Contrary to popular claims, the new human study reported in Cell Metabolism, suggests that restricting dietary fat intake can lead to greater body fat loss than carbohydrate restriction - even though a low-carb diet reduces insulin and increases fat burning.
Led by Kevin Hall, PhD at the US National Institutes of Health (NIH), the team noticed that noticed that despite claims about carbohydrate versus 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 vice versa.
Previous modelling suggested 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 physiological data upon which those beliefs are based are sometimes lacking," said Hall. "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."
Hall and colleagues confined 19 consenting adults with obesity to a metabolic ward for a pair of 2-week periods, over the course of which every morsel of food eaten was closely monitored and controlled
To keep the variables simple, the two 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.
At the end of the two dieting periods, Hall and his team found that the previous modelling had been 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.
But over prolonged periods the same model predicts that the body acts to minimise body fat differences between diets that are equal in calories but varying widely in their ratio of carbohydrate to fat.
"There is one set of beliefs that says all calories are exactly equal when it comes to body fat loss and there's another that says carbohydrate calories are particularly fattening, so cutting those should lead to more fat loss," commented Hall.
"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."
However, Hall warned against making sweeping conclusions about how to diet from the new study.
"We are trying to do very careful studies in humans to better understand the underlying physiology that will one day be able to help generate better recommendations about day-to-day dieting," he said. "But there is currently a gap between our understanding of the physiology and our ability to make effective diet recommendations for lasting weight loss."
Source: Cell Metabolism
Published online ahead of print, doi: 10.1016/j.cmet.2015.07.021
"Calorie for calorie, dietary fat restriction results in more body fat loss than carbohydrate restriction in people with obesity"
Authors: Hall, et al