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PUBLIC HEALTH Lifestyle Changes May Prevent Type 2 Diabetes The mass of evidence linking obesity, lack of exercise, and diet with type 2 diabetes is growing almost as fast as Americans' waistlines. The total effect of five lifestyle factors accounted for nearly 90 percent of the attributable risk of type 2 diabetes in a recent study. First author Frank Hu says obesity is the most important of these factors. Photo by Graham Ramsay Now, one of the largest, most comprehensive studies of lifestyle risk factors adds considerable weight to the conviction that most cases of type 2 diabetes could be prevented, particularly by losing weight. "In our study, if every woman ate a healthy diet, exercised a half hour a day, was not overweight, did not smoke, and drank a small amount of alcohol, over 90 percent of diabetes cases would have been eliminated," said Frank Hu, HSPH assistant professor of nutrition and first author of a prospective study of 3,300 new cases of diabetes among a cohort of nearly 85,000 women followed for 16 years. The study is published in the Sept. 13 New England Journal of Medicine. Walking the Walk "This is, of course, a hypothetical concept," Hu hastened to add, "because not everyone is going to follow such a lifestyle. But it indicates the potential of diet and lifestyle in preventing diabetes." Most of the study results are not news to the majority of people. The well-known lifestyle behaviors to ward off diabetes and other chronic diseases are not easy to maintain, however, even for the 85,000 middle-aged, health-conscious nurses in this study. Despite jobs that presumably expose them to the debilitating consequences of type 2 diabetes in their patients—blindness, kidney disease, heart disease and stroke, and nerve disease and amputations—less than 10 percent of the nurses followed longstanding health recommendations of moderate body weight, regular exercise, and a reasonably healthy diet. Only about 3 percent of the women were living healthy lives as measured by all five factors associated with low risk of diabetes in the study—good diet, exercise, moderate body weight, no smoking, and some alcohol. "This is one of the few areas where there are no mixed messages," said co-author JoAnn Manson, HMS professor of medicine and chief of preventive medicine at Brigham and Women's Hospital. "The evidence is compelling and overwhelming that type 2 diabetes is largely preventable through lifestyle modifications." Last month, the National Institutes of Health announced the premature end of a randomized clinical trial when early results showed that modest changes in diet and exercise sharply reduced the changes of type 2 diabetes within three years. Chaired by David Nathan, HMS professor of medicine at Massachusetts General Hospital, the 3,234-person Diabetes Prevention Program study was the first major trial to show that a low-fat diet and an average of about 20 minutes of exercise a day effectively delayed diabetes in a diverse American population of overweight people with impaired glucose tolerance. The results have not been published. In the new analysis from the Nurses' Health Study, weight control appeared to offer the greatest benefit. Hu and his colleagues estimate that 61 percent of the cases of type 2 diabetes could be attributed to overweight or obesity. That is typically defined by a body mass index of 25 or higher (about 140 pounds at a height of 5 feet, 3 inches or about 165 pounds at 5 feet, 8). Interestingly, a body mass index of 23, at the high end of normal (about 130 pounds at 5 feet, 3, or about 150 pounds at 5 feet, 8), had an increased risk of diabetes, compared with a body mass index of less than 23. "It implies that if obesity can be controlled, then the epidemic of type 2 diabetes can also be controlled," Hu said. "The two conditions go hand in hand." The Right Combination In the meantime, even if women in the study could not lose weight, obese women lowered their risk of type 2 diabetes by 24 percent with a combination of healthy diet and regular exercise. And, as the authors point out, diet and exercise are the primary factors in determining body weight. Among normal and overweight women, about half of the cases of diabetes could have been prevented by a combination of a healthy diet, more exercise, abstinence from smoking, and moderate alcohol consumption. Two dietary factors seemed to protect against type 2 diabetes—cereal fiber and a high ratio of polyunsaturated to saturated fat. Two other dietary factors were risky—trans fat and dietary glycemic load, which reflects the effect of diet on the blood glucose levels. Trans fat is formed during the partial hydrogenation of vegetable oils and found in margarine, shortening, French fries, cookies, cakes, and commercially baked products. Earlier this summer in a preliminary report, the same research team published the news that the type of dietary fat made a difference in women's risk of type 2 diabetes. It turns out that trans fatty acids increase the risk and polyunsaturated fatty acids reduce the risk for type 2 diabetes. Total fat was not a major player in the diabetes risk. The paper was published in the American Journal of Clinical Nutrition. "There should be more emphasis on the type of fat and the type of carbohydrates, which are not adequately reflected in the American Diabetes Association guidelines," Hu said. "The evidence is clear that there needs to be more emphasis on the quality of fat and carbohydrates than the quantity." In the latest study, moderate exercise was good, and more exercise was better. An average of at least a half hour of moderate or vigorous exercise daily—usually brisk enough to sweat—put women in the low-risk category. Just three out of the five factors—weight, diet, and exercise—conferred the majority of protection from type 2 diabetes. Not smoking was a bonus in reducing risk, and about half a drink a day seemed to cut risk further, although the researchers hesitate to advise women to drink. The reduced risk for the combined lifestyle factors was similar among women with and without a family history of diabetes and among white and nonwhite women (who made up about 3 percent of the cohort). Despite the dramatic findings regarding the combination of lifestyle factors, the researchers believe different genetic backgrounds may make certain people more susceptible to lifestyle risks. They are now analyzing a complex array of genetic factors and biochemical markers in the same population. Other authors of the study include Meir Stampfer, HSPH professor of epidemiology and nutrition and chair of Epidemiology; Graham Colditz, HSPH professor of epidemiology and HMS professor of medicine at BWH; Simin Liu, HMS assistant professor of medicine at BWH; Walter Willett, the Fredrick John Stare professor of epidemiology and nutrition at HSPH and chair of the School's Department of Nutrition; and Caren Solomon, HMS assistant professor of medicine at BWH. The study was funded by the National Institutes of Health and the American Diabetes Association. —Carol Cruzan Morton