BMJ  2005;331:108 (9 July), doi:10.1136/bmj.331.7508.108-a

Letter

Calcium and vitamin D in preventing fractures

Dietary intake of calcium needs to be considered

EDITOR—In their prospective randomised trial of calcium and vitamin D supplementation, Porthouse et al found no evidence that this reduced the risk of clinical fractures in women at risk.¹ However, several aspects need closer examination before a therapeutic role can be defined.

Although these women all had risk factors for hip factors, they were highly self selected. Of 48 987 originally invited, only 3314 (7%) were eventually randomised. Median follow-up overall was only 25 months, and the design of the recruitment process means that this must have differed between the two groups. In the "unequally allocated group," followed up for longer, the evidence of a benefit of the intervention is greater.

Vitamin D concentrations were not measured to see the prevalence of insufficiency and whether this degree of supplementation was able to produce a significant improvement in values. It would also have been informative to see whether improved calcium absorption reduced parathyroid hormone concentrations.

Perhaps the most important factor is dietary calcium. The self reported estimated intake in both groups was over 1000 mg daily, which is greater than in many other studies, and both groups received literature on adequate calcium and vitamin D intake. High quality data from the US Study of Osteoporotic Fractures Research Group on dietary calcium, intestinal calcium absorption, and hip fractures, which the authors do not cite, put these findings into context.²

Ensrud et al studied 5452 women with a mean follow-up of 4.8 years. Some 33% had a dietary calcium intake less than 400 mg daily. In this group, when fractional calcium absorption was below the mean, the risk of hip fractures was two and a half times greater than when absorption was above average. Therefore this combination of factors (low dietary calcium intake and low fractional absorption) seems to identify women most likely to benefit from calcium and vitamin D supplements.

Hammersmith Hospital, Imperial College, London W12 0NN julian.walters{at}imperial.ac.uk

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Competing interests: None declared.

References

1. Porthouse J, Cockayne S, King C, Saxon L, Steele F, Aspray T, et al. Randomised controlled trial of calcium and supplementation with cholecalciferol (vitamin D3) for prevention of fractures in primary care. BMJ 2005;330: 1003. (30 April.)[Abstract/Free Full Text]
2. Ensrud KE, Duong T, Cauley JA, Heaney RP, Wolf RL, Harris E, et al. Low fractional calcium absorption increases the risk for hip fracture in women with low calcium intake. Ann Intern Med 2000;132: 345-53.[Abstract/Free Full Text]

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