In a comprehensive review of recent randomized clinical trials and observational studies of diabetes and nutrition, Joslin Diabetes Center and Harvard School of Public Health investigators have identified specific foods and dietary patterns that are beneficial in preventing and controlling diabetes. The findings were published June 7, 2014 in Lancet.
"We undertook this review because we believe that most of the current dietary guidelines for patients with diabetes do not reflect recent evidence. Nutrition can be used as a medicine to prevent and control diabetes in a very effective way," says Osama Hamdy, M.D., Ph.D., Medical Director of Joslin's Obesity Clinical Program and Assistant Professor of Medicine at Harvard Medical School.
"With the global epidemic of type 2 diabetes, it's critical to invest in effective diabetes prevention and management," says Frank B. Hu, M.D., Ph.D., M.P.H., senior author of the study and Professor, Departments of Nutrition and Epidemiology, Harvard School of Public Health and Director, Boston Obesity Nutrition Research Center, Epidemiology and Genetics Core. "An unhealthy diet has long been considered a major contributor to the development of diabetes but only in the past two decades has this role been confirmed by prospective observational studies and clinical trials."
One major finding was that certain foods and dietary patterns can help prevent type 2 diabetes even without weight loss. "People who eat a Mediterranean diet, with foods such as olive oil, whole grains and leafy vegetables and fruits, have a lower risk of developing diabetes even when they don't lose weight," Dr. Hamdy says.
Foods such as oat cereal, yogurt and dairy products, green leafy vegetables, grapes, apples, blueberries and walnuts were associated with reduced diabetes risk. Drinking coffee and even decaffeinated coffee were also associated with lower type 2 diabetes risk.
Participants who followed a Mediterranean eating plan — without restricting calories — showed a greater improvement in glycemic control and insulin sensitivity than participants who ate other popular diets. In addition, overweight patients with newly diagnosed type 2 diabetes who followed the Mediterranean diet had less need for antihyperglycemic medications compared with participants on a low-fat diet.
Overall, a variety of eating plans, including the Mediterranean, low-carbohydrate/low glycemic index and high-protein diets, improved glycemic control and cardiovascular disease risk factors in patients with diabetes compared with control diets. This offers patients a range of options for diabetes management.
Foods associated with a higher risk of diabetes include red and processed meat, sugar-sweetened beverages, alcohol in excess quantities and refined grains, such as white flour.
Recent studies of fat intake and diabetes incidence support the notion that eating the right kind of fats is beneficial to health. This goes against years of advocacy of a low-fat/high-carbohydrate diet. "When people started eating less fat, they compensated by eating more refined carbohydrates, which stimulate insulin secretion and increase fat deposition. A major problem with the American diet is too much refined grains and added sugar, which are associated with the rise in obesity and type 2 diabetes," Dr. Hamdy says.
Current evidence shows that some fats, such as those from red and processed meats, are associated with higher cardiovascular risk while other fats such as those from vegetable oils and nuts are associated with lower risk. The Joslin nutritional guidelines for diabetes recommend a diet with relatively high amounts of healthy fats and protein but moderately low amounts of carbohydrates.
"We know now that the quality of fat and carbohydrates is more important than the quantity of fat and carbohydrates," Dr. Hamdy says.
Since 2005, participants in the Joslin Why WAIT (Weight Achievement and Intensive Treatment) program who have followed these guidelines and exercised regularly have lost weight and maintained it for five years and significantly reduced their diabetes medications by more than 50 percent. The program's nutritional guidelines have been adopted by many institutes and organizations across the globe. "The Why WAIT program works very well in practice," Dr. Hamdy says. "People enjoy what they eat while improving the quality of their diet."
"Now that we have clear evidence, we are striving to educate the American public about what foods to eat to prevent diabetes and improve overall health," he says.
Since almost all knowledge of dietary prevention and management of diabetes comes from research in developed countries, more studies are needed in less developed regions to help patients with diabetes worldwide improve their health. "We need to develop nutritional guidelines that take into account dietary patterns, accessibility to foods and types of agriculture in different regions of the world," Dr. Hu says.