Trying to perform significant nutritional research on humans may be the most difficult endeavor that clinical scientists ever attempt (1). Truly meaningful data about the relationship between what we eat and how it affects our health frequently takes many years to produces results. The gold standard of clinical research, the randomized control trial, is very difficult to perform when testing nutritional hypotheses. This is because of the variability in compliance that inevitably occurs when subjects are attempting to maintain a specific diet for long periods and the considerable costs that are associated with such a study. Hence, it is no wonder that there are multiple theories about what constitutes an appropriate diet for bone health. It is clear that there are minimum requirements for vitamin D and calcium to maintain healthy bones. This is best demonstrated by the discovery in the early twentieth century that infants and children need adequate dietary vitamin D and calcium to prevent rickets (2).
In 2010 the Institute of Medicine provided recommended daily dietary allowances for calcium and vitamin D based on their review of the available evidence (3). However, in which form we obtain either of these has been the subject of some debate. There is conflicting evidence that calcium supplements have adverse effects on health (4, 5), so many healthcare providers have been recommending that dietary calcium be obtained through food, which most often means dairy products like milk or its derivatives, cheese and yogurt. However, recently there have been commentaries in the popular press which question the utility of milk consumption in adults (6, 7). In support of these questions, a meta-analysis by Bischoff-Ferrari et al found no significant association between milk consumption and hip fractures in adults (8). Similarly, Michaëlsson et al found that milk consumption in adults in Sweden did not decrease the risk of fracture in men or women and may actually have increased fracture rates in women and mortality in both men and women (9).
The effect of milk consumption during adolescence on the risk of fractures in adulthood is probably clearer but is not without conflicting results. Multiple retrospective studies have reported that milk consumption in childhood and adolescence are inversely related to the risk of fractures in adulthood (10). However, a recent report by Feskanich et al found the opposite outcome in men, which the authors partially attributed to the greater height achieved by men who consume higher amounts of milk as adolescents (11).
So what do we do with all this data? I think there is a consensus that there are minimum requirements for vitamin D and calcium to maintain bone health during childhood, adolescence and adulthood as outlined in the recent Institute of Medicine report (3). Dairy remains an excellent source of these nutrients. However, exceeding the threshold needs may not provide additional benefit and may have adverse effects. Healthcare providers should be aware that excessive supplementation of any nutrient is not without risks and should make their patients aware of such risks.
Farmington, Connecticut, USA
We Are What We Eat
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