For years dieters were told that a calorie is a calorie, but a new study suggests people may burn more calories on a low-carb diet than on a diet rich in carbohydrates.
"These findings showed that all calories are not alike to the body," said study coauthor Dr. David Ludwig, who codirects the New Balance Foundation Obesity Prevention Center at Boston Children's Hospital. "Restricting carbohydrates might be a better strategy than restricting calories over the long term."
The new study did not focus on losing excess pounds, but rather on a factor that makes it hard to maintain weight loss: the fact that the body adapts as pounds are shed by slowing metabolism, which results in fewer calories being burned. And for most that means weight is regained.
Ludwig subscribes to a theory, known as the carbohydrate-insulin model, which suggests that increases in the consumption of so called high-glycemic foods - which raise blood sugar sharply right after they're eaten - trigger hormonal changes that increase hunger and lead to weight gain.
To see if metabolism and hunger can be shifted by types of foods people eat, Ludwig and his colleagues enlisted 164 overweight adults aged 18 to 65 who had already lost 10 per cent of their body weight, and randomly assigned them to one of three carb-varying diets for 20 weeks.
The volunteers' meals, provided by the researchers, had the same daily calorie count and all contained 20 per cent protein. But one group's diet consisted of 20 per cent fat and 60 per cent carbs, another got a diet with 40 per cent fat and 40 per cent carbs and the third group ate 60 per cent fat and 20 per cent carbs.
After tracking the volunteers' weight and measuring energy expenditure through the study period, it was clear that those who had consumed the lowest levels of carbs had burned the most calories. Perhaps just as important, their levels of the hunger-regulating hormones, ghrelin and leptin, were lower too.
Volunteers in the low-carb group burned 209 to 278 calories a day more than those on the high-carb diet, which meant they were burning 50 to 70 calories more a day for every 10 per cent decrease in carbs to their total energy intake, according to the report in The BMJ.
The volunteers with the highest insulin secretion at the start of the study had an even more dramatic difference in energy expenditure: those on the low-carb diet burned as much as 478 calories a day more than those consuming the highest level of carbs.
This kind of extra calorie burn would translate "into about 20 pounds of weight loss in a year among those on the low carb diet compared to those in the high carb group," Ludwig said.
The new study is "exciting and unique," said Dr. Rekha Kumar, an endocrinologist and obesity medicine specialist at NewYork-Presbyterian and Weill Cornell Medicine in New York City. "Most studies are looking at inducing weight loss," Kumar said. "This one is about weight maintenance. And it's asking, is there a particular macro nutrient composition that can result in a higher calorie burn?"
The fact that people with higher insulin levels "had the biggest impact allows you to say this is quite valid," said Kumar, who was not involved in the study. "That's because these are the people - the ones who have issues with blood sugar and insulin - that you would expect to respond."
The findings offer hope that modifying nutrients in the diet could affect energy expenditure, said Lisa Martich, a dietician specialist at Magee-Womens Hospital at the University of Pittsburgh Medical Center.
And it may end up being used as "another tool," in weight loss programs, said Martich, who was not involved in the study.
"I think there's a tendency to go all or nothing, saying just eat a low carb diet and it will keep the weight off," Kumar explained. "Maybe a low-carb diet can help, but so can increasing exercise."