Colorectal cancer is the second leading cause of cancer-related death in the US and the third most common form of cancer in men and women worldwide. Because colon and rectal cancers have similar features, they are often referred to together as colorectal cancer. Ninety-five percent of these cancers begin as benign polyps , which are growths of tissue on the cells that line the inside of the colon and rectum.

While there are screening tests that can detect and remove polyps, and well-documented lifestyle factors that reduce risk (regular physical activity; a diet high in fruits, vegetables, and whole-grains; and limited consumption of high-fat foods, especially red meat), researchers are looking at other factors that might reduce the incidence or recurrence of colorectal cancer. Calcium is one such factor.

Over the past two decades, studies have examined whether calcium intake has a protective effect against colorectal cancer. Although animal studies have indicated this may be the case, epidemiological studies in humans have been inconclusive. Researchers recently analyzed a group of studies that looked at the effect of calcium intake through diet and supplements on rates of colorectal cancer. Their results, which were published in the July 7, 2004 issue of the Journal of the National Cancer Institute , concluded that increased intake of milk and calcium is related to a lower risk of colorectal cancer.

About the Study

Researchers pooled the primary data of ten studies from North America and Europe. The combined studies comprised 534,536 participants. Each study reported a minimum of 50 people diagnosed with colorectal cancer, and assessed long-term dietary intake with a well-accepted food frequency questionnaire. Most studies had 6–16 years of follow-up data, and combined they documented 4,992 cases of colorectal cancer.

Researchers examined the following data from these studies: dietary calcium (calcium from food) and total calcium (from diet and supplements); intake of three groups of dairy foods (milk, cheese, and yogurt); vitamin D intake; and the site of the cancer in the colon or rectum. Men and women were evaluated separately.

The Findings

This analysis found that intake of dietary and total calcium seemed to reduce the risk for colorectal cancer, though consuming at least 1,000 milligrams per day was necessary to produce this beneficial effect.

Among the three groups of dairy products, milk had a strongest protective effect an observation that was highly consistent across the studies. Cheese, yogurt, and other dairy products had a weak, non-significant benefit. Researchers also found that the beneficial protective effects was limited to the distal (sigmoid) colon and the rectum.

Researchers acknowledged that they could not distinguish definitively between milk and calcium intake because the two are so strongly correlated in most of the studies. Milk appears to reduce the risk of colorectal cancer, but this may be influenced by calcium or other components in milk. Researchers also examined vitamin D intake in studies that provided data. Vitamin D, which is present in milk and enhances calcium absorption, may independently reduce the risk of colorectal cancer. In fact, the protective effect of total calcium intake seemed to depend on the presence of high levels of vitamin D.

How Does This Affect You?

These findings are supportive of other research showing that calcium intake may help prevent the recurrence of polyps. While not a controlled, clinical trial, this study does suggest that calcium and milk consumption is associated with a lower risk of colorectal cancer. Screening tests that find and remove pre-cancerous colon polyps can effectively reduce the risk and mortality of colorectal cancer, but it may be possible go one step further and reduce your risk of polyps in the first place by consuming calcium.

People with history of colorectal cancer, or who have risk factors for developing it, should consider increasing their consumption of calcium and milk. Discuss your plans to increase calcium intake with your physician, who make recommendations about the optimal amount and source of calcium. It’s important to note that excessive calcium supplementation can have adverse health effects, and that some studies indicate diets high in fiber and low in fat are required to optimize calcium’s preventive effects.

Increasing intake of calcium is simple and can be done by consuming dairy products (especially milk), green leafy vegetables, and certain fish, or by taking calcium supplements. In addition to reducing risk of colorectal cancer, increased calcium consumption will have a beneficial effect on bone health, especially in women.