values were below 600 mg/day. The results for these discordant twin pairs showed that in the 19 twin pairs with the greatest extreme in calcium intake (1390 vs. 330 mg/day), there was no statistically significant difference in BMD. There was, however, greater BMD that was in the direction of greater calcium intake (0.026 g/cm2 in the lumbar spine and 0.029 g/cm2 in the femoral neck). In the men with a mean 33-pack-year difference in smoking there was no difference in BMD. There was also no difference in BMD in the group discordant in endurance and game sports. In the group that was extremely different in resistance (power sports, weight lifting), the twins with the most resistance training had 0.009 g/cm2 and 0.048 g/cm2 improved BMD in the femoral neck and lumbar spine, respectively.
Bottom line. What comes out of this and similar studies is that genetics and early family influences appear to be the most important factors in controlling BMD. If there are things we can do to positively influence BMD, those done by the family during childhood would appear to be the most influential according to this study. Once we are adults, dietary or exercise changes appear to be much less significant concerning BMD. This means that men shouldn't feel guilty that there was something they could have done differently to have affected their diagnosis of osteoporosis. You don't choose your genes or the family you grow up in. It is wise, however, to follow nutritional and exercise guidelines that promote health from all standpoints, not just bone health. That is, this study's results notwithstanding, I would still recommend getting 1000-1500 mg/calcium day, getting 800 IU vitamin D or at least 15 minutes of sunshine daily, eating a healthy diet, and getting plenty of exercise. Don't expect that doing that will necessarily keep you from getting osteoporosis, but you will have done your part and left the rest to factors that you can't control--and thus should have no guilt because you did the best you could. And, cardiovascular disease, diabetes, obesity, and other diseases and conditions are controlled or helped by following healthy dietary and exercise guidelines, no matter what your genetic background.