A new study by the American Cancer Society shows calcium may lower the risk of colon cancer slightly for both men and women, while vitamin D might help cut risk in men.

The effect seemed to be strongest in people who took supplements, rather than getting these nutrients from foods. However, researchers said it is still too early to change recommendations about how much calcium or vitamin D to take in order to reduce colon cancer risk.

“Overall recommendations are tricky,” said lead researcher Marji McCullough, senior epidemiologist at the American Cancer Society, “because very high calcium and/or dairy intake has been found in some studies to increase prostate and possibly ovarian cancer risk.”

Still, the research could be helpful in the battle against colon cancer, the second leading cause of cancer deaths in the United States. The ACS estimates that nearly 150,000 Americans will get colorectal cancer this year, and more than 57,000 will die from it.

The study was published in the journal Cancer Causes and Control (Vol. 14, No. 1: 1-12).

McCullough and her colleagues studied more than 60,000 men and more than 66,000 women who were already participating in an ACS study of nutrition and cancer prevention. The participants were all between 50 and 74-years old when they enrolled in the study in 1992 and 1993. They were asked about their diet and supplement intake at the time they enrolled. The researchers used these questionnaires to estimate different nutrient intakes, including calcium and vitamin D.

By 1997, 421 men and 262 women in the group had developed colorectal cancer.

The researchers found that people who got calcium primarily from food did not have a lower risk of colon cancer. However, people who got calcium primarily from supplements did.

People who took calcium supplements had about a 30% lower risk of developing colon cancer than people who did not take supplements.

McCullough said the researchers weren’t entirely surprised by these results.

“Dairy products are a mix of several components, some with potentially beneficial effects, like calcium and vitamin D, and some less healthy, like saturated fat. One can speculate that the net effect may not be as strong as calcium alone,” she said.

In addition, men who got the most vitamin D also had a lower cancer risk. The protective effects were not seen in women, however. The researchers couldn’t fully explain this result, but said it might be due to the lower number of cancers in the group of women studied, or possibly to some aspect of sex hormone metabolism that is not yet understood.

The results of this study are consistent with previous research, which showed that taking calcium could help prevent the recurrence of colon polyps, a precursor to colon cancer.

But the amount of calcium that gave a protective effect in this study was less than the recommended daily allowance of 1200 milligrams a day for adults over age 50.

Risk started to decrease with as little as 700 milligrams of total calcium –-from food and supplements -- a day, and taking more than 1200 milligrams a day did not seem to give any greater protection. But the greatest reduction in risk was seen in people whose calcium came primarily from supplements rather than from diet.

This result “indicates that more may not always be better, or necessary, to reduce colon cancer risk,” McCullough said. It could also be important given the possible link between higher levels of calcium and prostate and ovarian cancers.

The researchers recommended further study to determine the optimal dose of calcium for colon cancer protection. Until then, people should consult their doctors about whether they need to take a calcium or vitamin D supplement.