This is the most repeated claim in low-carb and carnivore spaces, and it needs to be taken apart carefully, because half of it is true. Researchers added 50 g of fat to a standard breakfast. They measured the next 6 hours, then the next 18. Fat oxidation: no increase. Energy expenditure: no increase. The extra fat: stored. It made no difference whether the subject was aerobically fit. (Bennett et al., 1992, American Journal of Clinical Nutrition; R§26) This wasn't a one-off. Three years earlier, a respiration-chamber study added 106 g of fat per day. Twenty-four-hour energy expenditure: unchanged. Fat oxidation: unchanged. Energy balance tracked fat balance at r = 0.96, and didn't track carbohydrate balance at all. (Schutz et al., 1989, same journal; R§26) Two independent experiments, different labs, three years apart, same answer. At maintenance, eating fat does not raise the oxidation of your own fat. No deficit was asking for it. Now the half that's true. A high-fat, low-carbohydrate diet does raise the rate at which your body can oxidise fat. Within days. In a 33-study analysis of athletes, all 30 that measured it found the rate up by 28% to 200%. (R§29) And one crossover showed it's the fat doing it, not the missing carbohydrate. So the machinery retools fast. What doesn't change is how much body fat you lose at the same energy. That's set by the deficit, and by fat eaten minus fat oxidised. In a metabolic ward, an 80%-fat diet at the same energy slowed body-fat loss. (Hall et al., 2016; R§27) Rate up, balance unchanged. Two different quantities, and the argument only works if you keep them apart. There's also a mechanism working directly against you here, and almost nobody mentions it. Ketones suppress lipolysis through a receptor called HCA2. Tested head-to-head, ketone infusion abrogates growth-hormone-driven lipolysis. When ketones are high, the brake overrides the accelerator. Ketones made from your own fat throttle appropriately; you drew down the store to make them.