medwireNews: Dietary supplementation with high dose vitamin D3 plus calcium does not reduce the risk for cancer in healthy postmenopausal women, US study findings indicate.
However, the researchers point out that the women in their cohort had “higher baseline serum 25(OH)D [25-hydroxyvitamin D] levels compared with the US population” and may therefore be expected to derive less benefit, up to a point, from supplementation than women with lower baseline levels.
They add: “The higher intake of outside vitamin D3 supplementation in the placebo group in this study cohort (the majority of individuals would be considered to be vitamin D sufficient) might obscure any treatment effect in the intention-to-treat analysis.”
Joan Lappe (Creighton University Schools of Nursing and Medicine, Omaha, Nebraska) and colleagues randomly assigned 2303 healthy postmenopausal women age 55 years or older (mean 65.2 years) to receive 2000 IU of vitamin D3 and 1500 mg of calcium daily (n=1156) or placebo (n=1147) for 4 years.
At baseline, the mean 25(OH)D level was 32.8 ng/mL, and did not differ significantly between the groups. However, at each 6-monthly visit post baseline, 25(OH)D levels were significantly higher in the treatment group than in the control group, with respective levels of 42.5 and 30.9 ng/mL recorded at 4 years.
During the study, 45 (3.9%) women in the vitamin D3 plus calcium group and 64 (5.6%) in the placebo group were diagnosed with cancer (excluding nonmelanoma skin cancers), a difference that was not statistically significant.
And after excluding 106 participants who withdrew after randomization with no follow-up, there was no significant difference in cancer incidence between the two groups using Kaplan-Meier analysis, at rates of 0.042 and 0.060, respectively, over 4 years.
There was also no difference between the groups when breast cancer incidence was analyzed separately, but the researchers note that number of cases was small (19 in the treatment group vs 24 in the placebo group).
A posthoc analysis that excluded women who withdrew, died, or developed cancer during year 1 showed that participants in the intervention group had a significant 35% reduced risk for cancer compared with those in the placebo group.
This suggests that “cancers diagnosed early in the study may have been present upon enrolment and that time is needed for vitamin D supplementation to increase serum 25(OH)D level and to exert a potential effect on cancer development or progression,” Lappe et al remark in JAMA.
However, they caution: “[T]his finding, as well as the post hoc observation suggesting an inverse association between serum 25(OH)D levels and cancer, should be considered only exploratory and hypothesis generating, and require assessment in further studies.”
Commenting on the findings, JoAnn Manson and colleagues from Harvard Medical School in Boston, Massachusetts, USA say: “Although the current trial results do not preclude an important reduction in cancer risk, the absence of a clear benefit for this end point is consistent with the totality of current evidence on vitamin D supplementation, with or without calcium, for prevention of incident cancer.”
By Laura Cowen
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