Keto Flipped the Fat-Burning Switch Fast and Left Fat Loss Where It Was

The ward test. Hall KD, Chen KY, Guo J, et al. Energy expenditure and body composition changes after an isocaloric ketogenic diet in overweight and obese men. Am J Clin Nutr 2016;104(2):324–333. doi:10.3945/ajcn.116.133561. PMID 27385608. 17 men aged 18–50 with BMI 25–35 (mean age 33, weight 87.4 kg, BMI 28.8) on metabolic wards at four sites, no outside food; a 4-week high-carbohydrate baseline diet followed by 4 weeks of an isocaloric ketogenic diet with clamped protein. Not randomized: every participant followed the same diet sequence, with neither a comparison group nor a group given the diets the other way round. Fat intake rose from 108 to 242 g/d. 24-h RQ fell from 0.879 to 0.775 at the start of the ketogenic period and remained approximately constant, tracking the diet's food quotient. A rapid additional 1.6 kg weight loss was "likely primarily the result of body water loss" (fat mass −0.2 kg over the next 15 days); over the 28 days, 2.2 kg lost, 0.5 kg of it fat. Body fat loss slowed on transition to the ketogenic diet (urinary nitrogen excretion rose, indicating more protein use, until day 11); in the final 2 weeks the rates of weight and fat loss were similar to baseline. Energy expenditure rose transiently by only about 100 kcal/d; the ~300–600 kcal/d metabolic advantage predicted by the carbohydrate–insulin model was not supported. Limitation: unintentional weight loss, with an overall negative energy balance of about 300 kcal/d that the authors attribute to extra spontaneous activity on non-chamber days; Ludwig et al. 2021 describe the participants as underfed through a miscalculation, weighing about 2.3 kg less in the last 2 keto weeks than in the last 2 high-carbohydrate weeks, and add a modelled 50 kcal/d in a sensitivity analysis. The authors note the results may not translate to women. The Nutrition Sciences Initiative (NuSI) convened the team, helped formulate the hypothesis and provided partial funding; the investigators retained editorial control; no conflicts declared.

The same lab, pooled. Hall KD, Guo J. Obesity Energetics: Body Weight Regulation and the Effects of Diet Composition. Gastroenterology 2017;152(7):1718–1727.e3. doi:10.1053/j.gastro.2017.01.052. PMID 28193517. A review with a meta-analysis of 32 controlled feeding studies (563 subjects) in which all food was provided, calories were isocaloric and protein equal, with carbohydrate from 1% to 83% and fat from 4% to 84% of energy. Pooled weighted mean differences: energy expenditure 26 kcal/d and body fat loss 16 g/d greater with lower-fat diets (P < .0001), "so small as to be physiologically meaningless". The 16 g/d is a daily rate from studies of unstated length, not to be extrapolated. The theorised 400–600 kcal/d "metabolic advantage" of very low carbohydrate diets is, in the authors' words, not supported by the experimental evidence. When intake is not controlled, very low carbohydrate diets "may reduce appetite by promoting an increase in circulating ketones" and higher protein "may independently increase satiety", which "may help explain the short-term benefits for weight loss"; long-term diet studies show similar weight trajectories. Funding: NIH Intramural Research Program (NIDDK); KDH declares Nutrition Science Initiative funding to study ketogenic diets and energy expenditure, and a patent pending assigned to NIH. Same NIH group as the 2015, 2016 and 2021 ward studies cited here, so these are not independent confirmations.

How fast the fuel mix shifts. Hall KD et al. Effect of a plant-based, low-fat diet versus an animal-based, ketogenic diet on ad libitum energy intake. Nat Med 2021;27:344–353. doi:10.1038/s41591-020-01209-1. 20 adults, inpatient crossover, 2 weeks per diet: daily RQ 0.753 on the ketogenic diet vs 0.885 on the low-fat diet, with fat oxidation of 142 vs 49 g/d; the authors note this shift occurs within the first week of a ketogenic diet with no further change over the following weeks. Intake was ad libitum and differed by 689 kcal/d between diets, so this trial's fat-loss rates are not used here. Hall KD et al. Calorie for Calorie, Dietary Fat Restriction Results in More Body Fat Loss than Carbohydrate Restriction in People with Obesity. Cell Metab 2015;22(3):427–436. doi:10.1016/j.cmet.2015.07.021. On a metabolic ward, 19 adults with obesity ate 6-day reduced-carbohydrate (about 140 g/d, not ketogenic) vs reduced-fat diets at equal calories: only the carbohydrate cut raised net fat oxidation (+463 kcal/d), and its adaptation "achieved a plateau after several days", yet steady body fat loss was 53 g/d vs 89 g/d on the fat cut. On the objection that results would converge over longer periods, the authors write this "would require that the net fat oxidation rate somehow increase above the observed plateau", which neither the data nor their model simulations showed. Ludwig et al. 2021 note that several authors have proposed 2 to 3 weeks as the minimum time for metabolic adaptation, while "the time course of physiological adaptation has not been precisely determined."

A year, free-living. Gardner CD, Trepanowski JF, Del Gobbo LC, et al. Effect of Low-Fat vs Low-Carbohydrate Diet on 12-Month Weight Loss in Overweight Adults and the Association With Genotype Pattern or Insulin Secretion: The DIETFITS Randomized Clinical Trial. JAMA 2018;319(7):667–679. doi:10.1001/jama.2018.0245. PMID 29466592. 609 adults, assigned at random to a healthy low-fat or a healthy low-carbohydrate diet for 12 months. The low-carbohydrate group started at 20 g/d for 8 weeks, then added carbohydrate back, reaching 30% of energy at 12 months (vs 48%), so it was not ketogenic; no explicit calorie restriction was instructed. Respiratory exchange ratio was lower in the low-carbohydrate group at each time point after randomization (12-month change −0.027 low-carbohydrate vs −0.008 low-fat; difference 0.020, 95% CI 0.006 to 0.033). Body fat percentage change −2.15 vs −1.97 (difference 0.18, 95% CI −0.40 to 0.75) and weight change −6.0 vs −5.3 kg (low-carbohydrate vs low-fat) did not differ significantly. Fuel-mix and body-composition data were not collected for the first cohort. FitChef study page: https://fitchef.com/studies/dietfits-low-carb-vs-low-fat/

Calories burned, the live dispute. Ludwig DS, Dickinson SL, Henschel B, Ebbeling CB, Allison DB. Do Lower-Carbohydrate Diets Increase Total Energy Expenditure? An Updated and Reanalyzed Meta-Analysis of 29 Controlled-Feeding Studies. J Nutr 2021;151(3):482–490. doi:10.1093/jn/nxaa350. PMID 33274750. 29 trials, 617 participants, durations 1–140 days (median 4). Total energy expenditure on the lower-carbohydrate diet: −50.0 kcal/d (95% CI −77.4 to −22.6) in 23 trials of 17 days or less; +135.4 kcal/d (95% CI 72.0 to 198.7) in 6 trials longer than 17 days. These are energy expenditure figures, not fat loss, and "lower-carbohydrate" covers carbohydrate gaps of 8% to 77% of energy. 3 of the 6 longer trials used doubly labeled water; the authors note that dietary nonadherence "would tend to inflate DLW estimation of TEE on a lower- versus higher-carbohydrate diet". Funded by the New Balance Foundation; disclosures include low-glycemic-load diet book royalties (DSL). A commentary ran alongside it: Magkos F, Astrup A. J Nutr 2021;151(3):468–470, doi:10.1093/jn/nxaa423. FitChef's claim on this question (Low Certainty): cutting carbs probably raises energy expenditure by 50 to 150 calories per day, a finding genuinely disputed between two research camps using different measurement methods, and no controlled study has shown it translating into greater long-term fat loss (https://fitchef.com/claims/low-carb-metabolic-edge-real-or-not/).

Planned calories, free-living. Anagnostou A, Larumbe-Zabala E, Fiore J, Roberts J, Naclerio F. Eur J Nutr 2026;65(1):19. doi:10.1007/s00394-025-03862-z. PMID 41493485. 18 RCTs, low-carbohydrate (≤44% of energy) vs high-carbohydrate (≥45%) diets "designed to maintain equivalent total energy intake", excluding trials more than 200 kcal or 5% apart; all free-living, with self-reported intake, and two included trials listed as ad libitum. Fat mass, from 7 studies: Hedges' g −0.304 (95% CI −0.548 to −0.059; P = 0.015), "small but significant", with funnel asymmetry suggesting a possible small-study effect. The authors suggest nutrient partitioning, water retention or satiety as explanations. Co-funded by Crown Sports Nutrition and the University of Greenwich; no conflicts declared. Feng S, Liu R, Thompson C, et al. Am J Clin Nutr 2025;122(5):1461–1478. doi:10.1016/j.ajcnut.2025.09.012. PMID 40935153. 174 randomized trials (n = 11,481), carbohydrate-restricted diets at ≤45% of energy. In the authors' isocaloric class (energy margin about ±5%, method not stated): body fat percentage SMD −0.83 (95% CI −1.37 to −0.28), fat mass SMD −0.52 (95% CI −1.06 to 0.05; P = 0.07). The authors write that similar results between isocaloric and nonisocaloric trials "might suggest" macronutrient composition influences outcomes beyond caloric intake. No funding and no conflicts reported. Both report standardized effect sizes, not kilograms.

People who exercise. Leaf A, Rothschild JA, Sharpe TM, et al. International society of sports nutrition position stand: ketogenic diets. J Int Soc Sports Nutr 2024;21(1):2368167. doi:10.1080/15502783.2024.2368167. PMID 38934469. Narrative review. Fat oxidation averaged 1.5 g/min in male elite keto-adapted cyclists after 4 weeks, against 0.46 g/min in 300 adults from a different study, and ketogenic diets raised fat oxidation by 37% to 500% against control diets; keto-adaptation of daily fat oxidation occurs within a week in sedentary adults (citing Hall et al. 2016, so not an independent confirmation). Body composition, 19 controlled trials of 3–12 weeks: fat mass 0.3–5.3 kg lower on the ketogenic diet (2.3 kg average), statistically significant in five; the review judges the ketogenic diet "seems superior" for reducing body fat in exercising individuals, while noting that no study implemented strict dietary controls, keto groups ate 75 fewer kcal/d with protein 1.8 vs 1.3 g/kg, athletes' food logs underestimate intake by 19% on average, and a 5% variation in fat-free mass hydration can change DXA body fat percentage by nearly 3% (absolute). Several authors declare supplement-industry or ketone-related commercial interests, and the Society receives grants from sports nutrition companies. McSwiney FT, Wardrop B, Hyde PN, Lafountain RA, Volek JS, Doyle L. Metabolism 2018;81:25–34. doi:10.1016/j.metabol.2017.10.010. PMID 29108901. 20 male endurance athletes self-selected into a ketogenic (n = 9) or high-carbohydrate (n = 11) group for 12 weeks: body mass −5.9 vs −0.8 kg, body fat −5.2 vs −0.7 percentage points, fasting beta-hydroxybutyrate 0.5 mmol/L at week 12; per the ISSN review, calories and protein were not matched and the keto group was 10 kg heavier at baseline. A letter to the editor and the authors' reply were published (Metabolism 2018;83:e1–e2).