BOSTON — All diets that successfully reduce calorie intake achieve meaningful, but not dramatic, weight loss, regardless of whether they emphasize avoidance of fat or carbohydrates, according to a study published Feb. 26 in The New England Journal of Medicine.
The study sought to provide clarity in what has been a murky debate raging for years over whether diets reducing one kind of macronutrient or another, e.g., low-carbohydrate or low-fat diets, are more effective for weight loss. Given the continuing rise in obesity rates, such studies also have been interpreted to offer a glimpse into whether one macronutrient category, carbohydrates or fat, may be "blamed" for the nation’s weight gain.
In the study, participants were assigned to one of four diets: low fat-average protein (20% fat, 15% protein and 65% carbohydrate); low fat-high protein (20%, 25%, 55%); high fat-average protein (40%, 15%, 45%); and high fat-high protein (40%, 25%, 35%).
"The principal finding is that the diets were equally successful in promoting clinically meaningful weight loss and the maintenance of weight loss over the course of two years," the authors said. "Satiety, hunger, satisfaction with the diet and attendance at group sessions were similar for all participants."
The study, "Comparison of weight-loss diets with different compositions of fat, protein and carbohydrates" was written by a team of researchers led by Frank M. Sacks with the Department of Nutrition, Harvard School of Public Health and the Channing Laboratory at Brigham and Women’s Hospital and Harvard Medical School.
In introductory comments, the authors noted that previous studies have demonstrated widely inconsistent results when assessing whether cutting back on one macronutrient group or another matters. Adding to the confusion, many of the earlier studies had a number of flaws including small samples, underrepresentation of men and a lack of study blinding.
"The novelty of the diet, media attention and the enthusiasm of researchers could affect the adherence to any type of diet," they said. "The crucial question is whether overweight people have a better response in the long term to diets that emphasize a specific macronutrient composition."
The study was conducted between October 2004 and December 2007 at two sites: Harvard/Brigham and the Pennington Biomedical Research Center of the Louisiana State University System, Baton Rouge.
Researchers sought to recruit 400 overweight and obese subjects aged 30 to 70 at each site of whom about 40% would be men. Excluded were those with diabetes, heart disease or individuals taking medication that affects body weight. Through surveys, the researchers also excluded potential subjects with "insufficient motivation." Ultimately, the researchers screened 1,638 participants and randomly assigned 811 to diets with 645 (80%) completing the study.
In addition to varying the fat, protein and carbohydrate intakes, diet design for all groups included capping saturated fat intake at 8% and contained at least 20 grams of dietary fiber per day.
"Carbohydrate rich foods with a low glycemic index were recommended in each diet," the study said. "Each participant’s caloric prescription represented a deficit of 750 calories per day from baseline, as calculated from the person’s resting energy expenditure and activity level."
Several steps were taken to combat potential bias in staff members toward one diet or another.
"Blinding was maintained by the use of similar foods for each diet," the authors said. "Staff and participants were taught that each diet adhered to principles of a healthful diet."
Investigators and staff who measured outcomes did not know the diet assignment of participants.
Success was measured on weight loss and waist circumference reduction.
After two years, average weight loss was 8 lbs for those on the 25% protein diet and 6.6 lbs for those on the 15% protein diet. Weight loss averaged 7.3 lbs for those on the 40% fat and 20% fat diets.
"There was no effect on weight loss of carbohydrate level through the target range of 35% to 65%," the authors said. "The change in waist circumference did not differ significantly among the diet groups."
Similarly, all the diets reduced the risk factors for cardiovascular disease and diabetes.
Regardless of its design, the study encountered a number of execution difficulties. Subjects did not achieve targeted macronutrient intakes for the various groups, missing by as much as 14.4 percentage points (in the case of carbohydrate intake).
Participants who completed the study lost an average of 14 lbs at six months, which the authors said corresponded to a reduction of 225 calories in daily energy intake, a far more modest reduction than the targeted 750.
Discussing the study and the results, the authors said the subject population was "diverse with respect to age, income and geography." Additionally, they described subjects as "eager to lose weight and to attempt whatever type of diet they were assigned."
Still, despite this motivation and "intensive behavioral counseling," they noted subjects struggled achieving their macronutrient intake goals.
"Overall, these findings suggest that participants in weight loss programs revert to their customary macronutrient intakes over time but may nonetheless maintain weight loss," the study said.
Attendance of the counseling sessions was a strong predictor of success, with those attending about two thirds of the sessions losing an average of about 20 lbs over the two years.
While the overall weight loss was relatively modest, the most successful outcomes led the authors to conclude than any type of diet can be effective.
"When nonnutritional influences are minimized, as they were in our study, the specific macronutrient content is of minor importance," the authors said.
In an accompanying editorial in the Journal, Martijn B. Katan of the Institute of Health Sciences in Amsterdam, was troubled by how poorly the subjects adhered to their specific macronutrient regiments.
"The inability of the volunteers to maintain their diets must give us pause," he said, noting that subjects were highly educated, enthusiastic and carefully selected.
"The results would probably have been worse among poor, uneducated subjects," he said.
If the objective is larger weight reduction, as may be needed, it’s clear that diets generally will not succeed, Dr. Katan said.
"We do not need another diet trial; we need a change of paradigm," he said.