Weight-loss diets emphasizing different proportions of fat, protein, and carbohydrate content were found to be equally successful in a population-based study.
However, the author of an editorial commenting on this report argued that the dietary goals were only partly achieved.
In a direct comparison of four different diets, all study groups showed similar weight losses, decreases in waist circumference, and improvements in cardiovascular risk profiles. Satiety, hunger, satisfaction with the diet, and attendance at group support sessions also were similar across all four groups, regardless of the percentages of fat, protein, and carbohydrates the diets allowed.
“These divergent results suggest that any type of diet, when taught for the purpose of weight loss with enthusiasm and persistence, can be effective,” said Dr. Frank M. Sacks of Harvard School of Public Health, Boston, and his associates.
They assessed the diets of 811 subjects aged 30–70 years who had a BMI of 25–40 kg/m
Approximately one-third of the participants lost at least 5% of their initial body weight. Weight reductions differed by less than 0.5 kg across the four diet groups, and decreases in waist circumference differed by less than 0.5 cm. Cardiovascular risk factors such as cholesterol levels and blood pressure improved to the same degree in all groups.
“In conclusion, diets that are successful in causing weight loss can emphasize a range of fat, protein, and carbohydrate compositions.” Such diets can “be tailored to individual patients on the basis of their personal and cultural preferences and may therefore have the best chance for long-term success,” the investigators wrote (N. Engl. J. Med. 2009;360:859–73).
None of the diets in this study was particularly effective in the long term, Martijn B. Katan, Ph.D., of VU University, Amsterdam, said in an editorial comment accompanying this report. Weight loss averaged 6 kg at 6 months, but thereafter subjects tended to regain weight so that final losses averaged only 3–4 kg at 2 years. Even these relatively small losses might not have been sustained if the trial had continued, Dr. Katan said (N. Engl. J. Med. 2009;360:923–5).
“The participants were highly educated, enthusiastic, and carefully selected. They were offered 59 group and 13 individual training sessions [with expert professionals] over the course of 2 years. Nonetheless, their body mass index … averaged 31–32 and was moving up again” as the trial ended, he said.
Moreover, the macronutrient goals for each of the four diets were not reached. Protein intake was intended to differ among the four groups by 10% of energy, but it differed by only 1%–2%. Similarly, carbohydrate intake was supposed to differ by 30% among the groups, but both extremely low and extremely high carbohydrate intakes proved difficult to achieve, so the actual difference turned out to be only 6% of energy, Dr. Katan said.
The study investigators said that attendance at group behavioral counseling sessions “had a strong association with weight loss” and that the effect was seen across diet groups. Participants who attended two-thirds of their sessions during the 2 years lost about 9 kg, and regain after 6–12 months was about 20% of that seen in previous studies, the investigators reported. This suggests that “behavioral factors rather than micronutrient metabolism” have a greater influence on weight loss.
The editorial noted that it is difficult to quantify behavior as a factor in weight loss. “Cognition and feelings have a huge impact” on food consumption, Dr. Katan said. “Participants may eat less not because of the protein or carbohydrate content of a diet but because of the diet's reputation or novelty or because of the taste of particular foods in the diet.” Studies that can determine the effects of macronutrients on food intake and weight loss have used foods that necessarily look and taste the same such as “porridges (similar to oatmeal) and standardized snacks,” Dr. Katan explained. That was not done in the current study. “Few subjects would be willing to eat those foods for the several years that would be needed to examine long-term effects,” he wrote.
No relevant potential conflicts of interest were reported by Dr. Sacks or Dr. Katan.