To the Editor:
After reading the well-conducted and analyzed observational study by McGartland et al.(1) and its accompanying editorial,(2) we are concerned because the message sent is that the effect of soft drink (SD) consumption on bone health is entirely explained by dietary displacement of other nutrients, mainly from milk and dairy products, and that any untoward effect of SDs can be effectively counteracted by an adequate calcium intake. It should be emphasized that this explanation is nothing more than non-evidence-based opinion. The unproven hypothesis that the adverse effects of SDs on bone health are entirely caused by milk displacement has been approved by a consensus of experts,(3) but is supported only by circumstantial evidence such as the known salutary effect of calcium supplementation on BMD,(4) the absence of acute effects of SDs on renal handling of calcium in healthy volunteers,(5) and the inverse relationship between SD consumption and milk intake.(6) Fitzpatrick and Heaney(2) summarily exonerate SDs of having metabolic effects on their own, disregarding the two major pieces of published evidence that suggest the contrary is indeed the case. First, McGartland et al.(1) showed that the statistically significant relationship between SD intake and BMD of the heel in girls persists (although weakened), even after adjusting by liquid milk consumption and calcium intake among many other factors. The orthodox interpretation of this finding is that SD consumption has an effect on BMD that is independent of milk or calcium intake. Second, our group showed that rats drinking cola SDs ad libitum reduced their solid food intake by one-half and had impaired BMD. A pair-fed group of rats drinking tap water had a significant body weight reduction, but their BMD was not different from BMD of control rats and was significantly higher than BMD of rats drinking cola SDs.(7) This finding strongly supports that SDs have a metabolic effect on their own. Fitzpatrick had previously disqualified the pair-fed rats as an adequate comparison group,(3) but we still think that it is the best comparison group to test the hypothesis that the effects of SDs are caused by nutriment displacement.
Evidence continues to accumulate in support that SD consumption is associated with poor bone health. The association seems not to be restricted to a particular kind of SD. However, the pathophysiological explanation of how SDs affect bone and mineral metabolism has not been clarified. More research is needed to find out the mechanisms responsible for the observed disorders that may constitute public health problems, given the growing exposure of the world's population to SDs.