According to National Health and Nutrition Examination Survey data, more than one-third of adults in the United States are obese and more than two-thirds of adults with type 2 diabetes mellitus (DM) are obese.1 In light of overall increased life expectancy, the Centers for Disease Control and Prevention estimates that adults in the United States have a 40% lifetime risk of developing diabetes, as diabetes and obesity remain at epidemic levels.2
Weight loss in individuals who are overweight or obese is effective in preventing type 2 DM and improving management of the disease.3,4 Dietary changes play a central role in achieving weight loss, as do other important lifestyle interventions such as exercise, behavior modification, and pharmacotherapy. Achieving glycemic goals with diet alone is difficult, and for patients with DM who are also obese, it may be even more challenging.
Medical nutrition therapy, a term coined by the American Dietetic Association, describes an approach to treating medical conditions using specific diets. As developed and monitored by a physician and registered dietitian, diet can result in beneficial outcomes and is a front-line approach for patients with noninsulin-dependent diabetes.5 Medical nutrition therapy for patients with type 2 DM is most effective when used within 1 year of diagnosis and is associated with a 0.5% to 2% decrease in hemoglobin A1c (HbA1c) levels.6 This article reviews the role of diet in managing patients with both type 2 DM and obesity. Several diets are presented including what is known about their effect on weight loss, glycemic control, and cardiovascular risk prevention in patients with diabetes and obesity.
WEIGHT LOSS AND DIET FOR PATIENTS WITH OBESITY AND DIABETES
A person is overweight or obese if he or she weighs more than the ideal weight for their height as calculated by the body mass index (BMI; weight in kg/height in meters squared). A BMI of 25 to 30 is overweight and a BMI of 30 or greater is obese.7 The recommended daily caloric intake for adults is based on sex, age, and daily activity level and ranges from 1,600 to 2,000 calories per day for women and 2,000 to 2,600 calories per day for men. The lower end of the range is for sedentary adults, and the higher end is for active adults (walking 1.5 to 3 miles per day at 3 to 4 miles per hour, in addition to independent living).8
According to the American Diabetes Association (ADA), weight loss requires reducing dietary intake by 500 to 750 calories per day, or roughly 1,200 to 1,500 kcal/day for women and 1,500 to 1,800 kcal/day for men.3 For patients with obesity and type 2 DM, sustained, modest weight loss of 5% of initial body weight improves glycemic control and reduces the need for diabetes medications.9 Weight loss of greater than 5% body weight also improves lipid and blood pressure status in patients with obesity and diabetes, though ideally, patients are encouraged to achieve weight reduction of 7% or greater.10
Evidence of benefits from lifestyle and dietary modifications
The fact that patients with obesity and type 2 DM have increased risk of cardiovascular morbidity and mortality is well established.11 Multiple studies considered the effects of weight loss on cardiovascular morbidity and mortality. Our article focuses on dietary modifications, though most large, multicenter trials used both diet and increased physical activity to achieve weight loss. It is difficult to determine if diet or physical activity had the most effect on outcomes; however, results show that weight loss from dietary and other lifestyle interventions leads to change in outcomes.
Look AHEAD (Action for Health in Diabetes) trial. This large, multicenter, randomized controlled trial evaluated the effect of weight loss on cardiovascular morbidity and mortality in overweight or obese adults with type 2 DM. The 5,145 participants were assigned either to a long-term weight reduction intensive lifestyle intervention of diet, physical activity, and behavior modification or to usual care of support and education. At 1 year, the lifestyle intervention group had greater weight loss, improved fitness, decreased number of diabetes medications, decreased blood pressure, and improved biomarkers of glucose and lipid control compared with the usual care group.12 No significant reductions in cardiovascular morbidity and mortality were found, though an observational post hoc analysis of the Look AHEAD data suggested an association between the magnitude of weight loss and the incidence of cardiovascular disease.13
The diet portion of the intensive lifestyle intervention consisted of self-selected, conventional foods while recording dietary intake during week 1. In week 2, patients weighing less than 114 kg (250 lbs) restricted their intake to 1,200 to 1,500 kcal/day, and patients weighing 114 kg or more restricted their intake to 1,500 to 1,800 kcal/day. Fewer than 30% of calories were from fat, with less than 10% from saturated fat. During week 3 through week 9, meal replacement options and conventional foods were used to reach caloric goals. Participants then decreased the use of meal replacement and increased the use of conventional foods during week 20 through week 22.14
The mean weight loss for participants in the intensive lifestyle intervention group was 8.6% compared with 0.7% in the support and education group (P < .001). HbA1c decreased by 0.7% in the intervention group compared with 0.1% the support and education group (P < .001).12
Finnish Diabetes Prevention Study. This study evaluated lifestyle changes in diet and physical activity in the prevention of type 2 DM in participants with impaired glucose intolerance. Participants (N = 552) were randomly assigned to the control group or the intervention group where detailed instruction was provided to achieve weight loss of greater than 5%.15 The dietary goals included fewer than 30% of total calories from fat, with fewer than 10% from saturated fat, increased fiber consumption (15 g per 1,000 kcal), and physical activity of 30 minutes daily.15 During the trial (mean duration of follow-up 3.2 years), the risk of type 2 DM was reduced by 58% in the intervention group compared with the control group.15
Diabetes Prevention Program Research Group. A landmark study by the Diabetes Prevention Program Research Group randomized 3,234 participates with elevated plasma glucose levels to placebo, metformin, and lifestyle intervention arms.4 Those in the lifestyle intervention arm were educated about ways to achieve and maintain a 7% or greater reduction in body weight using a low-calorie, low-fat diet and moderate physical activity. Results based on a mean follow-up of 2.8 years found a 58% reduction in the incidence of diabetes for those in the lifestyle intervention arm.4