In a paper published this week in Cell Metabolism, researchers found that — contrary to popular claims — restricting dietary fat can lead to greater body fat loss than carbohydrate restriction.
That's despite the fact that a low-carb diet reduces insulin and increases fat burning.
In the study, 19 obese adult volunteers stayed in a metabolic ward for a pair of two-week periods. During that time, food intake was closely monitored and controlled.
At the end of the two dieting periods, body fat loss was greater when dietary fat was restricted compared with carbohydrate restriction, even though more fat was burned with the low-carb diet.
Study researcher Kevin Hall, PhD, who is also a metabolism researcher at the National Institute of Diabetes and Digestive and Kidney Diseases, has been using data from dozens of controlled feeding studies since 2003 to build mathematical models of how different nutrients affect human metabolism and body weight.
Hall said this study shows there are small differences between fat and carbohydrate restriction under controlled conditions.
That goes against the concept that all calories are equal when it comes to body fat loss.
Dr. Frank Hu, Ph.D., professor of nutrition and epidemiology at Harvard T.H. Chan School of Public Health, thinks there are more important factors for weight loss.
While calling the study interesting and “rigorously conducted,” Hu said it “doesn't really portray real life situations.”
“This was a small study and it was done [under] controlled lab conditions. It doesn't necessarily apply to the way people live,” said Hu, who is also a professor of medicine at Harvard Medical School. “In real life, people make choices and don't always adhere to a diet that fosters weight loss over the long run.”
Furthermore, Hu said a study that lasts only a few weeks might not produce a complete picture and confirm whether one diet is more beneficial than the other for long-term weight control.
“Clinical trials that have been published so far show that, in the short term, almost all diets can lead to weight loss,” he noted.
But the question is whether an individual can maintain the weight loss in the long run and that primarily depends upon other factors like behavior, he noted.
The answer isn’t that simple.
“It's important that people don't come away with the message that this study shows that low-fat diets are better than low-carb diets for fat loss in the real world. We tried to be very clear about that in the paper,” Hall said.
Rather, the study attempted to understand how the body adapts when people experience significant changes in their diets, he explained.
Other factors, like picking a diet that you can stick to, are far more important Hall noted.
Hall's ultimate advice: Go with what works best for you.
“If you think a low-carb diet is going to give you some sort of metabolic advantage for losing fat, this study shows this is probably not true. But if you think you can better stick to a low-carb than a low-fat diet, more power to you,” he said.
“Don't feel you're at a disadvantage because of a popular theory that low-carb diets offer some sort of metabolic advantage. Do what you can actually stick to.”
Next Hall and his associates will investigate how reduced-carbohydrate and reduced-fat diets affect the brain's reward circuitry as well as its response to food stimuli. He hopes these results might uncover why people respond differently to different diets.
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