Achieving the protection of high peak bone mass.

Abstract

Sixteen years ago, the National Osteoporosis Foundation (NOF) published a review of peak bone mass and of the factors that influence it [1]. In the ensuing years, literally hundreds of studies have been published on the topics of peak bone mass, its timing, and its contributing factors. A recent article in this journal contains the results of NOF’s effort at bringing that earlier review up-to-date [2]. It is useful to recall, in 2016, that both the current and the earlier reviews were based on two premises: (1) other things being equal, a more massive bone is structurally stronger than a more flimsy bone; and (2) bone mass in the adult years is influenced by how much bone an individual has been able to acquire during growth. This accumulated mass is designated Bpeak^ because it is the highest value an individual will normally accumulate over the course of his or her life. Put together, these premises postulate that going into the adult years having a greater bone mass provides useful protection from late life fragility fractures. It is the conjunction of these two premises that undergirds the concern about maximizing bone mass during growth. An unexamined presumption in both reviews (and in many of the research studies they summarize) is that bone mineral density (BMD), as measured by dual energy absorptiometry (DXA), adequately captures the amount of bone (i.e., bone mass) an individual has. Accordingly, measured peak BMD is commonly treated as effectively equivalent to peak bone mass. As a moment’s reflection will suffice to show, and as has been pointed out elsewhere [3], that presumption is incorrect. Density is not mass, but is, rather, mass per unit volume [or, with DXA, mass per unit area of the X-Ray shadow cast by the bone(s) being measured]. It is true that measured values for mass and density are often positively correlated. But that is hardly surprising, as mass is formally incorporated into the definition and measurement of density (i.e., see above). But correlation does not establish identity. It is important to make this distinction, as bone growth during maturation involves both accumulations of bone mass and expansion of bone volume [3, 4], two processes not always occurring in parallel. Thus, density can appear to decrease during some phases of growth at the same time as mass is increasing [3, 4]. Clark and colleagues [5, 6], cited in the current review, are careful to dissect apart the components of BMD prior to searching for correlations with putative causes. Not all authors have been so careful. Of more than passing interest today is how little the principal factors currently identified as influencing accumulation of a high peak bone mass have changed from the earlier synthesis [1], i.e., nutrition and mechanical loading (exercise). So, what then is the appropriate action outcome going forward? What is to be done? In asking ourselves this question, we are forced to confront yet another poorly examined presumption. Both the reviewers and the investigators whose work they review assume that public policy is science-driven. If policy remains unchanged, then perhaps it is because the science is not convincing enough. And the solution for that, obviously, is more research. * R. P. Heaney rheaney@creighton.edu