Dieters have long been told that to lose weight, you simply need to cut kilojoules. But new research suggests that some combinations of foods may burn more kilojoules than others.
When researchers compared a low-fat diet, a very low-carbohydrate diet and a low-glycemic-index diet, they found that people on very low-carb diets used the most kilojoules. But this type of diet also boosts stress hormones and inflammation, they found.
People on the low-glycemic-index diet - a plan designed to prevent spikes in blood sugar after eating - also burned more kilojoules than those on the low-fat diet, but fewer than those on the very low-carb diet.
"From a metabolic perspective, all kilojoules are not alike," said the study's senior author, Dr. David Ludwig, director of the New Balance Foundation Obesity Prevention Center at Boston Children's Hospital. "Attempting to severely restrict all fat or all carbohydrates is hard psychologically, and it's also the wrong approach biologically," he said. "When you try to force so many nutrients through one pathway, it may have downsides on the body."
Results of the study are published in the Journal of the American Medical Association.
Ludwig and his colleagues wanted to see if certain diets might lead to metabolic changes that could make it harder for dieters to maintain weight loss.
How the study was done
For the study, 21 overweight and obese young adults who had lost 10% to 15% of their body weight were put on one of three diets.
The low-fat diet derived 60% of its kilojoules from carbohydrates, 20% from fat and 20% from protein. The low-glycemic-index diet contained 40% carbohydrates, 40% fat and 20% protein. Low-glycemic foods tend to be less-processed foods, such as vegetables or whole grains. The low-carbohydrate diet, similar to an Atkins diet, contained 60% fat, 30% protein and 10% carbohydrates.
Each study participant completed four weeks on each diet in a randomly assigned order.
"The research subjects burned 1470kJ more on the low-carb diet than on the low-fat one," Ludwig said. "That's roughly equal to an hour of moderate physical activity."
"The difference between the low-glycemic diet and the low-fat diet was about 630kJ a day, favoring the low-glycemic diet," he added. "That's about an hour of light physical activity a day."
Ludwig said the low-fat diet had the worst effects on metabolic syndrome, a group of heart disease risk factors. Low-fat diets had adverse effects on insulin sensitivity, triglyceride levels and good cholesterol levels, he said.
The low-carb diet had drawbacks, too, he said, causing high stress hormones and inflammation that could increase the risk of heart disease.
Quality of food important
"I think what's been lost in the last 50 years of dieting is an understanding of food quality. We've been too simplistic with the idea of eliminating all fats or eliminating all carbohydrates," Ludwig said. "We should focus on the quality of foods and not try to eliminate a whole class of nutrients. The quality of the kilojoules going in can affect the number of kilojoules being burned off."
Dr. George Bray, professor of medicine at the Pennington Biomedical Research Center at Louisiana State University in Baton Rouge, expressed caution about the study conclusions.
"There are some interesting physiological responses in this study, but translating this information for possible long-term results is difficult to do," said Bray, co-author of an accompanying editorial. "Adherence is an important element. If a diet helps you adhere better, that will help you lose weight. [But] no diet is proven better than any other in the long-term."
In a related study in the same journal issue, researchers found that a stepped-care approach to weight loss may provide a more cost-effective way to lose weight. In standard weight-loss programs, the first weeks tend to be the most intensive. As time goes by, these programs get less intensive whether or not you lose weight.
In a stepped-care approach, however, weight-loss programs begin in a less intensive manner. If weight-loss milestones aren't achieved at a certain point, the program becomes more intensive.
Over 18 months, the stepped-care program cost about $785 per person while the standard program cost more than $1,350, the study found. Those in the standard intervention group lost slightly more weight.
"The message is the cost/benefit," Bray said. "In this study, they did as well or better by focusing on the people who didn't do quite as well and needed the extra help."
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