Not eating enough carbohydrates while bodybuilding limits your ability to train hard, recover fully, and build muscle at the rate you otherwise could. The effects range from flat-looking muscles and sluggish workouts to hormonal shifts that work against your goals. Current guidelines for bodybuilders recommend at least 3 to 5 grams of carbohydrates per kilogram of body weight per day, meaning a 180-pound (82 kg) lifter should aim for roughly 245 to 410 grams daily during a building phase.
Your Muscles Run on Glycogen
Resistance training is fueled almost entirely by glycogen, the stored form of carbohydrate inside your muscles. Every set you perform draws from that glycogen reserve. When you consistently eat too few carbs, those reserves never fully replenish between sessions, and the consequences compound over time.
Research published in Current Opinion in Clinical Nutrition and Metabolic Care found that maintaining carbohydrate restriction for 8 to 12 weeks blunts the hypertrophic response to resistance training. Anabolic signaling, protein synthesis, and the process of creating new muscle fibers are all impaired. Interestingly, raw strength on a single heavy rep may hold up reasonably well, but anaerobic output during higher-intensity, higher-volume work drops significantly. That means the training style most bodybuilders rely on, multiple sets in moderate rep ranges pushed close to failure, is exactly the kind that suffers most.
Your Body Starts Breaking Down Muscle for Fuel
When carbohydrate intake is very low, your body still needs to maintain blood sugar. The primary way it does this is through gluconeogenesis, a process where amino acids (the building blocks of protein) are converted into glucose. The main source of those amino acids is muscle tissue. Glycerol from stored body fat contributes as well, but amino acids carry the heavier load.
This creates a frustrating paradox for bodybuilders. You’re training to build muscle, but your metabolism is pulling amino acids out of muscle to compensate for the missing carbs. Even if you eat plenty of protein, some of that dietary protein gets diverted to fuel production instead of repair and growth. The net result is that your body is fighting against the very thing you’re in the gym to accomplish.
Hormonal Shifts Work Against You
Chronically low carbohydrate intake paired with hard training triggers a cascade of hormonal changes. One hypothesis gaining traction among researchers involves T3, the active thyroid hormone that regulates your metabolic rate. The proposed mechanism is that low carb availability, especially combined with high energy expenditure, elevates cortisol levels. Elevated cortisol then suppresses T3 production and interferes with the conversion of inactive thyroid hormone into its active form.
Low T3 doesn’t just slow your metabolism. It amplifies a broader pattern of hormonal disruption that can reduce levels of reproductive and anabolic hormones. For a bodybuilder, this means a hormonal environment that favors muscle breakdown and fat retention over the lean, muscular physique you’re chasing. The combination of elevated cortisol and suppressed anabolic hormones is one of the worst profiles for body composition.
Your Muscles Look Flat
This one matters to bodybuilders more than most athletes. Glycogen stored inside muscle cells pulls water in with it. Early research estimated that each gram of glycogen draws in roughly 3 to 4 grams of intracellular water. That water is what gives muscles their full, round appearance.
When glycogen stores are depleted from sustained low-carb eating, muscles lose that intracellular water and visibly shrink. You haven’t necessarily lost contractile tissue, but you look noticeably smaller and flatter. This is why competitive bodybuilders deliberately carb-load before stepping on stage: they’re maximizing glycogen and the water it holds to enhance muscle “fullness” and volume. If you’re eating low carb day after day, you’re living in the opposite state of that peak-week strategy.
Recovery Slows Down
Carbohydrates play a direct role in post-workout recovery. Replenishing glycogen after training is one piece, but carbs also trigger an insulin response that helps shuttle nutrients into damaged muscle cells and supports the repair process. Without adequate carbohydrate intake, the window between sessions where your muscles rebuild and adapt stretches longer. You may notice more persistent soreness, reduced motivation, and a general sense that you never feel “fresh” heading into your next workout.
Research comparing carbohydrate and protein supplementation around exercise found that carbs were effective at attenuating markers of muscle damage like creatine kinase, a protein released into the blood when muscle fibers are damaged. Skipping carbs doesn’t just cost you performance in the gym. It costs you the quality of what happens between sessions, which is where muscle is actually built.
What About Keto Bodybuilding?
Ketogenic diets are the most extreme version of carb restriction, typically limiting intake to under 50 grams per day. A systematic review and meta-analysis examining ketogenic diets in resistance-trained men and women found no significant difference in fat-free mass changes between keto and higher-carb groups. On the surface, that sounds like keto works just as well. But the details tell a more nuanced story.
Several of the individual studies included in that analysis showed that the non-ketogenic groups gained significant fat-free mass while the ketogenic groups did not. One study found that leg fat-free mass actually decreased in the keto group. A separate, larger meta-analysis of 13 studies with 244 participants found significant reductions in fat-free mass in the keto group. The overall takeaway is that while some people can maintain muscle on a ketogenic diet, the odds of building new muscle are consistently lower compared to eating adequate carbs.
The meta-analysis authors noted that the mean difference in fat-free mass favored the non-keto groups in most comparisons, even when the pooled result didn’t reach statistical significance. For a natural bodybuilder trying to maximize every possible advantage, that trend matters.
How Many Carbs You Actually Need
A narrative review published in Sports lays out a straightforward framework. After allocating calories to protein (1.6 to 2.2 grams per kilogram per day) and fat (0.5 to 1.5 grams per kilogram per day), the remaining calories should come from carbohydrates, with a minimum target of 3 to 5 grams per kilogram per day. For practical reference:
- 150-pound (68 kg) lifter: at least 204 to 340 grams of carbs per day
- 180-pound (82 kg) lifter: at least 246 to 410 grams of carbs per day
- 200-pound (91 kg) lifter: at least 273 to 455 grams of carbs per day
These numbers apply to the off-season or a lean-gaining phase. During a calorie deficit for contest prep, carbs inevitably drop, but the goal is to keep them as high as possible after protein and minimum fat needs are met. Cutting carbs first and hardest is a common mistake. Protein stays high, fats stay at a healthy minimum, and carbs fill whatever caloric room remains.
Timing also plays a role. Prioritizing a significant portion of your daily carbs in the meals before and after training ensures glycogen is available when you need it most and gets replenished quickly afterward. You don’t need to obsess over exact timing windows, but front-loading carbs around your workout is a simple way to get more out of the same total intake.