For many people, the question isn't whether or not exercise is good for health, but how much physical activity it really takes to get health and weight control benefits. The best answer once was that anything is better than nothing. But experts now advocate consciously carving out time for an hour of moderate activity every day.

A recent study in the New England Journal of Medicine reported on preventing heart disease in women past menopause with no known heart disease. The most active women had more than a 50 percent reduction in the risk of having a heart attack or stroke, compared to those who were the least active. Yet even a small increase in activity above that of the least active group reduced the risk of such problems by more than 25 percent. A few years ago, the same journal reported virtually identical statistics from the very large Nurses' Health Study. Even women who did not become physically active until middle age, or older, had a lower risk of heart disease than those who remained sedentary. In general, these studies find heart-related benefits start accruing with even one to three hours of walking a week, and adding more exercise adds even more protection.

Diabetes is another health problem that exercise helps ward off. Studies in Medicine and Science in Sports and Exercise and the New England Journal of Medicine report 58 to 68 percent less development of diabetes when people engage in moderate physical activity for at least 30 minutes a day, five to seven days a week. Some of this risk reduction is related to exercise's impact on weight control, but studies show that even after adjusting for such effects, exercise itself still lowers risk.

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Many people don't realize that keeping physically active is now considered an essential part of a lifestyle that lowers the risk of cancer. As reported in a Journal of Nutrition article on the American Institute for Cancer Research (AICR) international research conference last year, convincing evidence showed that regular exercise could reduce the risk of colon cancer by 40 to 50 percent, and breast cancer 30 to 40 percent. Reduced risk of other cancers may also occur.

To reduce cancer risk, any amount of exercise is clearly better than none. But AICR recommends a total of an hour of moderate activity (like brisk walking) each day, and an hour of vigorous activity each week. Breaking up your routine into brief sessions can help reach this goal.

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AICR's goals were originally higher than the general government health guidelines that called for 30 minutes or more at least five days a week. But the latest recommendation, from the National Academy of Sciences (NAS), also calls for accumulating an hour of moderate exercise a day. (For those who prefer vigorous exercise, like jogging, the report notes that 20 to 30 minutes, four or more days a week, is sufficient.)

According to a new NAS report, a major reason for the increase to an hour of moderate exercise a day is that the previously recommended 30 minutes a day is not enough to maintain a healthy weight for most adults. Hormonal and metabolic changes due to excess weight are being linked to many health problems today.

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Even small increases in exercise can produce a loss of several pounds over the course of a year. But, for those Americans who are overweight, exercise alone will not produce results without reducing calorie consumption. And studies show that reducing calories without exercise can cause loss of not just fat, but also muscle, which is not conducive to long-term weight maintenance or health. Studies also show that, although regular exercise alone may not be enough to help us lose weight, without it, adults gradually lose muscle and gain body fat each year.