In a paper published August 13 in Cell Metabolism, 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. “Compared to the reduced-fat diet, the reduced-carb diet was particularly effective at lowering insulin secretion and increasing fat burning, resulting in significant body fat loss,” said Kevin Hall, Ph.D., NIDDK senior investigator and lead study author. 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. “But interestingly, study participants lost even more body fat during the fat-restricted diet, as it resulted in a greater imbalance between the fat eaten and fat burned. 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. "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," Hall says. "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." “But the real world is more complicated than a research lab, and if you have obesity and want to lose weight, it may be more important to consider which type of diet you’ll be most likely to stick to over time.” 
 
 The NIDDK, a component of the NIH, conducts and supports research on diabetes and other endocrine and metabolic diseases; digestive diseases, nutrition and obesity; and kidney, urologic and hematologic diseases.  NIH study finds cutting dietary fat reduces body fat more than cutting carbs 
 
 In a recent study, restricting dietary fat led to body fat loss at a rate 68 percent higher than cutting the same number of carbohydrate calories when adults with obesity ate strictly controlled diets.
– Think carbs are a dieter's enemy? It turns out restricting one's fat intake leads to a 68% greater loss of body fat than restricting the same amount of calories through carbohydrates in obese adults on strictly controlled diets—and this is in spite of the fact that a low-carb diet reduces insulin and increases fat burning. So says a physicist turned metabolism researcher at the National Institute of Diabetes and Digestive and Kidney Diseases who wanted to test a mathematical model he devised that projected similar results. Reporting in the journal Cell Metabolism, Kevin Hall says that while many believe that cutting carbs reduces fat faster, none had actually tested the impact of cutting carbs while keeping fat at a base line or vice versa. So he checked 19 consenting non-diabetic adults with obesity into to a metabolic ward for two 2-week periods, controlling and accounting for every single bite they ate. As his model predicted, people lost more body fat when their dietary fat was restricted than when carbs were, even though a low-carb diet does indeed burn more fat. But his model also predicted that in the long term, these differences would become less pronounced for diets that have the same number of calories, according to the press release. "Over the long term, it's pretty close," Hall says. He used data from dozens of controlled feeding studies from decades of nutrition research to develop the model, but he cautions that the study is both small and highly controlled, meaning people's unrestricted diets may cause them to eat differently. "The real world is more complicated than a research lab, and if you have obesity and want to lose weight, it may be more important to consider which type of diet you’ll be most likely to stick to over time," he tells the National Institutes of Health. (See how carbs mess with our metabolism.)