I think this topic will apply to several posts. Quick question before you read on: how many different fuels can the human body actually run on? Most people say three. The answer is seven. Drop your guess in the comments, then check yourself. The body follows one rule: it oxidises first whatever it cannot store. That gives you a queue, from first used to last. 1. Alcohol. Zero storage capacity, so the body deals with it immediately, and while it does, everything else waits. In infusion studies, alcohol alone cut fat oxidation by 87%. Add glucose after the alcohol, and fat oxidation drops to almost nothing for 90 minutes. Interesting twist: the liver exports most of alcohol's energy as acetate, which the rest of the body then uses. 2. Ketones. No storage tank either. Made by the liver from fat when glucose runs short — and the liver, remarkably, cannot use the ketones it makes. It lacks the enzyme. They are produced purely for export. In prolonged fasting, they supply up to 60–70% of the brain's energy, the adaptation that stretches survival without food from weeks into months. 3. Carbohydrate and protein — the contested middle. You hold roughly 400–500 g of glycogen, about 6,700 kJ. One hard day. Protein has no storage depot, so every gram used for energy means dismantling working tissue. Disposing of a protein-rich meal also produces the biggest rise in heat output of any fuel — the body literally vents the surplus as warmth. 4. Lactate. Not a waste product — that idea died decades ago. Working muscle exports lactate, and the heart and even the brain take it up and use it as fuel. The catch: it is recycled glucose carbon, so it moves energy around the body without adding any. 5. Short-chain fatty acids. The fuel almost nobody names. You cannot digest fibre — but the bacteria in your colon can, and they ferment it into fuels that supply roughly 10% of your daily energy. The cells lining your gut draw 60–70% of their energy from these. 6. Fat. Last in the queue, biggest tank by a distance: around 15 kg in a typical adult, roughly 565,000 kJ. Stored at 96–98% efficiency. While carbohydrate is abundant, fat oxidation is actively suppressed — a mechanism first described by Randle in 1963. And from a study we verified in full this week: the greatest amount of fat is oxidised at moderate exercise intensity, not at really low or maximal intensity. At 85% effort, fat use is no higher than at 25%. Now the part that reframes everything.

Alan reeves and Lyle McDonald As you'd expect nice historical anecdotes on luminaries of diet and bodybuilding Fill your boots https://youtu.be/Cx5Zzg82714?is=p0sFRouz_31scF3D