Building muscle requires calories because your body needs energy to construct new tissue, fuel your workouts, and maintain the hormonal environment that makes growth possible. A kilogram of new muscle stores roughly 1,200 to 1,240 calories worth of energy in its protein, fat, and glycogen alone, and the actual metabolic cost of assembling that tissue is even higher. Without enough incoming energy, your body shifts into conservation mode and actively suppresses the processes that drive muscle growth.
Muscle Is Made of Energy
Skeletal muscle is about 20% protein and 75% water, with small amounts of fat and stored carbohydrate (glycogen). The energy locked inside one kilogram of muscle tissue breaks down to roughly 810 calories from protein, 335 to 360 calories from fat, and 70 to 105 calories from glycogen. That adds up to about 1,200 to 1,240 calories stored in every kilogram of muscle you build.
But storing energy in new tissue is only part of the equation. Your body also burns calories during the construction process itself, powering the molecular machinery that links amino acids together into muscle protein. Estimates for the total energy cost of depositing one kilogram of muscle range from about 1,445 to 1,780 calories once you account for this overhead. Those calories have to come from somewhere. If your diet doesn’t supply them, your body has limited reason, and limited resources, to build.
How Your Cells Decide to Build or Conserve
Inside your muscle cells, a signaling system acts as a central switch for growth. When energy is available, this system (driven by a protein complex called mTOR) ramps up protein synthesis, telling cells to get bigger and stronger. When energy runs low, a competing sensor called AMPK detects the shortage and essentially flips the switch in the other direction. AMPK activates pathways that break down stored fuel to recover energy, and it directly inhibits mTOR through multiple mechanisms to slow down the energy-expensive process of building new protein.
Think of it as a built-in priority system. Your body treats survival functions like breathing, brain activity, and organ maintenance as non-negotiable. Muscle growth is a luxury. When calories are scarce, your cells redirect resources toward keeping you alive rather than adding tissue you don’t strictly need. This isn’t a subtle effect. AMPK suppresses the growth signal at two separate points in the pathway, making it a powerful brake on hypertrophy when you’re underfed.
Calories Protect Your Protein From Being Burned as Fuel
Your body can use protein for energy, and it will if it has to. When you’re fasting or eating too few calories, your liver converts amino acids into glucose to keep your brain and organs running. This process pulls amino acids away from muscle repair and growth and sends them to be burned as fuel instead.
Eating enough carbohydrates and fats prevents this. Classic nutrition research showed that feeding carbohydrates to fasting subjects cut their protein losses by about 50%, bringing daily protein breakdown down to roughly 40 grams. This is called the “protein-sparing” effect. When your body has plenty of glucose and fat to burn, it no longer needs to raid your amino acid supply for energy. Those amino acids stay available for their primary job: repairing and enlarging muscle fibers after training. Without adequate non-protein calories, even a high-protein diet partly defeats its own purpose because a significant chunk of that protein gets diverted to energy production.
Energy Intake Shapes Your Hormonal Environment
Calories don’t just provide raw materials. They also regulate the hormones that control whether your body is in a growth state or a breakdown state. Long-term calorie restriction lowers testosterone, the primary anabolic hormone in men, independently of how much body fat someone carries. Research on men practicing sustained calorie restriction found significantly lower total testosterone, free testosterone, and IGF-1 (a growth factor that stimulates muscle and tissue repair) compared to men eating at maintenance or above.
The mechanism is straightforward: building complex molecules from simple ones requires energy and substrates. When food intake drops, your body downregulates the hormones that stimulate cell growth and proliferation as a feedback response to the reduced availability of fuel. Estradiol, another hormone involved in bone and muscle health, also drops. The net result is a hormonal profile that favors tissue conservation over tissue building. You can still train hard in a deficit, but your body’s internal chemistry is working against hypertrophy the entire time.
Your Workouts Need Fuel Too
Building muscle requires progressively challenging your muscles with resistance training, and that training runs on stored carbohydrate. Glycogen, the form of glucose packed into your muscles, is the primary fuel for resynthesizing the high-energy phosphate compounds that power intense contractions. Research has demonstrated that reduced muscle glycogen impairs both the force your muscles can produce and your capacity to do repeated sets of resistance exercise.
Some studies show that eating carbohydrates before training increases total work capacity, though results vary. The practical takeaway is that chronically undereating, especially carbohydrates, can erode your ability to train at the intensity and volume needed to trigger growth. Interestingly, research suggests that low glycogen doesn’t directly block the molecular signals for muscle growth. The problem is more practical: if you can’t complete enough quality sets because you’re running on empty, the growth stimulus itself is weaker.
Why High-Protein Diets Cost Extra Calories
If you’re eating a high-protein diet to support muscle growth (and you should be), a larger fraction of your food calories gets burned during digestion itself. Protein costs 20 to 30% of its calorie content just to digest and process, compared to 5 to 10% for carbohydrates and 0 to 3% for fat. This is called the thermic effect of food.
In practical terms, if you eat 800 calories of protein in a day, 160 to 240 of those calories are lost to the digestion process before they ever reach your muscles. Research confirms that this elevated thermic effect persists over time and doesn’t diminish as your body adapts to the diet. This means that when you’re eating for muscle growth, your actual calorie needs are somewhat higher than the simple math might suggest, because protein-heavy meals are metabolically expensive to process.
How Much of a Surplus You Actually Need
The total energy cost of building one kilogram of muscle falls somewhere between 1,445 and 1,780 calories, depending on which estimate you use. A realistic rate of muscle gain for a trained natural lifter is roughly 0.25 to 0.5 kilograms per month. Working backward, that means the energy specifically needed for new muscle tissue is relatively modest, perhaps 350 to 900 extra calories spread across an entire month, or roughly 12 to 30 extra calories per day.
In practice, though, the surplus needs to be larger than that bare minimum. Your body isn’t perfectly efficient at partitioning extra calories toward muscle. Some will go to supporting the increased training demands, some to the elevated thermic effect of a high-protein diet, and some inevitably to fat storage. Most evidence-based recommendations for natural lifters land in the range of 200 to 500 extra calories per day above maintenance. Leaner individuals and beginners can often get away with a slightly larger surplus because their bodies partition nutrients toward muscle more effectively, while more advanced trainees benefit from keeping the surplus smaller to limit unnecessary fat gain.
No study has pinpointed a single optimal surplus for everyone. The key principle is that your body needs to sense energy abundance, both at the cellular level to activate growth pathways and at the hormonal level to maintain an anabolic environment. Eating at maintenance can support some muscle growth, especially in beginners, but a modest surplus removes the metabolic friction and gives your body the clearest possible signal that it’s safe to invest in new tissue.