Three RCTs (n=1,485) were included. The analysis was based on 1,279 patients with complete follow-up.
In terms of the quality of the studies, two studies reported adequate methods of randomisation and one did not mention the method used. All studies involved blinding, although the level of blinding was unclear. All three studies reported the use of ITT analysis. The drop-out rates ranged from 9 to 19.5%. Two studies checked compliance using food diaries; the method of compliance was not reported in the other study.
Calcium supplementation was associated with a statistically significant reduction in the risk of recurrence of adenomas compared with placebo (RR 0.80, 95% CI: 0.68, 0.93, p=0.004). No statistically significant heterogeneity was found (p=0.75). The corresponding RD was 7% (95% CI: 1.0, 12.0) and the NNT was 14 (95% CI: 8, 100).
The subgroup analysis showed a non statistically significant trend towards a reduced risk of adenoma recurrence for proximal polyps (RR 0.80, 95% CI: 0.61, 1.04, p=0.09), distal polyps (RR 0.86, 95% CI: 0.69, 1.08, p=0.20) and polyps over 9 mm (RR 0.78, 95% CI: 0.50, 1.22, p=0.28).