Background: Frequent use of personal, nonprotocol calcium supplements obscured an adverse effect of coadministered calcium and vitamin D (CaD) on cardiovascular risk in the Women's Health Initiative (WHI).
Objective: We investigated the effects of the use of personal calcium or vitamin D supplements on other outcomes in the WHI CaD Study (WHI CaD) by using the WHI limited-access clinical trials data set.
Design: The WHI CaD was a 7-y, randomized, placebo-controlled trial of CaD (1 g Ca/400 IU vitamin D daily) in 36,282 community-dwelling, postmenopausal women. The incidence of total cancer (excluding nonmelanoma skin cancers), breast and colorectal cancers, hip and total fracture, and mortality was assessed by using Cox proportional hazards models.
Results: In the WHI CaD, interactions between the use of either personal calcium or vitamin D supplements and CaD were found for total, breast, and colorectal cancers but not for fracture or mortality. In 15,646 women (43%) who were not taking personal calcium or vitamin D supplements at randomization, CaD significantly decreased the risk of total, breast, and invasive breast cancers by 14-20% and nonsignificantly reduced the risk of colorectal cancer by 17%. In women taking personal calcium or vitamin D supplements, CaD did not alter cancer risk (HR: 1.06-1.26).
Conclusions: For women in the WHI CaD who were not taking personal calcium or vitamin D supplements at randomization, CaD decreased the risk of total, breast, and colorectal cancers and did not change the risk of fractures or total mortality. The nonskeletal effects of CaD may be more important than the skeletal effects and should be considered when evaluating these supplements. The WHI CaD trial is registered at clinicaltrials.gov as NCT00000611.