A new study by Harvard researchers and colleagues shows that eating fewer calories leads to weight loss, regardless of where those calories come from.
Many popular diets emphasize either carbohydrate, protein, or fat as the best way to lose weight. Yet there have been few studies lasting more than a year that evaluate the effect on weight loss of diets with different compositions of those nutrients.
In a randomized clinical trial led by researchers at the Harvard School of Public Health (HSPH) and Pennington Biomedical Research Center of the Louisiana State University System, a comparison of overweight participants assigned to four different diets over a two-year period showed that reducing calories achieved weight loss regardless of which of the three nutrients was emphasized.
“This is important information for physicians, dietitians, and adults, who should focus weight loss approaches on reducing calorie intake,” said Frank Sacks, professor of cardiovascular disease prevention at HSPH and lead author of the study.
The NEJM issue includes an accompanying editorial on the study’s findings.
The trial included 811 men and women who were randomly divided into four diet groups with different target nutrient compositions:
• Low-fat, average protein: 20 percent of calories from fat, 15 percent of calories from protein, 65 percent of calories from carbohydrate
• Low-fat, high protein: 20 percent fat, 25 percent protein, 55 percent carbohydrate
• High-fat, average protein: 40 percent fat, 15 percent protein, 45 percent carbohydrate
• High-fat, high protein: 40 percent fat, 25 percent protein, 35 percent carbohydrate
The participants were diverse in age, sex (62 percent women, 38 percent men), geography, and income. The diets followed heart-healthy principles, replacing saturated with unsaturated fat, and were high in whole cereal grains, fruits, and vegetables.
Each participant received a diet prescription that encouraged a 750-calorie reduction per day; however, none were less than 1,200 total calories per day.
Participants were asked to do 90 minutes of moderate exercise each week. They recorded their daily food and drink intake in a food diary and in a Web-based program that provided information on how closely they were meeting their dieting goals.
Individual counseling was provided every eight weeks over two years and group sessions were held three out of four weeks during the first six months and two out of four weeks from six months to two years.
The results showed that, regardless of diet, weight loss and reduction in waist circumference were similar. Participants lost an average of 13 pounds at six months and maintained a 9-pound loss at two years. Weight loss primarily took place in the first six months; after 12 months, all groups began to slowly regain weight, a finding consistent with other diet studies. However, the extent of weight regain was much less – about 20 percent – of the average regain in previous studies. Waistlines were reduced by an average of 2 inches at the end of the two-year period.
Most risk factors for cardiovascular disease improved for dieters at six months and two years. HDL (“good”) cholesterol increased and LDL (“bad”) cholesterol, triglycerides, blood pressure, and insulin decreased. The metabolic syndrome, a group of coronary heart disease risk factors including high blood pressure, insulin resistance, and abdominal obesity, also decreased.
The main finding from the trial was that diets with varying emphases on carbohydrate, fat, and protein levels all achieved clinically meaningful weight loss and maintenance of weight loss over a two-year period. “These results show that, as long as people follow a heart-healthy, reduced-calorie diet, there is more than one nutritional approach to achieving and maintaining a healthy weight,” said Elizabeth G. Nabel, director of NHLBI.
Another important finding was that participants who regularly attended counseling sessions lost more weight than those who didn’t. Dieters who attended two-thirds of sessions over two years lost about 22 pounds as compared to the average weight loss of 9 pounds. “These findings suggest that continued contact with participants to help them achieve their goals may be more important than the macronutrient composition of their diets,” said Sacks.