Dietary Fat vs. Carbohydrate for Reducing Body Fat

The study compared the effects of cutting carbs vs. cutting fat in a very carefully controlled environment. Dr. Kevin Hall, NIDDK.

Choosing a weight-loss program can be confusing. Some programs recommend restricting fat, while others recommend cutting carbohydrates. It can be difficult to separate fad from fact, and to determine what’s safe and effective.

A research team led by Dr. Kevin Hall of NIH’s National Institute of Diabetes and Digestive and Kidney Diseases (NIDDK) conducted a study under carefully controlled conditions to determine the way the body responds to fat vs. carbohydrate restriction. They designed the study using a mathematical model the team had previously developed. The findings appeared online on August 13, 2015, in Cell Metabolism.

The team enrolled 10 men and 9 women with obesity. Participants averaged 35 years of age, did not have diabetes, and had an average body mass index (BMI) of 36 (30 or greater is considered obese).

The participants stayed in the metabolic clinical research unit on the NIH campus for 2 visits of about 2 weeks each. Having participants reside in a metabolic ward 24 hours/day allowed the researchers to carefully control the volunteers’ diet and meticulously monitor their energy expenditure.

For the first 5 days of each stay, the volunteers were given a balanced diet (50% carbohydrate, 35% fat, and 15% protein) of about 2,740 calories/day, which matched energy expenditure. Over the next 6 days, they received a diet with 30% fewer calories. The amount of protein remained the same. On one visit, the calorie decrease came solely by cutting carbohydrates. On the other visit, the calories were cut only from dietary fat. The diets were given in random order with 2-4 weeks between visits.

While in the metabolic ward, the participants walked on a treadmill for 60 minutes each day. Part of the time, they resided in a metabolic chamber to measure their energy expenditure and the relative balance of fat and carbohydrate they used to produce energy.

The scientists found that when participants ate the reduced-carb diet, they lowered production of the hormone insulin. This diet caused a shift in metabolism; participants increased fat oxidation (“burning”) and decreased carbohydrate oxidation. While on the reduced carb diet, they lost about 53 grams of body fat per day.

When participants consumed the reduced-fat diet, they had no observed changes in insulin production or fat burning. However, they lost about 89 grams per day of body fat—68% more than when they cut the same number of calories from carbohydrates.

The researchers note that given the short time frame of the study, it would be difficult to extrapolate the findings to a longer time frame. More research is needed to assess the long-term effects of fat and carb reduction in the body.

“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. 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,” Hall says. “Our data tell us that when it comes to body fat loss, not all diet calories are exactly equal.”