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Clinical Nutrition Research Center Department of Nutrition Sciences 231 Webb Building 1675 University Boulevard University of Alabama at Birmingham Birmingham, AL 35223-3360 E-mail: weinsier{at}shrp.uab.edu
Dear Sir:
We appreciate the comments of Weaver and Heaney, whom we respect as being internationally recognized experts in the field of calcium metabolism and bone disease. They raise a valid concern that randomized controlled trials (RCTs), especially double-blind trials, are needed to establish causal relations in clinical investigations and that even large longitudinal cohort studies may not be equivalent for this purpose. As they indicate, there are both strengths and weaknesses in longitudinal observational studies, notably, the ability of such studies to detect hard endpoint outcomes such as bone fractures and their limited ability to accurately assess dietary intake. However, there are also shortcomings in the few RCTs on the effect of dairy foods on bone status.
In our review (1) we classified 2 RCTs (2, 3) in the favorable-effect category because each study showed less bone loss in the dairy-supplemented group. On the other hand, at the end of the intervention neither trial resulted in greater bone mass in the dairy-supplemented than in the nonsupplemented group. Four of the 5 RCTs classified in the favorable-effect category were not blinded and did not have a placebo control. Hence, confounding variables were not always removed. This is evident in one RCT of adolescent girls in which the dairy-supplemented group had a 50% greater energy intake than did the control group and in which energy intake correlated significantly with greater bone mineral content (3). A further potential shortcoming of the available RCTs was summarized in Heaney's (4) recent review article in which he states that "all controlled manipulations of calcium intake produce a bone remodeling transient, generally expressing itself during the first year of treatment." The average length of the 5 RCTs assigned to the favorable-effect category was 1.5 y, with a range of 14 wk to 3 y; none included a baseline period of adaptation to the intervention. Thus, there is the risk that the positive effects seen in these RCTs may be, as Heaney (4) pointed out, inflated by being a compound of the remodeling transient plus an improvement in bone balance. By contrast, observational studies do not alter customary calcium intake, thereby avoiding the confounding problems of the remodeling transient.
Nevertheless, assuming that observational studies should not be given level A strength-of-evidence status equal to that of RCTs, downgrading the one observational study in the unfavorable-effect category (5) to level B status does not change the results. That is, the reported ratio of favorable to unfavorable effects for the stronger-evidence categories A and B remains low and unchanged at 2:1 (6 favorable and 3 unfavorable outcomes). We believe that the more important point is that there are too few carefully designed studies of the effects of dairy foods on bone health.
Weaver and Heaney conclude that adequate dairy consumption supports good bone health. We agree with their conclusion, if qualifying terms are added to clarify that adequate consumption of certain types of dairy foods appears to be supportive of good bone health among select age, race, and sex groups. Two important caveats are as follows. 1) A clear distinction must be made between milk and its derived dairy products in discussions of dairy foods and bone status. Many dairy foods, particularly processed cheese products and cottage cheese, have markedly different ratios of calcium to potassium, sodium contents, and renal acid loads than does milk, and may have markedly different effects on the way calcium is metabolized. 2) There are too few studies in males and ethnic minorities for conclusions to be drawn about the effect of any dairy food on bone health in these groups, which together represent more than one-half of the US population. Hence, the body of scientific evidence is inadequate to support a recommendation for daily intake of dairy foods to promote bone health in the general US population.
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