The prevalence of obesity and its associated morbidity risks have increased in the United States in recent years. In addition, more attention is being paid to syndrome X, or insulin-resistance syndrome, which is defined as a combination of obesity and impaired glucose tolerance. Approximately one fourth of adults in the United States have this syndrome and are at a substantially increased risk for disability, morbidity, and mortality.
Because of these issues, an increased emphasis is being placed on weight reduction strategies. Diets low in fat and high in complex carbohydrates have been recommended as a way to prevent obesity and promote weight loss. Complex carbohydrates are metabolized less efficiently than fats, and high-fat diets may promote greater energy intake because they are more palatable and have higher energy density. Most successful diets with high levels of complex carbohydrates also have limited caloric intake. No studies have assessed high-carbohydrate diets with no limitations on the amount of energy consumed. Hays and colleagues assessed the impact of an ad libitum diet low in fat and high in complex carbohydrates on body weight, body composition, and appendicular fat distribution in older persons with glucose intolerance. In addition, they evaluated the impact of exercise on patients consuming a diet high in complex carbohydrates.
Adult patients who were overweight, non-smokers, and sedentary, and who had been at a stable weight for the past six months were eligible for the study. In addition, participants had to have glucose intolerance as defined by a plasma glucose concentration of 140 to 200 mg per dL (7.8 to 11.1 mmol per L) two hours after a 75-g oral glucose load. Participants were not allowed to take any medications that might affect their glucose metabolism.
Before the study began, all participants received an extensive medical history, physical examination, laboratory evaluation, and exercise tolerance assessment. For one week, all participants were placed on the same diet. After that week, they were randomized into three groups: (1) control, (2) high complex-carbohydrate diet alone, and (3) high complex-carbohydrate diet plus endurance exercise.
The control group was placed on a diet of 41 percent fat, 14 percent protein, 45 percent carbohydrates, and 7 g of fiber. Patients in the complex-carbohydrate groups were given diets consisting of 18 percent fat, 19 percent protein, 63 percent carbohydrates, and 26 g of fiber. The active group performed endurance exercises four days per week for 45 minutes at 80 percent peak oxygen consumption. All participants were provided 150 percent of their estimated energy needs and instructed to consume as much as they wished until they were no longer hungry. Outcome measures included total food consumed, body composition, resting metabolic rate, and substrate oxidation.
The study included 34 patients with a mean age of 67 years. No significant differences were evident among the three groups with regard to demographics, sex, and fasting and two-hour post–glucose-load blood glucose concentrations. When comparing the three groups, the total food intake or change in energy intake did not differ over time. The two groups consuming high levels of complex carbohydrates lost significantly more body weight and a higher percentage of body fat than the control group. In addition, the two groups consuming high levels of complex carbohydrates had a significant reduction in thigh fat when compared with the control group. The high intake of carbohydrates did not decrease the resting metabolic rate or reduce fat oxidation.
The authors conclude that a high-carbohydrate diet with no attempt at energy restriction or intake results in loss of body weight and fat in older men and women. A diet high in carbohydrates with no restrictions on energy intake does not decrease the metabolic rate, which is a common problem in patients who restrict caloric intake. At no time during the study did the participants complain of being hungry, which is an important consideration when recommending diets to promote weight loss.