A study from the US National Institutes of Health presents some of the most precise human data yet on whether cutting carbs or fat has the most benefits for losing body fat. In a paper published August 13 in Cell Metabolism¹, the researchers show how, contrary to popular claims, restricting dietary fat can lead to greater body fat loss than carb restriction², even though a low-carb diet reduces insulin and increases fat burning. Since 2003, Kevin Hall, PhD--a physicist³ turned metabolism researcher at the National Institute of Diabetes⁴ and Digestive and Kidney Diseases--has been using data from dozens of controlled feeding studies conducted over decades of nutrition research to build mathematical models of how different nutrients⁵ affect human metabolism and body weight. 

 

He noticed that despite claims about carbohydrate⁶ versus⁷ fat restriction for weight loss, nobody had ever measured what would happen if carbs were selectively cut from the diet while fat remained at a baseline or vice⁸ versa. His model simulations showed that only the carb-restricted diet would lead to changes in the amount of fat burned by the body, whereas the reduced-fat diet would lead to greater overall body fat loss, but he needed the human data to back it up.

 

"A lot of people have very strong opinions about what matters for weight loss, and the physiological⁹ data upon which those beliefs are based are sometimes lacking," Hall says. "I wanted to rigorously test the theory that carbohydrate restriction is particularly effective for losing body fat since this idea has been influencing many people's decisions about their diets."

 

Studying the effects of diet on weight loss is often confounded by the difficulty in measuring what people actually eat--participants may not adhere to meal plans, misjudge amounts, or are not truthful¹⁰ in follow-up surveys. To counter this, Hall and colleagues confined 19 consenting adults with obesity¹¹ to a metabolic¹² ward¹³ for a pair of 2-week periods, over the course of which every morsel¹⁴ of food eaten was closely monitored and controlled.

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