MONDAY, Feb. 23, 2009 (HealthDay News) -- High dietary intake of calcium may reduce the incidence of colorectal cancer, especially for women, but has no apparent effect in reducing other malignancies, a U.S. National Cancer Institute study finds.
Why calcium should influence cancer risk differently in women versus men isn't clear, said Yikyung Park, a staff scientist at NCI who led the study. "One can speculate that hormonal or metabolic factors contribute to this difference," she said.
Park and her colleagues relied on data for nearly 500,000 men and women who participated in the U.S. National Institutes of Health-AARP Diet and Health Study. Participants filled out a food questionnaire when they enrolled and then were followed for an average of seven years.
"In both men and women, dairy food and calcium intakes were inversely associated with cancers of the digestive system," the researchers reported in the Feb. 23 issue of the Archives of Internal Medicine.
The top one-fifth of women with the highest intake averaged 1,881 milligrams of calcium per day. This group experienced a 23 percent lower risk of colon cancer than those women in the lowest fifth of intake, who averaged 494 milligrams daily. The comparable reduction for men was 16 percent.
The U.S. Institute of Medicine recommends a daily calcium intake of 1,200 milligrams for adults 50 and older, roughly the amount found in three cups a day of the dairy products that are the main sources of calcium. Other sources of calcium include sardines and green, leafy vegetables.
Calcium has been shown to reduce abnormal growths and induce normal turnover of cells in the gastrointestinal system, the report noted.
The study was done because "calcium has been hypothesized to play different roles in different cancer sites, but testing has been incomplete, inconsistent and limited," Park said.
One expert said the study is an important one. "This is the first paper looking at calcium, dairy products and all cancers combined," said Marji McCullough, strategic director of nutritional epidemiology at the American Cancer Society. The findings, she said, "were consistent with the previous literature."
For example, a controlled trial reported last year found no protective effect of calcium intake against breast cancer. The new report confirms that finding, and also finds that the nutrient offers no protective effect against prostate cancer.
The NCI study results "are consistent with guidelines for a healthy diet," McCullough said. "But it is important for people to understand that they shouldn't go overboard on calcium."
No additional protective effect was found for calcium intakes greater than 1,300 milligrams a day, according to the NCI study.
Current calcium recommendations are best met by dietary sources rather than supplements, McCullough added, in part because diet offers more than just calcium. "Calcium and vitamin D and are highly correlated in the diet, and it is difficult to isolate a single component," she said. "It may be that a combination of nutrients is important."
The combination of calcium of vitamin D is important, since vitamin D facilitates calcium's absorption by the digestive system. The skin makes vitamin D naturally through exposure to sunlight.
Another report in the same issue of the journal finds that that a combination of vitamins B6, B12 and folic acid appears to reduce the risk among women of age-related macular degeneration, a major cause of vision loss for older Americans.
A controlled trial including more than 5,400 women 40 and older found a 34 percent lower incidence of the eye disorder in women taking the vitamins compared to those taking an inactive placebo, said the report by researchers at Harvard Medical School and Brigham and Women's Hospital, Boston.
There's more on dietary sources of calcium at the U.S. Library of Medicine.