Effects of Ad Libitum Low Carbohydrate Versus Low Fat Diets on Body Weight and Fat Mass

Abstract

Background: To describe the effects of ad libitum low-fat (LF) and low-carbohydrate (LC) diets on body weight and composition. Methods: Twenty adults without diabetes spent 4 continuous weeks as inpatients at the NIH Clinical Center where they were fed ad libitum either an animal-based, LC diet (75% fat, 10% carbohydrates, 15% protein) or a plant-based, LF diet (75% carbohydrates, 10% fat, 15% protein) for 14 days each, in random order. Participants were served three meals daily and additional snacks amounting to 200% of their calculated energy requirements. Subjects were instructed to eat as much or as little as they desired. Body weight was measured using a calibrated scale and body fat and fat-free masses were measured using dual-energy X-ray absorptiometry, respectively. Results: We enrolled 9 women and 11 men, aged (mean±SE) 29.9±1.4 y with BMI=27.8±1.3 kg/m2. Participants lost weight on both diets, with 1.77±0.32 kg lost with the LC diet (p<0.0001) and 1.09±0.32 kg lost with the LF diet (p=0.003) which was not significantly different from the LC diet (p=0.15). Most of the of the weight loss with the LC diet was due to changes in fat-free mass (-1.61±0.27 kg; p<0.0001) whereas the LF diet did not result in a significant change in fat-free mass (-0.16±0.27 kg; p=0.56). The LC diet did not result in a significant change in body fat (-0.18±0.19 kg; p=0.34) whereas the LF diet resulted in 0.67±0.19 kg of fat loss (p=0.001). The difference in the average rate of body fat loss between the diets was 35±14 g/d (p=0.019). Conclusions: While participants lost similar amounts of weight on both diets, only the LF diet led to significant body fat loss. Early weight loss with the LC diet is likely due to reductions in body water, glycogen, and gastrointestinal contents and did not reflect a similar state of negative energy balance as compared with the LF diet.