Reply to the Effect of Vigorous Physical Activity and Body Composition on Cortical Bone Mass in Adolescence

To the Editor: We would like to express our thanks to the ALSPAC research group for their interest in our systematic review and to commend them on the Avon Longitudinal Study of Parents and Children (ALSPAC). We are grateful for this opportunity to clarify the approach we adopted for the systematic review and, specifically, our inclusion criteria, as we believe that this is at the heart of their comments. For the systematic review, we focused on studies that reported bone strength as the primary outcome (p. 2163). That is, only studies that reported values for bone strength met the criteria for inclusion, regardless of whether the authors reported bone mass or structure. Upon review, we accept that line two in the first study selection and inclusion paragraph could more appropriately have stated, “We included studies that: (1) … (2) reported bone strength, with or without measures of bone structure and/or bone mass.” In their 2011 article, Sayers and colleagues1 illustrated associations between intensity of physical activity (PA) and strength-strain index (SSI; Fig. 1), an estimate of bone strength. However, they did not report actual values for SSI. Specifically, in Table 2, Sayers and colleagues provided values for BMC, Ct.Ar, BMD, Ps.Pm, and Es.PmPs.Pm but not for SSI. The authors stated that the overall objective of their study was to evaluate the association between BMC and PA through its influence on lean and fat mass. Thus, although we carefully considered this study, because the authors did not report values for bone strength, we were unable to include it in our review. Unfortunately, the article by Deere and colleagues (published in September 2012) did not appear in our last electronic database search (17 January 2013). The article was indexed in MEDLINE (OvidSP) on 26 December 2012 with a revision date of 7 November 2013. Despite being missed in our initial search, the article by Deere and colleagues was similar to Sayer's 2011 publication in that estimated bone strength values (cross-sectional moment of inertia [CSMI]) in relation to PA were only provided in Fig. 1. Further, the primary aim of the analysis by Deere and colleagues was to examine the association between PA and hip aBMD (by DXA). Large, well-designed, cross-sectional pediatric bone studies such as ALSPAC have made significant contributions to our field of research. However, cross-sectional studies by their very nature are considered level 3 evidence, as defined by the Oxford Centre for Evidence-Based Medicine and, therefore, by definition provide limited evidence. It is the purview of randomized controlled trials (level 2 evidence)2 to assess causality. As readers well know, the highest level of evidence-based recommendations are gleaned from systematic reviews.2 Further, it is challenging to accurately assess the intensity of children's PA. We agree with the ALSPAC group that this is especially true using subjective measures of PA such as questionnaires. However, as discussed in our review, most of the cross-sectional studies of recreational PA we reviewed used questionnaires to assess PA (7/10 studies). Further, PA was derived differently across these questionnaires and the PA outcomes also varied across studies.3 We also agree that PA measured objectively by accelerometry is now considered a highly reliable, accurate, and practical tool,4 and applaud the ALSPAC group for using accelerometry in their cross-sectional study. However, measuring PA in children using accelerometry is not without its own unique challenges.4 For example, of the three cross-sectional studies we reviewed that used accelerometers to assess PA,5-7 each utilized different cut-points to categorize PA intensity. Sardinha and colleagues used >2000 to 3999 counts per minute (cpm) to represent moderate PA in pre- and early pubertal boys and girls.7 In comparison, Janz and colleagues considered moderate PA as ≥3000 cpm.6 Thus, we contend that there is indeed still much to learn about how best to assess PA in children using accelerometry and, as a next step, how bone responds to PA during growth. In closing, the preponderance of studies we reviewed that examined the association between PA and bone strength were cross sectional, and very few of these studies used objective measures of PA. Together, these findings lend support to our conclusion that “we have much to learn about the specific response of bone mass versus bone structure to PA, and the relative contribution of each to bone strength in the child and adolescent skeleton.”

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