Many popular diets for weight loss emphasize one energy source over the other. This study from Harvard University's School of Public Health titled, "Comparison of Weight-Loss Diets with Different Compositions of Fat, Protein, and Carbohydrates" was conducted to determine if varying the energy source of calories (carbohydrates, fat or protein) of overweight peoples diet's affected their weight loss.
The authors chose a longitudinal experimental design, monitoring the same group of people over a two year period. The fact that many previous longitudinal studies have used a much shorter time frame, makes this study particularly important. It was a double blind study so that neither the subjects, nor the staff and investigators administering the diet knew to which group each subject was assigned.
Eight hundred and eleven over weight individuals participated in the study, 38% men and 62% women. They were randomly divided into one of four experimental groups based on the percentage of calories from each energy source: Low-Fat, Average Protein (20% fat, 15% protein, 65% carbohydrate), Low-Fat, High Protein (20% fat, 25% protein, 55% carbohydrate), High-Fat, Average Protein (40% fat, 15% protein, 45% carbohydrate), and High-Fat, High Protein (40% fat, 25% protein, 35% carbohydrate).
After obtaining baseline data on each subject of resting energy expenditure and activity level, each was given a diet prescription designed to lower their caloric intake by 750 Calories/day, but not lower than 1200 Calories/day. In addition, each subject was asked to perform a moderate endurance exercise for 90 minutes/week. Subjects recorded their food intake and exercise on a web based computer program for analysis. Weight Loss and waist circumference were recorded at baseline, 6, 12, 18 and 24 months. Individual counseling sessions were offered once every 2 months and group sessions were offered 3 of 4 weeks during the first 6 months of the study and every other week throughout the balance of the study.
Their results at 24 months showed that all the diets were effective at losing weight and waist circumference, but there was no difference between the diet groups, indicating that any of the four diets are equally effective. Collectively, subjects in the diet groups lost an average of 13 pounds at 6 months, but had regained some of that weight back at 24 months, showing a net weight loss of only 9 pounds. Waist circumference measurements for the combined diet groups at 24 months were about 2 inches less than baseline data. These results are consistent with other long term studies showing the maximum weight loss at 6 months.
The facts that there was no difference between any of the diet groups for energy intake and physical activity, and they all lost a similar amount of weight, suggests that the important factor in losing weight by dieting is simply reducing calories eaten.
Another interesting finding of this study is the subjects who went to at least two-thirds of the counseling sessions lost an average of 22 pounds at two years compared to the combined group who only lost an average of 9 pounds. These results suggest that having a support group during dieting can help you achieve a much greater weight loss and keep it off for at least 2 years.