More than two dozen studies have shown that women who exercise have a 30 percent to 40 percent lower risk of breast cancer than their sedentary peers. The female hormone estrogen seems to play a key role. Women with high estrogen levels in their blood have increased risk for breast cancer. Since exercise lowers blood estrogen, it helps lower a woman’s breast-cancer risk. Exercise also reduces other cancer-growth factors such as insulin.

Even older women need to be concerned about estrogen, because after menopause the hormone is produced by fat cells. Women who exercise have less fat and therefore produce less estrogen. With more than 150,000 new breast-cancer cases reported in the United States each year, preventing cancer through exercise is one of the best ways a woman can take charge of her health.

Exercise plays a dramatic role in preventing cancer of the colon and rectum. Nearly 150,000 Americans are diagnosed with colorectal cancer each year, and nearly 50,000 die from the disease. Encouragingly, more than three dozen studies show exercisers reduce their risk of colon cancer by 20 percent or more compared to sedentary people, and the benefits are seen in both men and women, although the effect is greater in men. Changes in digestive acids and other substances also occur with exercise, and these changes are believed to provide some protection from colon cancer. Decreases in body fat, insulin and other growth factors also may contribute to exercisers’ lower colon-cancer risk. Current research is also uncovering new ways in which physical activity cuts cancer risk—from reducing chronic inflammation to improving DNA repair.

How much exercise is too much?

According to national activity guidelines, a good goal is to exercise at least 30 minutes a day on most days of the week. To get the most benefit, though, aim for about an hour a day. Moderate-intensity activities such as brisk walking may be sufficient, although there is more benefit with increased intensity.