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New analysis of the Women's Health Initiative.
New analysis of the historic Women’s Health Initiative suggests that long-term supplementation of vitamin D and calcium can reduce hip fractures, but at the expense of a greater risk of developing kidney stones.
Researchers from 11 U.S. institutions analyzed data from 36,282 women who participated in the Women’s Health Initiative clinical trial and 68,719 women who participated in a companion observational study. All participants were postmenopausal. Those who participated in the clinical trial were assigned to 1 g of calcium carbonate and 400 IU of vitamin D3 daily or placebo for an average intervention of 7 years.
Studies have centered on Women’s Health Initiative data for a multitude of reasons, but this occasion called for an investigation of hip fracture risk, followed by secondary markers including total fracture, colorectal cancer, and other health conditions.
When analyzing a subset of women from the clinical trial who were not taking calcium and/or vitamin D supplements at baseline, the researchers identified a 38% reduced risk of hip fracture (35% when combined with the observational study). The results were supported by a significant increase in hip bone mineral density with supplementation versus placebo at years 2, 5, and 8 of follow-up. In weighing the totality of participant data-not just subjects who were taking supplements at baseline-researchers only identified a “nominally significant risk reduction” for hip fracture with supplements versus placebo.
Further analysis of all subjects revealed what appeared to be a 17% increased risk of kidney stone formation with vitamin D and calcium supplementation, deemed by the researchers to be a modest risk. (Kidney stone risk is a recurring concern in published research on this topic.) Supplement use did not appear to significantly affect heart health, cancer, or death by any cause.
“Our further analyses of Women’s Health Initiative calcium and vitamin D trial data, through the end of the intervention period, along with corresponding and combined analyses using the WHI observational study, yield few clear clinical effects for daily 1,000 mg calcium plus 400 IU of vitamin D,” concluded the researchers.
More research is warranted to investigate the health effects of vitamin D and calcium, especially at other supplementation doses. This analysis of the Women’s Health Initiative was supported by a grant from the National Osteoporosis Foundation and was published ahead of print in Osteoporosis International.