Staying fit may be more important than staying trim for a woman’s risk of heart disease - but the opposite appears true when it comes to diabetes, according to two studies released Tuesday.

In one, researchers found that among 906 middle-aged and older U.S. women, those who were more physically fit were less likely than out-of-shape women to have narrowed heart arteries or to suffer a heart attack or stroke over four years.

However, neither body mass index (BMI) nor abdominal obesity were in and of themselves predictors of artery disease or its complications.

In contrast, the second study, of close to 38,000 women, found that BMI was far more important than exercise levels in the risk of developing type 2 diabetes.

Both reports are published in this week’s issue of the Journal of the American Medical Association.

People have long been advised to exercise and keep their weight down in order to cut the risks of both cardiovascular disease and type 2 diabetes. And though the two new studies differ on the relative importance of fitness and “fatness,” experts say the overall implication is straightforward: Get regular exercise.

Exercise is the “common denominator” in both sets of findings because it boosts fitness and promotes weight loss, Steven N. Blair of the Cooper Institute in Dallas writes in an accompanying editorial.

In their own research, Blair and his colleagues have found that it is not only possible to be “fat but fit,” but that fitness can also counter the ill effects of excess weight. In one study, they found that obese adults who were at least moderately fit had about half the risk of dying from cardiovascular disease as their lean but out-of-shape peers.

According to Blair, the question of whether fitness is more or less important than body weight in a given disease is “largely academic.”

The bottom line, he concludes, is that adults should strive to accumulate at least 30 minutes of moderate exercise on most, if not all, days of the week - through activities like brisk walking, biking, swimming, gardening and housework.

In the new heart disease study, researchers led by Dr. Timothy R. Wessel of the University of Florida College of Medicine in Gainesville followed 906 women who had chest pain or other signs of possible heart disease at the study’s start.

All of the women underwent imaging tests to look for blockages in the arteries feeding the heart. Their BMI and fitness levels, as estimated from questionnaires on activity levels, were also recorded.

The researchers found that women who were deemed fit were less likely to show artery narrowing, and were at lower risk of having complications such as heart attack and stroke over the next four years. Out-of-shape women were 46 percent more likely than fit women to suffer a cardiovascular complication.

High BMI was linked to several diseases that pose a threat to the heart, including high blood pressure and diabetes, but was not in itself associated with coronary artery disease or its complications.

Wessel and his colleagues echo Blair’s advice on exercise, noting that its wide-ranging benefits, including weight loss, make it an “ideal therapy” for heart disease.

The second study included 37,878 middle-aged and older women whose BMI and activity levels were recorded at the outset. Over an average of seven years, 1,361 women developed type 2 diabetes.

While physical activity lowered the risk of diabetes somewhat, BMI was a much more important factor, according to the researchers, led by Dr. Amy R. Weinstein of Beth Israel Deaconess Medical Center in Boston.

They found that compared with normal-weight women, those who were overweight had more than three times the risk of diabetes, while obese women had a nine-times greater risk.

The researchers speculate that exercise may not fully counter the effects of excessive body fat when it comes to diabetes. For one, they note, it’s thought that fat tissue may release substances that affect the metabolism of insulin - the blood-sugar-regulating hormone that is rendered ineffective in type 2 diabetes.

Exercise remains an important part of preventing diabetes, the authors conclude. But they add that it should be coupled with weight loss to gain the greatest benefit.

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SOURCE: Journal of the American Medical Association, September 8, 2004.