High intakes of calcium and vitamin D were associated with lower breast cancer risk in premenopausal women in a large prospective study of more than 30,000 women.
The decreased risk appears to be most pronounced with aggressive breast tumors, study researchers wrote.
Animal studies have suggested that calcium and vitamin D may protect against breast cancer, but epidemiologic studies of the issue in humans have yielded conflicting results.
Jennifer Lin, Ph.D., of Harvard Medical School, Boston, and her associates used data from the Women's Health Study to examine intakes of the two nutrients in relation to breast cancer risk in more than 10,000 premenopausal and 20,000 postmenopausal subjects who were followed for an average of 10 years.
During that interval, 276 premenopausal and 743 postmenopausal women developed incident cases of invasive breast cancer.
Mean intakes of total calcium and vitamin D were 1,021 mg/day and 353 IU/day, respectively.
Among premenopausal women, there was a moderate association between lower risk of breast cancer and higher consumption of dietary and supplemental calcium and vitamin D.
When the women were divided into quintiles based on consumption, the multivariate hazard ratios in the highest quintile group relative to the lowest one were 0.61 for total calcium and 0.65 for total vitamin D intake.
This association was most pronounced in women who had cancers larger than 2 cm, poorly differentiated tumors, or positive lymph nodes (Arch. Intern. Med. 2007;167:1050–9).
In contrast, among who were postmenopausal, consumption of calcium and vitamin D were not inversely associated with breast cancer, and tumor characteristics did not influence the relationship.
The reason why these nutrients may be linked to breast cancer risk only before menopause remains unknown, Dr. Lin and her associates said.
Research suggests that vitamin D may inhibit “late events of breast tumorigenesis,” and it also may enhance apoptosis and reduce the proliferation of tumor cells.
Similarly, calcium is thought to slow the progression of breast cancer by inhibiting the secretion of certain proteins that play a key role in the growth of advanced tumors and metastasis, they added.
These nutrients appear to lower breast cancer risk, but only before menopause. Lynda Banzi/Elsevier Global Medical News