Calorie for Calorie, Dietary Fat Restriction Results in More Body Fat Loss than Carbohydrate Restriction in People with Obesity

Abstract

Dietary carbohydrate restriction has been purported to cause endocrine adaptations that promote body fat loss more than dietary fat restriction. We selectively restricted dietary carbohydrate versus fat for 6 days following a 5-day baseline diet in 19 adults with obesity confined to a metabolic ward where they exercised daily. Subjects received both isocaloric diets in random order during each of two inpatient stays. Body fat loss was calculated as the difference between daily fat intake and net fat oxidation measured while residing in a metabolic chamber. Whereas carbohydrate restriction led to sustained increases in fat oxidation and loss of 53 ± 6 g/day of body fat, fat oxidation was unchanged by fat restriction, leading to 89 ± 6 g/day of fat loss, and was significantly greater than carbohydrate restriction (p = 0.002). Mathematical model simulations agreed with these data, but predicted that the body acts to minimize body fat differences with prolonged isocaloric diets varying in carbohydrate and fat.

Fig. 1

Overview of the study design. Adults with obesity were admitted to the metabolic unit at the NIH Clinical Center where they received a eucaloric baseline diet for 5 days followed by a 30% energy restricted diet achieved either through selective reduction of fat (RF) or carbohydrate (RC) for a period of 6 days. Subjects spent 5 days residing in metabolic chambers and had a dose of doubly labeled water (DLW) administered on the first inpatient day. Body composition was assessed by dual energy x-ray absorptiometry (DXA) during baseline and at the end of the reduced energy diets. Subjects returned after a 2–4 week washout period to undergo the opposite RC or RF diet following the same 5 day baseline phase. The order of the RC and RF diet periods was randomized.

Fig. 2

Changes in daily diet, insulin secretion, and energy metabolism. (A) The reduced carbohydrate (RC) diet achieved 30% energy restriction via selective reduction in carbohydrate intake (CI) whereas the isocaloric reduced fat (RF) diet resulted from selective reduction of fat intake (FI). Protein intake (PI) was unchanged from baseline on both diets. (B) Insulin secretion throughout the day was assessed by 24hr urinary C-peptide excretion and was significantly reduced only following the RC diet. (C) The 24hr respiratory quotient was practically unchanged during the RF diet but fell during the RC diet indicating an increased reliance on fat oxidation. (D) Energy intake was reduced equivalently during the RC and RF diets. (E) Energy expenditure as measured in the metabolic chamber (24hr EE) decreased minimally with the RC and RF diets. (F) Energy balance was similar between RF and RC diets. (G) Net fat oxidation increased substantially during the RC diet and reached a plateau after several days, whereas the RF diet appeared to have little effect. (H) Net carbohydrate oxidation decreased during the RC diet and was relatively unchanged during the RF diet apart from a slight initial increase on the first day. (I) Net protein oxidation was not significantly altered by the RF or RC diets. Mean ± 95%CI. * indicates a significant difference from baseline at p=0.001; ** indicates a significant difference between RC and RF at p<0.0001.

Fig. 3

Macronutrient balance and body composition changes. (A) Daily fat balance was negative for both the RF and RC diets indicating loss of body fat. The RF diet led to consistently greater fat imbalance compared with the RC diet. (B) Net carbohydrate balance was more negative for the RC diet compared to the RF diet and returned towards balance at the end of the study with both diets. (C) Protein balance tended to be lower for the RC diet compared to the RF diet. (D) Cumulative fat balance indicated that both the RF and RC diets led to body fat loss, but the RF diet led to significantly more fat loss than the RC diet. (E) Fat mass change as measured by DXA revealed significant changes from baseline, but did not detect a significant difference between RF and RC diets. (F) The RC and RF diets both led to weight loss, but significantly more weight was lost following the RC diet. (G) Mathematical model simulations of 6 months of perfect adherence to the RC and RF diets predicted slightly greater fat mass loss with the RF diet compared with the RC diet. (H) Simulating 6 months of adherence to a 30% reduced-energy diets varying in carbohydrate and fat percentage, but with protein fixed at baseline, indicated that weight loss was linearly related to carbohydrate content, but fat mass was non-monotonic and relatively unaffected by carbohydrate content. (I) Model simulated changes in fat balance and average total energy expenditure (TEE) were reciprocally related and non-monotonic with respect to carbohydrate content. The experimental RC and RF diets are indicated by the vertical dashed lines. Mean ± 95%CI. ** indicates p<0.001 between RC and RF. * indicates p=0.004 between RC and RF.