Keep carbs high enough to fuel hard days while cutting
Medium confidence
Cutting carbohydrate very low during a diet makes hard runs and lifts go worse, and it makes muscle harder to hold onto. Keep enough carbohydrate to fuel the hard days and you protect both. The measured effects are small and come mostly from ketogenic diets, so treat this 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, with a more conservative deficit the leaner the athlete. It sets protein at 2.2-3.0 g/kg body weight per day, spread over 3 to 6 meals (0.40-0.55 g/kg per meal), to hold onto lean mass. Carbohydrate is matched to the athlete's activity level (2-5 g/kg/day), and fat is kept at 20-30% of energy or at least 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 changes in meal timing and frequency appear to have little effect on fat loss or lean-mass retention, so hitting daily totals matters more; 3 to 6 meals a day, with 0.4 to 0.5 g/kg of protein before and after resistance training, likely captures any timing benefit.
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Carbohydrate-rich diets came out more favorable than the ketogenic diet for time-trial performance (-3.3%), 1-RM strength (-5.7%) and fat-free mass (-0.8 kg), while the ketogenic diet gave more total and fat-mass loss. The authors conclude the ketogenic diet is not suitable for enhancing strength or high-intensity cyclic performance and carries a small negative effect on fat-free mass. All credible intervals crossed zero.
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Compared with non-ketogenic diets, ketogenic diets lowered body mass (-3.67 kg) and fat mass (-2.21 kg) but also lowered fat-free mass (-1.26 kg). The authors conclude that adherence to a ketogenic diet may have a negative effect on fat-free mass that resistance training does not offset.
n=244
Why we call confidence medium
Mechanism reviews (Margolis 2023) show low carbohydrate raises amino-acid burning, blunts the muscle-building response and lowers high-intensity output. Two meta-analyses of controlled trials point the same way for very low (ketogenic) intakes: Koerich 2023 finds carbohydrate-rich diets beat ketogenic diets for time trials, 1-RM strength and fat-free mass in athletes, and Ashtary-Larky 2022 finds ketogenic diets with resistance training lost about 1.3 kg more fat-free mass. The effects are small with wide intervals, the evidence comes mostly from ketogenic diets and strength-trained people rather than runners, and the ISSN position stand (Aragon 2017) finds many diets give similar body composition when calories and protein are matched, so confidence sits at medium.
Where it applies
Runners and athletes dieting while still training at meaningful intensity. The trial evidence comes mostly from athletes and strength-trained adults on ketogenic diets.
Does not apply to: low-intensity or low-volume training, where carbohydrate needs are modest.
Last reviewed Oct 14, 2026. See how we score.