Ad Libitum Energy Intake Differences Between a Plant-Based, Low-Fat and an Animal-Based, Low-Carbohydrate Diet: An Inpatient Randomized Crossover Study

Abstract

Abstract Objectives To investigate differences in ad libitum energy intake when inpatient adults were exposed to diets with equal calories and protein but varied widely in the ratio of carbohydrate to fat. Methods 16 adults without diabetes were admitted to the Metabolic Clinical Research Unit in the NIH Clinical Center for four continuous weeks and were randomized to receive either a plant-based, low-fat (LF) diet or an animal-based, low-carbohydrate (LC) diet for two weeks, followed by the alternate diet for two weeks. The LF diet was ∼75% carbohydrate and ∼10% fat, whereas the LC was ∼10% carbohydrate and ∼75% fat. The LF diet had ∼4-fold more fiber and was ∼60% of the energy density of the LC diet. Both diets were matched for protein and the presented calories were double each subject's maintenance energy requirements, as calculated from their measured resting energy expenditure multiplied by 1.6. Participants received three daily meals, had continuous access to snacks, and were instructed to eat as much or as little as they wanted. Leftovers were weighed to determine food intake. ProNutra software was used to calculate energy and nutrient intake. Results The study enrolled 9 men and 7 women with an age (mean ± SE) of 29 ± 1.7 years and body mass index (BMI) of 27.5 ± 1.5. During exposure to the LF diet, participants consumed 726 ± 84 kcal/d less than during the LC diet (P < 0.0001). The composition of the food intake closely matched the presented diets, with the LF diet consumption of 75.2 ± 0.2% carbohydrate, 10.7 ± 0.2% fat, and 14.0 ± 0.3% protein whereas the LC diet consumption was 9.9 ± 0.1% carbohydrate, 74.5 ± 0.2% fat, and 15.6 ± 0.2% protein. During the second week of the LC diet, when ketosis had been fully established (capillary β-hydroxybutyrate = 1.4 ± 0.08 mM), intake decreased by 295 ± 52 kcal/d compared to the first week (P < 0.0001) whereas during the second week of the LF diet intake was not significantly changed (−51 ± 52 kcal/d; P = 0.33). Nevertheless, energy intake remained 611 ± 68 kcal/d lower during the second week of the LF diet than the LC diet (P < 0.0001). Conclusions Exposure to the LF diet resulted in significantly lower ad libitum energy intake compared to the LC diet, potentially due to its lower energy density and greater fiber content. Funding Sources Intramural Research Program of the National Institute of Diabetes and Digestive and Kidney Diseases.