Some of the most precise human data yet on whether cutting carbs or fat has the most benefits for losing body fat has been presented by US National Institutes of Health. The researchers show how, contrary to popular claims, restricting dietary fat can lead to greater body fat loss than carb restriction, even though a low-carb diet reduces insulin and increases fat burning.
Since 2003, Kevin Hall, PhD—a physicist turned metabolism researcher at the National Institute of Diabetes 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 nutrients affect human metabolism and body weight. He noticed that despite claims about carbohydrate vs fat restriction for weight loss, nobody had ever measured what would happen if carbs were selectiely cut from the diet while fat remained at a baseline or vice 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. “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.”
To counter dining bias, Hall and colleagues confined 19 consenting adults with obesity to a metabolic ward for a pair of 2-wk periods, over the course of which every morsel of food eaten was closely monitored and controlled. To keep the variables simple, the 2 observation periods were like two sides of a balance scale: during the first period, 30% of baseline calories were cut through carb restriction alone, while fat intake remained the same. During the second period the conditions were reversed. Each day, the researchers measured how much fat each participant ate and burned and used this information to calculate the rate of body fat loss.
RESULTS: At the end of the 2 dieting periods, the math model proved to be correct. Body fat lost with dietary fat restriction was greater compared with carbohydrate restriction, even though more fat was burned with the low-carb diet. However, over prolonged periods the model predicted that the body acts to minimize body fat differences between diets that are equal in calories but varying widely in their ratio of carbohydrate to fat. “Our results showed that, actually, not all calories are created equal when it comes to body fat loss, but over the long term, it’s pretty close.”
For now, the best diet is the one that you can stick to. His lab will next 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 inform why people respond differently to different diets. http://www.sciencedirect.com/science/article/pii/S1550413115003502
http://www.eurekalert.org/pub_releases/2015-08/cp-ldr080615.php
Body fat loss was calculated as the difference between daily fat intake and net fat oxidation measured while residing in a metabolic chamber. Whereas carbohydrate restriction led to sustained increases in fat oxidation and loss of 53 ± 6 g/day of body fat, fat oxidation was unchanged by fat restriction, leading to 89 ± 6 g/day of fat loss, and was significantly greater than carbohydrate restriction (p = 0.002). Mathematical model simulations agreed with these data, but predicted that the body acts to minimize body fat differences with prolonged isocaloric diets varying in carbohydrate and fat. Credit; Cell Metabolism, Hall et al.: “Calorie for calorie, dietary fat restriction results in more body fat loss than carbohydrate restriction in people with obesity”
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