Do you need carbs to train hard?
For hard endurance and repeated high-intensity work, yes — muscle glycogen is the main fuel, and running low measurably hurts performance. For a few hard sets in the gym, you need far less than the carb-loading talk suggests.
Hard efforts run on carbohydrate. Once the pace or the load climbs, your muscles can’t pull energy from fat fast enough, so they lean on glycolysis — which burns through muscle glycogen, the stored form of carbs. Drain that tank and the highest-intensity work is the first thing to fall apart, because the fast-twitch fibers doing it are also the most glycogen-hungry.
So for anything long or repeatedly hard, carbs clearly matter. A 2024 meta-analysis pooling 136 studies found that taking in carbohydrate during endurance exercise reliably improves performance, and the benefit grows the longer the event runs. The major position stands agree, and put daily targets on a sliding scale: roughly 3-5 grams per kilogram of bodyweight on easy days, climbing toward 8-12 g/kg through heavy endurance training, plus 30-60 grams an hour once a session passes the hour mark. That is a lot of carbohydrate — and for that kind of work, it’s earned.
The high end is where it’s real
The cleanest way to see glycogen’s role is to take it away. Ketogenic and low-carb, high-fat diets get athletes burning impressive amounts of fat, but a meta-analysis of endurance athletes found no performance payoff, and the highest-intensity, glycolytic efforts tend to suffer rather than improve. When the work is hard and sustained, there’s no fat-adapted shortcut around carbohydrate.
Where the folklore overshoots
The gym version — you need carbs to lift heavy — is weaker than it sounds. A 2022 systematic review found that for a typical resistance session, up to about ten sets per muscle in a fed state, extra carbohydrate didn’t reliably improve performance. It only started to matter in very high-volume, glycogen-depleting workouts, or when lifters trained fasted. A normal mixed diet already covers a few hard sets.
So match the carbs to the job. Long endurance and dense high-intensity days genuinely demand them, so fuel up. A short, heavy strength session doesn’t need a carb strategy — just enough food overall to recover and grow.