The research followed 811 overweight or obese people, 62% of whom were women, enrolled at one of two study sites: Harvard School of Public Health in Boston or the Pennington Biomedical Research Center of Louisiana State University in Baton Rouge. The participants were assigned to one of four diets: low-fat, average-protein; low-fat, high-protein; high-fat, average-protein; and high-fat, high-protein.
The diets ranged from 1,200 to 2,400 calories a day based on each individual's body mass index and gender, but everyone was asked to cut about 750 calories a day from what they normally ate. All the diets were low in saturated fat, the kind linked to heart disease and found in many fried or processed foods. Participants were asked to do 90 minutes a week of moderate exercise. They kept a food diary, and a Web-based program provided feedback on how close they had come to their goals. Individual and group counseling sessions were held over the two-year study.
"We were trying to focus on just those three nutrients -- fat, protein and carbohydrates -- and keep everything else, such as saturated fat and fiber, as consistent as possible," said Catherine M. Loria, project scientist at the National Heart, Lung and Blood Institute, which funded the research. "This shows people can just focus on counting calories. They have a lot of flexibility. It's a great finding."
The study refutes the notion that any one nutrient has a special power to accelerate weight loss, said Dr. Frank M. Sacks, lead author of the study and a professor of cardiovascular disease prevention at Harvard. "We used to think there could be a biological effect of certain diets. That is probably not true."
There may be a strong behavioral effect in the success of a diet, however. The people who attended two-thirds or more of the counseling sessions over the two years lost an average of 22 pounds, compared with the average loss of 9 pounds.
The study was highly anticipated because previous research on diets over the last two decades has come to dramatically different conclusions.
"Some studies showed a very low-fat, strict vegetarian diet was best," Sacks said. "Others had Atkins diets doing better. So the question we had was, how do we reconcile all that?"
Many of the previous studies lasted six months or less, enrolled small numbers of people (usually women) and sometimes involved feeding participants prepared meals instead of allowing them to follow the diet on their own in real-life conditions, Gardner said.
