Reducing Dietary Fat May Beat Cutting Carbs for Decreasing Body Fat

SAN DIEGO — Not all calories are equal, and it may be easier to reduce body fat by eating less fat than by eating fewer carbohydrates, researchers reported at ENDO 2015.

“Despite a great deal of research on the effectiveness of various weight loss diets, we wanted to understand the metabolic adaptations that occur when cutting dietary carbohydrate vs. fat while keeping dietary protein constant. In particular, we were interested in how the body adjusts its source of fuel when faced with a selective cut in fat vs. carbohydrate from the baseline diet,” said lead study author Kevin Hall, PhD, who is a senior investigator at the National Institute of Diabetes and Digestive and Kidney Diseases (NIDDK) of the National Institutes of Health (NIH) in Bethesda, Maryland.

He and his colleagues studied 10 men and 9 women with obesity. The average age of the volunteers was 24 years and average BMI was 36.

All the volunteers were admitted to a metabolic ward and resided there 24 hours per day. All food was strictly controlled, and the daily activities of the volunteers were monitored. All the volunteers were fed a eucaloric baseline diet (consisting of 50% carbohydrate, 35% fat and 15% protein) that gave them the exact number of calories they needed to maintain their body weight for 5 days.

For the next 6 days, the participants were randomly assigned to one of two groups where they received a 30% reduced-energy diet by having either their fat or carbohydrate intake restricted.

After a 2- to 4-week washout period, all volunteers were readmitted, and they repeated the same 5-day eucaloric diet. Those who had eaten 6 days of reduced-fat diet in the first phase now ate a reduced carbohydrate diet, and those who had eaten the reduced carbohydrate diet now ate the reduced fat diet.

The researchers measured the amount of fat eaten and the amount of fat burned, and the difference between them determined how much fat was lost from the body during each diet. Compared with the reduced carbohydrate diet, the reduced fat diet led to a roughly 67% greater body fat loss.

“The clinical importance is that we found that restricting dietary carbohydrates leads to more body fat loss than restricting dietary fat in people with obesity when food intake is strictly controlled and an equal number of calories are removed from the diet in both cases,” Dr. Hall told Endocrinology Advisor.

Nutrition recommendations for obese adults often conflict as to whether restricting fat or carbohydrates is better for body fat loss. However, this is the first study to investigate whether the same degree of calorie reduction, either through restricting only fat or restricting only carbohydrates, can lead to differing amounts of body fat loss in obese men and women. 

The findings suggest fat calories may not be the same calories from carbohydrates when it comes to promoting increased adipose tissue lipolysis and fat oxidation.

“The most important take-home message is that, calorie for calorie, cutting dietary fat from the diet leads to metabolic imbalances that result in more body fat loss than cutting dietary carbohydrate in people with obesity,” Dr. Hall said.

“Of course, an important caveat is that the study subjects had no choice over their diet since they were inpatients on the metabolic ward at the NIH Clinical Center during the study and had to eat all of the food provided,” he added. “We did not assess whether one diet would be easier to stick to than the other over the long term.”

Reference

1. Hall K et al. Abstract THR-553. Presented at: The Endocrine Society’s 97th Annual Meeting & Expo (ENDO 2015); March 5-8, 2015; San Diego.