WASHINGTON (AP) - Most cases of the most common form of diabetes can be prevented by moderate exercise and a healthy diet, studies show.

One study indicates that such lifestyle changes could head off over 90 percent of type 2 diabetes. Another says that the changes could reduce the risk of getting the disease by 58 percent even among people who already show signs of developing it.

"There is no question that diabetes is a lifestyle disease, and can be prevented by lifestyle modifications," said Dr. Frank B. Hu of the Harvard School of Public Health, lead author of a major study in the Sept. 13 issue of the New England Journal of Medicine.

Hu and his colleagues looked at data on 84,941 women in the Nurses' Health Study, a database that Harvard researchers began to track in 1976. None of the women had diabetes at the start of the study; 3,300 had it after 16 years.

Type 2 diabetes, the class that accounts for 90 percent or more of all diabetes cases, occurs when the body can't properly utilize insulin in getting nutrients to cells. The number of diabetes cases has been growing sharply, and federal data say about 16 million Americans have some form of the disease.

The Harvard researchers concluded 91 percent of cases among the people they studied could have been prevented by avoiding overweight, being active, eating right, not drinking and not smoking.

Being overweight was the single strongest predictor of the metabolic disorder - 61 percent of cases could be attributed to weight problems.

But other factors had powerful effects, even among the overweight. Overweight women cut their risk 24 percent simply by at least walking regularly and favoring foods that were high in fiber but low in partially hydrogenated oils. Such foods include french fries and commercial baked goods.

Benefits of physical activity went up with the amount of exercise. Those who did seven or more hours a week had almost a 30 percent lower risk than did women who put in less than one half hour a week.

Benefits of healthy eating followed a similar pattern. Researchers created a diet scorecard to account for variations in eating, and found those who scored best had about half the diabetes risk of those who ate worst.

A separate study, the Diabetes Prevention Program, considered the effect of eating and exercise patterns on 3,234 people who already had a strong indication they were progressing toward diabetes. These people had high levels of sugar in the blood.

A moderate intensity program of walking 30 minutes a day, coupled with a low-fat diet that shaved off 5 percent to 7 percent of their body weight, reduced their risk of progressing to diabetes by 58 percent.

Participants had been followed for 3 years when the study was stopped a year ahead of schedule because outside experts who were monitoring the program felt the results were conclusive. Results of the National Institutes of Health study were announced Aug. 8.

Lifestyle changes caused the reduced risk, said the Diabetes Prevention Program's chair, Dr. David Nathan of Massachusetts General Hospital in Boston.

The changes are within the capacity of the average American, Nathan said. "We were able to take a high-risk group, and they were able to accomplish what we asked of them," he said. "It worked wonderfully."

Even people who don't have high blood sugar should be able to summon the motivation, Nathan said. The people in his study could not have known that exercise and diet would work so well, but people now should know, he said. The next step is to find ways to encourage a healthier lifestyle among those who most need it - those who are genetically at risk of developing diabetes, said Dr. Francine Kaufman, head of endocrinology at Childrens Hospital Los Angeles. "This is a disease in which the underlying genetic background is critical, and if you have these genes, the impact of lifestyle turns these genes on or keeps them suppressed," Kaufman said.

Type 2 diabetes, once a disease of the old, has been working its way to young and younger patients, and now is showing an alarming increase among young people, Kaufman said.

Workplaces and communities should make it easier to exercise, by creating safe areas in which to exercise and encouraging such things as "walk at lunch programs," Kaufman said. And the food industry should do better at spelling out the nutritional risks in products, as well as cutting back on the portion sizes, she said.

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