The women's health initiative calcium–vitamin D trial: overview and baseline characteristics of participants
Introduction
Osteoporosis is a major public health issue leading to significant morbidity, loss of independence and excess loss of life. It has been estimated that 13 to 17 million postmenopausal women have low bone mass or osteoporosis 1., 2.. This is associated with almost 1.5 million fractures annually, including 300,000 hip fractures 2., 3.. A review of the evidence by Cumming in 1990 (4) and subsequently published randomized clinical trials have shown that calcium and/or vitamin D supplements may play a role in the prevention and treatment of osteoporosis by slowing the rates of bone loss in postmenopausal and elderly women 5., 6., 7., 8., 9.. However, there are a limited number of calcium and/or vitamin D trials 10., 11., 12., 13. and observational studies [review of evidence by Cumming and Nevitt (14) and Kanis (15)], to support a role for calcium and vitamin D supplementation in the reduction of hip and other fractures. It has also been suggested that calcium and vitamin D supplementation may play a role in the reduction of colorectal cancer incidence 16., 17., 18., 19., 20., 21.. To address these major health concerns of postmenopausal women, the Calcium-Vitamin D (CaD) trial of the Women's Health Initiative (WHI) was designed to test the primary hypothesis that women who are randomized to receive calcium and vitamin D supplementation will have a lower risk of hip fracture and secondarily, a lower risk of all fractures and colorectal cancer than women receiving corresponding placebo. The objective of this paper is to describe the 36,282 women enrolled in the WHI CaD trial cohort with an emphasis on risk factors for osteoporotic fracture and colorectal cancer.
Section snippets
Enrollment of study participants
Enrollment into the CaD component was delayed by one year to avoid undue participant burden at entry into WHI. Participants in the Diet Modification (DM) and/or Postmenopausal Hormone Therapy (PHT) component(s) were invited to join the CaD trial at the first or second annual follow-up visit (see Anderson's article and Hays' article in this issue for details of the design, recruitment, and screening process). Informed consent was obtained at the CaD randomization visit. The Institutional Review
Results
Because the focus of the CaD trial is on hip and all fractures and colorectal cancer, the results presented below focus on the baseline characteristics of the CaD cohort that are likely to affect risk of these outcomes. Data presented reflect the characteristics of the CaD trial cohort at entry into the WHI clinical trial at baseline unless otherwise noted.
Discussion
The WHI CaD Trial is the largest randomized, placebo-controlled clinical trial with the longest duration of exposure and follow-up testing the impact of calcium plus vitamin D supplementation on the incidence of hip and all fractures. There have been many published randomized clinical trials testing the effects of calcium supplementation on changes in BMD 4., 5., 6., 7., 8., 9., an intermediate variable for treatment efficacy for osteoporosis. However, few randomized clinical trials have
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