Article type
Year
Abstract
Introduction: The parameter reported in all studies assessing the effect of increasing calcium intake is the average annual rate of loss in bone density calculated over the duration of the trial. The underlying assumption is that the rate can be extrapolated beyond the observed duration of the trial. As calcium is a normal part of the diet, this approach includes the 'wash-in' period in the active treatment group which lasts for 6-12 months. Hence, this approach could lead to incorrect estimates of the longterm usefulness of calcium in reducing bone loss. We investigated whether the rate of bone loss differs in the first and second year of the trials.
Methods: Trials published in English using calcium supplements or dairy products as the treatment and change in bone density as the outcome were identified from reviews and a Medline search conducted in June 1994. Inclusion criteria were: a concurrent, randomised control group, intention-to-treat analysis and at least 2 years of follow-up in post-menopausal women. Combination therapy arms involving drugs with strong effects (e.g. oestrogen) were excluded. The baseline, 1 year and 2 year results were read by overlaying a transparent grid on the enlarged graphs. A weighted mean slope for each year was calculated and the difference between them tested under several covariance assumptions.
Results: Of 12 studies meeting the inclusion criteria, 6 included a graph or a table showing the effects by year. The percent of baseline bone density lost in the second year was greater than in the first year in the femoral neck: -1.28% (95% confidence interval: -0.61 to -1.95) and the spine: -0.88 (95% CI: -0.4 to -1.33). These differences are quite substantial in relation to the postulated benefit of calcium. There was no difference for the proximal radius: 0.03% (95% CI:-0.46 to 0.52). Differences in the other bones varied and had wide confidence intervals. These results were not sensitive to changing the covariance assumptions.
Discussion: These results indicate that, at least for the femoral neck and spine, conducting a formal meta-analysis using currently published results is likely to over-estimate the longterm effectiveness of calcium supplementation in retarding bone loss.
Methods: Trials published in English using calcium supplements or dairy products as the treatment and change in bone density as the outcome were identified from reviews and a Medline search conducted in June 1994. Inclusion criteria were: a concurrent, randomised control group, intention-to-treat analysis and at least 2 years of follow-up in post-menopausal women. Combination therapy arms involving drugs with strong effects (e.g. oestrogen) were excluded. The baseline, 1 year and 2 year results were read by overlaying a transparent grid on the enlarged graphs. A weighted mean slope for each year was calculated and the difference between them tested under several covariance assumptions.
Results: Of 12 studies meeting the inclusion criteria, 6 included a graph or a table showing the effects by year. The percent of baseline bone density lost in the second year was greater than in the first year in the femoral neck: -1.28% (95% confidence interval: -0.61 to -1.95) and the spine: -0.88 (95% CI: -0.4 to -1.33). These differences are quite substantial in relation to the postulated benefit of calcium. There was no difference for the proximal radius: 0.03% (95% CI:-0.46 to 0.52). Differences in the other bones varied and had wide confidence intervals. These results were not sensitive to changing the covariance assumptions.
Discussion: These results indicate that, at least for the femoral neck and spine, conducting a formal meta-analysis using currently published results is likely to over-estimate the longterm effectiveness of calcium supplementation in retarding bone loss.