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Neal D. Barnard, MD
Physicians Committee for Responsible Medicine
Washington, DC 20016
We thank Dr Hanley and Ms Fenton for their thoughtful comments on our review. By way of clarification, of the 58 studies that we reviewed, 11 were excluded for not controlling for weight, pubertal status, or exercise, and 10 focused specifically on the effects of calcium supplements.1 Of the remaining 37 studies, 4 had prospective designs that specifically addressed the effects of dairy calcium rather than total dietary calcium on bone health in children and young adults.2–5 Of these, 3 were randomized, controlled trials and 1 was a cohort study. In a 2-year trial that doubled calcium intake from 750 to 1640 mg of calcium per day with additional servings of dairy products, no difference was observed in bone mineral density (BMD) or change in bone density over time with the dairy-product treatment.2
In the cohort study, no association between existing dairy calcium intakes and change in BMD was noted over 1 year.5 The correspondents suggest that this may have been because of generally high calcium intakes among the children in this study. If so, these findings are particularly relevant to North American children, whose dairy-product intake is also high; dairy-product intake is, in fact, the leading source of both total fat and saturated fat in the diets of youth aged 2 to 19.6
In the other 2 randomized, controlled trials, the observed benefits of dairy-product consumption on bone may be overstated. In one study in which mean calcium intake of 11-year-old girls was increased from 738 to 1437 mg/day for 1 year, neither bone mineral content (BMC) nor BMD of the lumbar spine was significantly different between dairy-product–treated and –untreated girls. An increased rate of gain in total BMC and BMD of the lumbar spine (there was no such difference at the femoral neck or radius) failed to translate into significant differences at the study's conclusion.3 In the other study, the total increase in bone density was slightly greater in 12-year-old girls in the treated group as compared with those in the control group (9.6% vs 8.5%; P = .017), but between-group differences in resultant bone mineral measures were not reported.4 In summary, in the 3 randomized, controlled trials, after 1 to 2 years of approximately doubling calcium intake in adolescent girls, the only differences noted were in changes in the rate of BMD or BMC gain at some but not all sites, with no significant differences in the resultant measures of BMD or BMC of the treated and untreated groups.2–4
Similarly, the 4 additional recent trials of the effects of milk, milk powder, or enriched milk supplementation on bone health in children and teens cited by Hanley and Fenton either did not report7,8 or found no difference between the supplemented and unsupplemented groups9,10 in any measure of resultant BMC or BMD. However, 3 of these studies (2 with Chinese grade-school children and 1 with teenage girls from New Zealand) report an increased rate of BMC or BMD gain at some sites in dairy-product–treated children as compared with controls.7,8,10 Of these, the New Zealand study is perhaps the most informative because it reports on 2 years of dairy-product supplementation and 1 year of posttreatment follow-up. One year after the treatment ended, no effect of dairy-product treatment was evident. In fact, according to the figures in this report, girls who had been in the milk-treated groups lost BMD at all sites measured between the end of the treatment period and 1-year follow-up, whereas untreated girls continued to gain BMD or stayed steady.7
Perhaps more to the point, if public-policy recommendations and government programs were not already pushing children and adults to consume large quantities of dairy products (3–4 servings per day), there would be no need to discuss the difference between "no evidence of an effect" versus "evidence of no effect." The limited and controversial evidence that does exist, such as from the above-mentioned 8 studies, has been interpreted far too generously, translating into unsupported policy recommendations. As the correspondents point out, and as illustrated by the New Zealand study,7 the medical and scientific communities still do not know whether this small, albeit measurable and possibly confounded effect on rate of BMC or BMD gain was seen in a subset of a small number of randomized, controlled trials and will translate into a benefit for bones in later life. This leap to policy, in the absence of clear supporting evidence of milk's benefit, draws clinical attention away from more comprehensive research on how to effectively promote long-term bone health among youth.
Of the 37 studies that met our inclusion criteria, only 9 demonstrated modest positive benefits on BMD or BMC in children or adolescents. However, even those with the strongest designs did not demonstrate a between-group difference in any measure of BMC or BMD. Rather, they found only small changes in rates of gain during treatment periods that did not translate into significant differences in final values.
REFERENCES
Related articles in Pediatrics:
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