In a study published recently in the British Journal of Nutrition, Major et al. ⁽Reference Major, Alarie and Doré¹⁾ reported findings from a double-blind placebo-controlled randomised trial of Ca and vitamin D (1200 mg Ca and 10 μg vitamin D per d) on the ability of obese women to lose weight. The study was restricted to obese women with habitual Ca intake < 800 mg/d. The key-reported observation relates to a small subset of the women with unusually low habitual Ca intake ( < 600 mg/d). In this subgroup (n 7), the authors report a modest enhancement of weight loss with supplement v. placebo. These findings cannot be fully interpreted because the nature of the placebo is not stated. At this dose, the supplement or placebo would have a mass of at least 3000 mg if supplied as calcium carbonate. The composition of the placebo would seem to be important given the modest treatment effect. It is possible that the placebo could contain some active ingredient. It is also possible that the placebo ingredient could cause less indigestion than is generally caused by calcium carbonate. It seems to be an important oversight to not provide details of the placebo used and it would be helpful if the authors were to disclose this valuable piece of information.

We wrote to the authors to ask them to supply further information about the composition of the placebo. The authors replied that: ‘We know what is the composition of the placebo but this information is confidential.’

Furthermore, the sensitivity of the subset analysis to the exact choice of cut-off in terms of habitual Ca intake is uncertain. This cannot be assessed from the work as published. It would be helpful if the authors provided some information, such as a scatter-plot showing the relationship between weight loss and habitual intake in each study arm, so that this could be assessed.

We have no conflicts of interest.