Compared to those women taking a placebo, cancer risk of any kind over 10 years decreased by 60 percent in those taking both calcium and vitamin D, and by 47 percent in those taking calcium alone.
Moreover, when they excluded those cancers that occurred in the first year of the study — based on the assumption that these cancers were likely present at the start of the study — the benefit of calcium plus vitamin D appeared even more dramatic: a 77 percent decrease in cancer risk.
These are stunning results. Even if the authors could not definitively say calcium reduced cancer risk, a number of other studies bore out calcium's protective effect. The authors can say with a strong degree of confidence that combining calcium with vitamin D is highly beneficial.
Experimentally, calcium and vitamin D have been shown to exert their anti-cancer effect by interfering with the action of a hormone called insulinlike growth factor, or IGF.
The IGF hormone stimulates breast cancer cells — as well as cells of other types of cancer — to divide. Calcium and vitamin D interact with IGF to disrupt such cell growth; in fact, vitamin D can effectively block IGF's effect.
Other factors might also come into play. In laboratory animals, for example, diets low in vitamin D and calcium increased the number of breast tumors. Amazingly, diets rich in these nutrients caused the disappearance of many worrisome breast cells, the types of cells that can eventually become cancerous.
While the Harvard study evaluated self-reports of usual food intake from dietary questionnaires, the Nebraska study actively gave patients calcium and vitamin D pills. This has important implications: Cancer risk can be reduced both by calcium and vitamin D in the diet, as well as by using over-the-counter supplements of these nutrients.
One should note, however, that both studies were carried out exclusively among women, and thus might not fully apply to men. However, given previous studies suggesting a reduction in male cancers (e.g., prostate cancer) by calcium and vitamin D, it is likely that this effect is not bound by gender.
Moreover, some of the cancers prevented among the women in the Nebraska study included cancers that also affect men, such as colon and lung cancers and leukemia.
Calcium is largely derived from the diet and is found in dairy products, leafy green vegetables, fortified juices and nuts. Many people, particularly women, also take calcium supplements to strengthen their bones.
Vitamin D is found in oily fish (salmon, sardines), as well as fortified foods such as milk and some cereals. Another major source of vitamin D for most people is exposure to sunshine.
Although the recommended daily allowance of vitamin D is 400 IU, some authorities suggest that up to 1000 I,U might be necessary to achieve optimal blood levels, especially when sun exposure is rare (e.g., during the winter or among home-bound individuals).
Of course there are other important benefits of adequate vitamin D and calcium intake: the prevention of osteoporosis, a thinning of the bones that increases bone fragility making bones more likely to break.
So follow mom's advice: Drink your milk, finish your spinach and get plenty of fresh air and sunshine. And dietary supplements of calcium and vitamin D are also important. If you are a woman who has not gone through menopause, you likely will decrease your risk of breast cancer.
And no matter what your age, you will improve the health of your bones, and probably your overall cancer risk as well.
Dr. John Spangler is a professor of family medicine at Wake Forest University School of Medicine.