For millions of Americans on a diet, eating the right combination of foods could be key to keeping off unwanted pounds. Diets that limit processed carbohydrates such as breakfast cereals and bagels may enable longer-lasting weight loss compared with other diets with the same number of calories, according to a study published Tuesday in the Journal of the American Medical Association.
“It says that from a metabolic perspective all calories are not alike,” said Dr. David Ludwig, director of the New Balance Foundation Obesity Prevention Center at Boston Children’s Hospital. “The quality of the calories going in affects the number of calories going out.”
Ludwig oversaw the study to examine how three popular diets might change the body’s metabolism and its ability to maintain weight loss.
“We know that many people can lose weight for a few weeks or months, but most people have difficulty maintaining that for a long time,” he said. As dieters cut their food intake, their bodies naturally burn energy more slowly, making more efficient use of limited calories. Slower metabolism can work with increased hunger to contribute to weight re-gain. The authors found that different diets slowed the burning of calories to varying degrees.
Twenty-one overweight and obese young adults participated in the research at Children’s and at Brigham and Women’s Hospital. For the first phase of the eight-month study, participants ate a typical American diet but with portion-size limits intended to lower their weight by about two pounds per week.
After participants reached stable, reduced weights, they tried each of three test diets for four weeks at a time. All diets provided the same number of calories but differed in their protein, fat, and carbohydrate content. At the end of the study, participants reported no changes in feelings of hunger or well-being while on the different diets.
The study found that a low-fat diet, favored by many weight-conscious consumers, contributed to the greatest reduction in metabolic rate, meaning they were more at risk of putting pounds back on in the long term—though the study didn’t track participants long enough to observe weight gain. This diet also was associated with the biggest drop in insulin sensitivity—a risk factor for diabetes.
By comparison, participants burned about 300 more calories per day while on an ultra-low-carbohydrate diet similar to the Atkins diet. “That’s equivalent to about an hour of moderate physical activity without lifting a finger,” said Ludwig.
Despite these metabolic gains, participants also had higher circulating levels of chemicals associated with biological stress and inflammation while on the ultra-low-carbohydrate diet. Ludwig suspects that the diet, which consisted of 90 percent protein and fat, may be hard on the body in the long term.
A third diet, which Ludwig called a “happy compromise” was designed to minimize blood sugar spikes and crashes between meals. Participants burned about 125 calories more per day on this diet compared with their time on the low-fat diet.
This diet, called the low-glycemic index diet, didn’t restrict carbohydrates overall but was rich in carbohydrates that are digested slowly in the body. Such foods include fruits and leafy vegetables, as well as minimally processed grains. The low-glycemic index diet was not associated with increased biochemical indicators of bodily stress.
“This continues to overturn the idea that low fat is the one best diet we have and everybody should be on it,” said Stanford University nutrition scientist Christopher Gardner, who was not involved with the study.
Gardner praised the research team for preparing meals for all the participants for seven months. Many other diet studies rely on participants to choose their own food according to study guidelines.
“It’s an elegant, clever study. It answers a very specific question in a way that other people haven’t addressed,” he said. However, Gardner notes that the results may not be generalizable to people outside the study since the experimental diets do not resemble typical meals.
Ludwig said the results provide clues that can inform consumers’ choices about their diets.
“You don’t have to go to the extreme of eliminating all carbohydrates,” he said. “By simply focusing on quality of carbohydrates we can get similar advantages to low-carbohydrate diets but without the potential downsides.”