In a new study from Brigham and Women's Hospital and Harvard Medical School in Boston, a high intake of calcium and vitamin D through food sources and nutritional supplements was linked to modestly lower risk of breast cancer in premenopausal women.
The link appeared strongest for the most aggressive tumors, and it was not seen after menopause.
"Calcium and vitamin D are important for overall health and, additionally, they may help prevent breast cancer," she tells WebMD.
Evidence 'Fairly Consistent'
Roughly 31,000 women enrolled in the larger Women's Health Study were included in the analysis by Lin and colleagues. The findings were published May 28 in the Archives of Internal Medicine.
All of the women were aged 45 or older, and two-thirds were postmenopausal. The women completed questionnaires at study entry and periodically after that were designed to determine their medical history and lifestyle, including the foods they ate and supplements they took.
Over an average of 10 years of follow-up, 276 premenopausal and 743 postmenopausal study participants developed breast cancer.
Premenopausal women with the highest intakes of calcium and vitamin D had modestly reduced risk of breast cancer compared with women who got the lowest amount of the nutrients through food and supplemental sources.
The findings are similar to those reported in 2002 by another group of Harvard researchers. In that study, calcium and vitamin D through dairy sources were associated with a decreased risk of breast cancer before, but not after, menopause.
Dietary calcium and vitamin D were found to lower breast cancer risk in a cancer prevention study reported by researchers from the American Cancer Society (ACS).
ACS nutritional epidemiologist Marji McCullough, ScD, RD, who reported the findings, tells WebMD that more study is needed to understand how vitamin D and calcium influence breast cancer risk.
"The evidence of a modest protective benefit [for dietary vitamin D and calcium] is fairly consistent, but we still don't know if premenopausal and postmenopausal women benefit equally," she says.
What About the Sun
Current dietary recommendations call for people aged 50 and under to consume just 200 international units (IU) of vitamin D a day, with 400 IU recommended for those between the ages of 51 and 70, and 600 IU recommended after age 70.
Many experts now agree that these levels are too low. Longtime vitamin D researcher Cedric Garland, DrPH, says most people should get between 1,000 IU to 1,500 IU a day.
Excessive vitamin D can lead to toxicity. You should talk to your doctor about the use of supplements prior to taking them.
Dairy products and oily fish like salmon and tuna are some of the best food sources for vitamin D, but it would be difficult to get that much vitamin D in foods alone.
The easiest way for the body to get vitamin D is through sun exposure, because ultraviolet rays from the sun trigger the natural synthesis of vitamin D.
An 8-ounce glass of milk contains only 100 IU of vitamin D. By comparison, someone who spends 10 to 15 minutes in the sun on a sunny day without sunscreen can absorb 2,000 to 5,000 IU of vitamin D if 40% of their body is exposed, Garland says.
Recommending sun exposure is controversial because of the risk of skin cancer, and Garland tells WebMD that it is possible to get all the vitamin D the body needs through foods and dietary supplements.
"Ideally a mix is good, with some vitamin D coming from food, some from supplements, and some from sun if people can handle sunlight," he says.
Considering the Evidence
The American Cancer Society, the National Council on Skin Cancer Prevention, and other interested health groups from the U.S. and Canada met early last year to consider the evidence on sun exposure, vitamin D, and health.
The coalition concluded that the evidence is "strong" linking vitamin D to a decreased risk of bone fractures in the elderly. With regard to cancer risk, the group concluded that "a growing body of evidence" suggests a protective benefit for some cancers.
The coalition noted that the risks of unprotected exposure to the sun as a source of vitamin D had to be weighed against the benefits.
"To minimize the health risks associated with ultraviolet B (UVB) radiation exposure while maximizing the potential benefits of optimum vitamin D status, supplementation and small amounts of sun exposure are the preferred methods of obtaining vitamin D," the group concluded.
McCullough says 10 minutes a day of unprotected sun exposure in the spring and summer is plenty for most people, and more than this is too much.
"We need to make sure that people don't interpret this as meaning that going to the beach and sunbathing for hours without sunscreen is a good idea," she says.