A little more than a glass of milk a day can reduce the risk of cancer of the colon and rectum, according to the most comprehensive study ever done on the subject. It is actually a study of studies, with researchers at the Harvard School of Public Health and Brigham and Women's Hospital in Boston lumping together data from 10 studies of more than 534,000 people in the United States and Europe. "We were able to look at a wider range of calcium and milk intake than in other studies, which gave us greater statistical power," explained Stephanie Smith-Warner, an assistant professor of nutritional epidemiology at the Harvard School of Public Health and a member of the team reporting the results in the July 7 issue of the Journal of the National Cancer Institute. While several studies have shown that calcium intake reduces the recurrence of intestinal polyps, growths that can become cancerous, individual study results about the effect on cancer have been "inconclusive," she said. The analysis showed that "people who drank more than one eight-ounce glass of milk a day had a 15 percent reduction in risk compared to those who drank less than two glasses of milk a week," she said. It is the calcium in milk that is primarily responsible for this "modest" reduction in risk, Smith-Warner said. Calcium prevents the proliferation of cells that line the intestinal tract, stopping potential overgrowth that can make the cells become cancerous. Even though the analysis found that people with the highest calcium intake, from food and supplements, had a 22 percent lower risk of colorectal cancer, "the question of calcium supplements is still not resolved," Smith-Warner said. One reason is that the analysis showed a threshold effect, with the beneficial effect leveling off at a daily intake of 900 milligrams of calcium, roughly the amount in three glasses of milk, she said. Another is that "some studies suggest that high calcium intake may increase the risk of prostate cancer," Smith-Warner said. "We still need to understand the full range of effects of calcium intake." Colorectal cancer is the fourth most common malignancy in the United States. The American Cancer Society estimates there will be 106,370 new cases of colon cancer and 40,570 new cases of rectal cancer this year, with a combined total of 56,370 deaths. "The results of this study have brought us closer to understanding how to reduce the risk of this disease," said study co-author Eunyoung Cho, a researcher at Brigham and Women's Hospital and instructor in medicine at Harvard Medical School . Calcium is not the only risk factor to be considered, Cho added. Doctors should tell patients that other protective measures are "a healthy lifestyle including exercise, maintenance of body weight and a diet low in red meat and high in fruits, vegetables and whole grain foods." The study is unlikely to dissuade those who insist that meat and dairy products can lead to cancer. Vegans and groups like the People for the Ethical Treatment of Animals have contended that these foods can contribute to colon cancer because they contain little to no fiber, which has been shown to be protective against the disease.