A new study led by researchers at the National Institutes of Health (NIH) recently reported that a low-fat diet induces a greater reduction in body fat than a low-carbohydrate diet in obese patients. The study was published in the journal Cell Metabolism and is entitled “Calorie for Calorie, Dietary Fat Restriction Results in More Body Fat Loss than Carbohydrate Restriction in People with Obesity.”

Obesity is a serious public health problem worldwide and is associated with the development of medical conditions like heart disease, type 2 diabetes and certain types of cancer. Weight loss diets are often based on a restriction of either carbohydrates or fat. Low-fat diets were quite popular in the latter part of the 20th century, although nowadays, low-carbohydrate diets have regained popularity. This phenomenon has been supported by studies suggesting that since the primary regulator of adipose tissue fat storage is insulin, then a decrease in insulin secretion through a reduction on the intake of refined carbohydrates should induce a greater body fat loss in comparison to a restriction on dietary fat.

In order to properly compare both diets, researchers conducted an in-patient metabolic balance study at the National Institute of Diabetes and Digestive and Kidney Diseases (NIDDK), part of the NIH. Prior to the study, the team used a mechanistic mathematical model of human macronutrient metabolism to design the study and predict the results.

In total, 19 non-diabetic obese individuals were enrolled in the study. The participants stayed at the facility 24 hours a day in two visits of two weeks each; they all ate the same food and performed the same activities, including daily exercise. In the first five days of each visit, participants ate a balanced diet, followed by six days with diets containing 30% fewer calories, either by a restriction in total carbs or total fat from the diet. The amount of protein was the same throughout the different diets. Upon the second visit, participants switched diets.

Researchers found that, in agreement with the mathematical model simulation, only the low-carbohydrate diet led to significant alterations in the metabolic mechanisms, namely a reduction in carbohydrate oxidation (or burning) and increased fat oxidation, accompanied by a reduction in insulin production. On the low-fat diet, fat oxidation actually remained unchanged but led to a greater loss in body fat than the low-carbohydrate diet, although the number of calories was the same in both diets. The low-fat diet did not alter insulin production.

“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,” explained the study’s lead author Dr. Kevin Hall in a press release. “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. These findings counter the theory that body fat loss necessarily requires decreasing insulin, thereby increasing the release of stored fat from fat tissue and increasing the amount of fat burned by the body.”

The team reported that restricting dietary fat intake resulted in a rate of body fat loss 68% higher compared to a restriction of the same number of carbohydrate calories in obese adults under strictly controlled diets.

Using their model, researchers predicted however, that in the long-term, the body may eventually mitigate the differences in body fat loss when the diets have the same number of calories. The authors state that further studies are required to determine the long-term effect of fat and carb restriction in diets.

“This NIH study provides invaluable evidence on how different types of calories affect metabolism and body composition,” noted the NIDDK Director Dr. Griffin P. Rodgers. “The more we learn about the complicated topic of weight loss, the better we can find ways to help people manage their health.”

“Our data tell us that when it comes to body fat loss, not all diet calories are exactly equal,” concluded Dr. Hall. “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.” Currently, it is estimated that more than two-thirds of the American adults are overweight or obese.