Ecologic studies comparing women in areas of high vs. low sun exposure, animal studies, and some case control studies, particularly in young women, suggest that it can, especially if the blood levels are higher than we usually see with conventional doses of supplements. However, these initial investigations are but first steps.
Studies have shown that women who have lower body fat from diet and exercise are also likely to have higher levels of vitamin D—but we don't know why. It could because of what they eat, or because they are exercising outside. We also are not sure of the target blood level of vitamin D or the age that vitamin D supplementation needs to start to prevent breast cancer. Further, overly high levels of vitamin D can result in side effects such as kidney stones, so it's not as if we can just tell women to take as much vitamin D as they like.
Other findings are mixed. Two randomized studies have compared the benefit of calcium and vitamin D to a placebo. The first was a small study in postmenopausal women. The vitamin D dose was 1100 (IU). This study suggested that vitamin D could reduce a woman's risk of getting all types of cancers, including breast cancer, but the numbers of women who actually got cancer was too small to say that vitamin D reduced a specific type of cancer.
The second study, the Women's Health Initiative (WHI), randomized postmenopausal women to take calcium and 400 (IU) of vitamin D or placebo for seven years. Women in both the placebo and treatment arms also were allowed to take up to 1000 IU of non-study vitamin D. This study did not find any relationship between vitamin D intake, blood levels of vitamin D, and breast cancer risk. One possible explanation may be that the women with higher vitamin D levels were also more likely to be thinner and also to exercise. Since these factors can also reduce breast cancer risk, it is hard to know which is more relevant.
Vitamin D is important in the maintenance of healthy bones, muscle, and immune system and probably several other types of tissue as well. Currently, the recommended daily allowance of vitamin D is 200 units a day for children and adults up to age 50; 400 units a day for adults aged 51-70, and 600 units a day for adults age 70 and over. However, many doctors now believe that this is insufficient, and are recommending an adult daily vitamin D intake of 800-1,000 units a day.
You can get this by taking a supplement or by increasing the number of vitamin D-rich foods you eat. Spending 15 minutes a day in the sun without sunscreen will probably not harm you unless you are very fair or have a personal or family history of skin cancer.
It is not recommended that you try to get a lot of vitamin D by, for example, spending a day in the sun on the beach without sunscreen. And remember: More is not always better. The maximum recommended dose of vitamin D is 2,000 units a day. Too much vitamin D can cause nausea, vomiting, constipation, and weakness. It can also cause heart rhythm abnormalities and kidney problems.
Bottom-line: We currently do not know whether the vitamin D we get through sun exposure has a different impact on our health than the vitamin D we get in supplements. And we can't assume that randomized controlled trials will find that vitamin D helps reduce cancer risk. Let's not forget that we recently saw the results of a study that showed that regular multi-vitamin use does not appear to reduce women's risk of cancer or other diseases—and how many of us were taking multivitamins just for that reason?
National Institutes of Health: Office of Dietary Supplements Fact Sheet on Vitamin D
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