Energy expenditure and body composition changes after an isocaloric ketogenic diet in overweight and obese men12

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Background: The carbohydrate–insulin model of obesity posits that habitual consumption of a high-carbohydrate diet sequesters fat within adipose tissue because of hyperinsulinemia and results in adaptive suppression of energy expenditure (EE). Therefore, isocaloric exchange of dietary carbohydrate for fat is predicted to result in increased EE, increased fat oxidation, and loss of body fat. In contrast, a more conventional view that “a calorie is a calorie” predicts that isocaloric variations in dietary carbohydrate and fat will have no physiologically important effects on EE or body fat.

Objective: We investigated whether an isocaloric low-carbohydrate ketogenic diet (KD) is associated with changes in EE, respiratory quotient (RQ), and body composition.

Design: Seventeen overweight or obese men were admitted to metabolic wards, where they consumed a high-carbohydrate baseline diet (BD) for 4 wk followed by 4 wk of an isocaloric KD with clamped protein. Subjects spent 2 consecutive days each week residing in metabolic chambers to measure changes in EE (EEchamber), sleeping EE (SEE), and RQ. Body composition changes were measured by dual-energy X-ray absorptiometry. Average EE during the final 2 wk of the BD and KD periods was measured by doubly labeled water (EEDLW).

Results: Subjects lost weight and body fat throughout the study corresponding to an overall negative energy balance of ~300 kcal/d. Compared with BD, the KD coincided with increased EEchamber (57 ± 13 kcal/d, P = 0.0004) and SEE (89 ± 14 kcal/d, P < 0.0001) and decreased RQ (−0.111 ± 0.003, P < 0.0001). EEDLW increased by 151 ± 63 kcal/d (P = 0.03). Body fat loss slowed during the KD and coincided with increased protein utilization and loss of fat-free mass.

Conclusion: The isocaloric KD was not accompanied by increased body fat loss but was associated with relatively small increases in EE that were near the limits of detection with the use of state-of-the-art technology. This trial was registered at clinicaltrials.gov as NCT01967563.

keyword

body composition
energy expenditure
ketogenic diet
insulin
carbohydrate
fat
macronutrients

ABBREVIATIONS

AFT
awake and fed thermogenesis
BD
high-carbohydrate baseline diet
DXA
dual-energy X-ray absorptiometry
EE
energy expenditure
EEchamber
total daily energy expenditure measured during repeated stays in metabolic chambers
EEDLW
average energy expenditure measured by doubly labeled water
EEexercise
energy cost of cycling exercise at a clamped intensity
EEsedentary
energy expenditure when not moving
FFA
free fatty acid
KD
low-carbohydrate ketogenic diet
PAEchamber
physical activity expenditure on days inside the metabolic chamber
PAEnonchamber
physical activity expenditure on days outside the metabolic chamber
rCO2
CO2 production rate
RQ
respiratory quotient
SEE
sleeping energy expenditure
SPA
spontaneous physical activity inside the metabolic chamber

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1

Supported by the Nutrition Sciences Initiative. This work was also supported in part by the Intramural Research Program of the NIH, the National Institute of Diabetes and Digestive and Kidney Diseases (KDH, KYC, and MLR), NIH grant UL1 TR00040 (Columbia Clinical and Translational Science Award; MR and RL), and Nutrition Obesity Research Center Grant P30DK072476 (ER).

2

Supplemental Figures 1–3 and Supplemental Tables 1 and 2 are available from the –Online Supporting Material– link in the online posting of the article and from the same link in the online table of contents at http://ajcn.nutrition.org.