Keep carbs high enough to fuel hard days while cutting
Medium confidence
Cutting carbohydrate too low during a diet makes hard runs and lifts feel worse, and it makes muscle harder to hold onto. Keep enough carbohydrate to fuel the hard days and you protect both. The evidence here is mostly short-term and mechanistic, so treat it as a well-reasoned guide rather than a settled rule.
Why it works
When your muscles run low on carbohydrate, their quick fuel, the body burns more amino acids for energy. Amino acids are the building blocks of muscle. That same shortage quiets the signals that build and hold muscle, and it lowers how hard you can push.
What it means in practice
If you're running a deficit, take most of it from fat and keep carbohydrate high enough to fuel your hard sessions. Cutting carbohydrate to the floor is the part that costs you.
The evidence
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When carbohydrate runs low, the body oxidizes more amino acids and blunts the muscle-building response to resistance training. High-intensity and anaerobic output also fall, even though maximal strength is less affected. Margolis and Pasiakos (2023) read this as a case for keeping carbohydrate adequate when the training is hard. It is a narrative review of short-term, mechanistic studies, so the direction holds more weight than the magnitude.
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This narrative review recommends losing about 0.5-1.0% of body weight per week, easing the deficit as the athlete leans out. It sets protein at roughly 2.3-3.1 g/kg of fat-free mass per day (about 1.6-2.4 g/kg body weight) to hold onto lean mass. Carbohydrate stays high enough to fuel training (around 2.5 g/kg/day or more), and fat holds above roughly 0.5 g/kg/day. These are expert recommendations synthesized from the literature, not pooled trial results.
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This review of the contest-prep literature for drug-free bodybuilders recommends setting calories so that body weight drops about 0.5 to 1% per week, which best preserves muscle during a cut. Within that calorie level it recommends most competitors eat 2.3 to 3.1 g/kg of lean body mass per day of protein, 15 to 30% of calories from fat, and the rest of calories from carbohydrate. It also concludes that meal timing and frequency matter far less than hitting these daily totals, and that adherence over the long prep is the main driver of results.
Why we call confidence medium
The support comes from mechanism studies and reviews. Low carbohydrate raises amino-acid oxidation, meaning the body burns amino acids for fuel. It also blunts the muscle-building response and lowers high-intensity performance. Most of this evidence is short-term or mechanistic rather than long field trials, which is why the confidence sits at medium.
Where it applies
Runners and athletes dieting while still training at meaningful intensity.
Does not apply to: low-intensity or low-volume training, where carbohydrate needs are modest.
Last reviewed Jun 19, 2026. See how we score.