A new study is raising questions about the age-old belief that a calorie is a calorie.
The research finds that dieters who were trying to maintain their weight loss burned significantly more calories following a low-carb diet than they did on a low-fat diet.
But some experts say the findings are preliminary.
The study, funded by the National Institutes of Health, was designed to see whether changing the type of diet people followed helped with weight maintenance, because dieters often regain lost weight.
So scientists had 21 obese participants lose 10% to 15% of their initial body weight (about 30 pounds). After their weight had stabilized, each participant followed one of three diets.
One was low-carb, like the Atkins diet, meaning it was heavy on fish, chicken and beef but cut back on breads and potatoes; a second was low-fat with whole-grain products, fruits and vegetables; the third was a low-glycemic index diet similar to a Mediterranean diet, with vegetables, fruit, beans and healthy fats (olive oil, nuts).
Participants were given food prepared by diet experts. The diets had the same number of calories, but fat, protein and carbohydrate contents varied. Researchers used state-of the-art measure to test the calories being burned off.
In results published in this week's Journal of the American Medical Association, participants burned about 300 calories more a day on a low-carb diet than they did on a low-fat diet.
"That's the amount you'd burn off in an hour of moderate-intensity physical activity without lifting a finger," says senior author David Ludwig, director of the New Balance Foundation Obesity Prevention Center at Boston Children's Hospital.
Participants burned 150 calories more on the low-glycemic-index diet than the low-fat diet.
The reason for the low-carb advantage is unclear, he says. "We think the low-carb and low-glycemic index diets, by not causing the surge and crash in blood sugar, don't trigger the starvation response. When the body thinks it's starving, it turns down metabolism to conserve energy."
The authors note a downside to the low-carb diet: It appears to raise some risk factors for heart disease.
Marion Nestle, a nutrition professor at New York University, says longer studies without such controlled meals have shown "little difference in weight loss and maintenance" among diets. More research is needed to show that results are applicable in real life, she says.
"In the meantime, if you want to lose weight, eat less."
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