Almost 40 percent of adults in the U.S. get no leisure time physical activity, according to the Centers for Disease Control and Prevention (CDC). While more than 60 percent of those recently surveyed get some physical activity during their leisure hours (defined as ever getting ten minutes or more), only a little over 30 percent of the population actually meet current recommendations that define "regular" exercise. Studies presented at this year's annual American Institute for Cancer Research (AICR) conference on nutrition and cancer suggest this is bad news for cancer risk.

The CDC report is based on interviews with more than 65,000 adults. To qualify as "exercising regularly," someone must either engage in light-to-moderate activity (causing light sweating or a slight to moderate increase in breathing or heart rate) for 30 minutes or more at least five days a week, or engage in vigorous activity (causing heavy sweating or large increases in breathing or heart rate) for 20 minutes or more at least three days a week.

According to recent research, strength training, like weight training or calisthenics, prevents the loss of muscle that otherwise occurs as we age. It is also thought to play a significant role in weight control since muscle burns more calories than other body tissue does. Strength training also seems to offer quality-of-life benefits, including the ability of older adults to get around independently. Almost 23 percent of the adults in the CDC study reported at least some strength-training exercise, although the numbers declined significantly with age.

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The 32 percent of adults in the CDC report categorized as regular exercisers represent a slight increase in recent years.

Unfortunately, that still leaves almost 70 percent of American adults at increased risk of cancer, heart disease, high blood pressure and diabetes, according to a variety of research reports in the last few years.

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At the recent AICR research conference, Dr. Christine Friedenreich discussed the relationship of physical activity and cancer risk. She noted that regular exercise appears to reduce overall cancer risk 30 to 40 percent. Exercise offers cancer protection in a variety of ways, including its effects on the hormones and growth factors that seem to be involved in cancer development. Studies most strongly support a reduction in colon cancer risk (by 40 to 50 percent) and breast cancer (by 10 to 30 percent), but prostate risk is also probably reduced by regular physical activity, and other cancers may also be affected.

Friedenreich recommends at least 30 minutes of moderate-to-vigorous activity at least five days a week. The latest CDC survey shows that many aren't even getting that. Both AICR and the International Agency for Research on Cancer (part of the World Health Organization) suggest working toward an hour a day of moderate activity (which includes brisk walking) to lower cancer risk and maintain a healthy weight.