Clearing up confusion around the vitamin supplement findings from the Women’s Health Initiative Dietary Modification Trial
Many people were surprised and confused by the report from the Women’s Health Initiative Dietary Modification Trial (WHI) that vitamin D supplements had no apparent benefit for hip fracture or colon cancer risks.(1) In this study, over 36,000 women were randomly assigned to receive either a placebo or 400 IU of vitamin D plus 800 mg of calcium daily. During an average of seven years of follow-up, the women who received the supplements did not appear to gain protection against hip fractures or colon cancer. However, the study had serious limitations.
First, women who were already taking vitamin D or calcium supplements were not excluded or discouraged from taking them during the trial; by the end of the study nearly 70% of both groups were taking calcium supplements and nearly 40% were taking vitamin D supplements on their own. Further, only about 60% of participants were taking their assigned treatments regularly. Thus, the difference in blood vitamin D levels during the trial between the treatment and placebo group was very small. Interestingly, women with higher blood levels of vitamin D before the start of the study had less than half the risk of colon cancer compared to those with the lowest levels.
When the WHI study started, 400 IU of vitamin D seemed like a reasonable dose. However, this is now regarded as very little; other studies have shown a reduction in fracture risk with 700-800 IU per day, but not with 400 IU per day. Moreover, for prevention of colon cancer, seven years may well be too short to show a benefit of supplements because we know that this disease takes decades to develop.
For all of these reasons, the WHI report should not influence thinking about the effects of vitamin D or calcium on health, and for those who might benefit, a vitamin D supplement of 800 to 1000 IU per day still makes sense.
1. Jackson RD, LaCroix AZ, Gass M et al. Calcium plus vitamin D supplementation and the risk of fractures. N Engl J Med. 2006; 354:669-83.