Nutrition

Fuel for Muscle Growth: Calories, Protein & Macros

Calories, protein, fat and carbohydrates: the right amounts, at the right time, are needed from the right sources to optimise muscle growth.

Muscle can be built while you're in a calorie deficit, but a moderate surplus offers the best environment for maximising growth.
All three macronutrients are important. Proteins provide the building blocks for muscle, while carbohydrates and fat provide the fuel.
However, the quality of your macros matters. Whole foods and animal proteins are generally higher-quality, but there are other options.

The kilocalorie (kcal for short) is the unit of energy for food. Understanding the relationship between the amount of energy you eat (your "energy intake") and the amount of muscle you grow is a little more complicated than the "eat big, get big" mantra.

Once you've got the calories roughly right, it's balancing the protein, carbs, and fat, and then the quality of that protein that actually determines how much of that energy turns into muscle.

Defining energy balance

Energy balance is the fundamental equation that determines body mass. It's calculated by subtracting energy expenditure from energy intake. When intake exceeds expenditure, that's an energy surplus. When expenditure exceeds intake, that's an energy deficit. When the two are approximately equal, that's energy balance.

This simple equation gets more complicated once you factor in that both sides of the equation change day to day, are nearly impossible to measure accurately, and are influenced by other processes (including muscle growth itself). Energy expenditure comes in four forms:

Planned exercise
Any other movement that isn't planned exercise
Digesting food
Maintaining basic bodily functions. Your heart, lungs, stomach, and intestinal muscles don't contract themselves (they need energy too).

You can measure energy expenditure with expensive lab equipment. But in practice, it's normally just estimated. The simplest way is based on body weight, though more complex equations exist.

Measuring energy intake has its own problems. Errors in judging portion size, along with variations in food composition, nutritional content, and cooking method, all affect how accurate your intake estimate really is. Even with kitchen scales, diligent records, and a good tracking app, measuring intake precisely remains genuinely difficult.^[1]

Fortunately, over the long run, your hypothalamus (and several other bodily systems) keeps a fairly accurate running tally of how many calories you're eating and burning, through a process known as "biofeedback" or appetite regulation. A measuring tape and a reliable set of bathroom scales are usually all you need to track the result of what you eat over time.

Energy Surplus: Necessity or Luxury for Muscle Growth?

Resistance training combined with higher energy intakes generally leads to greater muscle hypertrophy than lower calorie intakes. This doesn't mean gaining muscle while eating fewer calories is impossible — just more difficult.^[2]

It's also worth noting that more calories doesn't always mean more muscle. The exact amount of energy needed to maximise growth can't be estimated with an equation, as it depends on individual needs and your macronutrient intake

Running a small to moderate surplus (roughly 10–20% above your estimated maintenance) is a sensible place to start. Eating well beyond this tends to just add body fat, which for most people isn't the goal.

Muscle Growth in an Energy Deficit

You can lose fat and gain muscle at the same time while in an energy deficit. But you'll likely gain less muscle, and gain it less easily, than if you were in a surplus or at maintenance levels.

This becomes especially true with severe calorie deficits (over 500 calories a day), where gaining muscle becomes very difficult, and simply maintaining muscle mass (or minimising its loss) becomes the more realistic goal.

Protein intake is the key determining factor. In an energy deficit, higher protein intakes are required (above 1.4 g/kg/day) because maintaining muscle becomes more metabolically "expensive": your body will break muscle down for energy to keep vital organs running (a process known as muscle protein breakdown).^[3]

The Three Macronutrients

Protein, carbohydrate, and fat each serve a distinct role in muscle growth, but they're rarely consumed in isolation. Most meals, and even most individual foods, contain some combination of all three. But how these macronutrients interact creates a unique environment within your muscle that either supports or hinders growth.

Protein: the muscle builder

Protein acts as both a building block and a signalling molecule for muscle growth; it's the single most important nutritional factor for maximising it. There's no total agreement on the ideal amount, but most experts think that 0.8 grams per kg body mass a day is the bare minimum, with little added benefit from eating beyond 2.4 grams per kg body mass a day, regardless of how much you're training.^[4]

The practical "sweet spot" for most people sits around 1.4–2.0 grams per kg body mass a day — trending toward the higher end of that range, or beyond it, when you're in an energy deficit.

Split across three main meals and two snacks, this works out to roughly 0.3–0.4 g per kg per meal and 0.15–0.2 g per kg per snack — or, more simply, aim for at least 20 g of high-quality protein every time you eat.

Protein quality: why source matters

Protein "quality" is arguably more important than hitting a raw gram target, and depends on two factors: digestibility (how readily your body actually absorbs it) and how completely it supplies each of the nine essential amino acids (EAA).

Several scoring systems exist to measure this, the most common being the Digestible Indispensable Amino Acid Score (DIAAS). Sources are scored from 0 (lowest quality) upward, with "excellent" proteins scoring over 1.0 and "low" quality proteins scoring under 0.75. A score of 1.0 means the protein supplies 100% of your digestible EAA needs; a score of 0.7 means only 70% — so you'd need to eat more of it to close the gap.^[5]

Simply eating more of a low-quality protein isn't always practical, but there are better strategies available. As a rule of thumb, animal sources (i.e., meat, fish, dairy, eggs) are "complete" proteins, containing all nine digestible EAAs in adequate amounts.

Plant proteins are often "incomplete," typically falling short in one of two amino acids: cereals (wheat, rice, maize) tend to be low in lysine, while legumes (beans, pulses, soy) tend to be low in methionine. Combining the two (e.g., rice and lentils, beans on toast, hummus and pita) creates a complete amino acid profile through a strategy known as protein "complementation".^[6]

Processing is a second lever on protein quality. Despite its reputation, processing isn't inherently bad. There's a real difference between minimal processing and "ultra-processing" Soaking, drying, cooking, and fermenting are all technically processing, done for millennia, and some of these methods actually improve nutritional profile, palatability, and protein quality.

Fast and slow proteins

Digestion speed matters too. Different proteins release amino acids into your bloodstream at different rates: "fast" versus "slow" digesting.

Whey and casein, the two principal dairy proteins (together accounting for over half of global protein supplement sales), are a well-known example: whey digests quickly, spiking blood EAA levels and stimulating muscle growth rapidly, while casein forms a gel-like substance in the stomach and releases more slowly. Both are high-quality proteins delivering similar total EAA, just at different speeds.^[7]

At higher doses (over 20 g), this difference in speed doesn't affect muscle growth much. But at lower doses, faster proteins tend to stimulate growth more efficiently, while slower proteins are better at maintaining protein balance and delaying hunger across longer gaps between meals (over six hours, like overnight).

Blending the two is a smart way to capture both benefits; this is how they occur naturally anyway. Dairy products are the best source of these proteins if you're going for whole foods.

Carbohydrates: fuelling performance and recovery

Carbohydrates supply the energy needed to fuel your training sessions and support recovery. They also "spare" muscle from being broken down for energy, and reduce muscle protein breakdown.

After you eat carbohydrate it's stored in muscle as glycogen. Enough for roughly an hour or so of moderate-to-hard exercise. So, under-eating carbohydrates will likely impair your ability to train at full capacity and, in turn, impair muscle growth (indirectly). Carbohydrates also stimulate insulin release, a hormone that supports muscle growth, which is one reason carbs are so often advised to be eaten alongside protein.^[8]

At a minimum, aim for around 20 g of carbohydrate alongside protein at each meal or snack. This is usually enough to stimulate insulin and cover basic energy needs. Broader recommendations run considerably higher, from 3–5 grams per kg body mass per day up to 6–10 g/kg/day for those doing a lot of exercise. The top end of that range is generally reserved for endurance athletes.

Fats: The overlooked supporter of muscle growth

Dietary fat plays a supporting role in muscle growth that's often overlooked. Beyond providing energy, fat helps male hormones, maintains the structure of your cells, and helps with the absorption of certain vitamins.

Testosterone is important for muscle maintenance and growth in both men and women. It requires certain fats (including cholesterol and saturated fat) to be produced, and diets with adequate fat intake tend to sustain healthier testosterone levels.^[9]

As a general guide, aim for 15–30% of daily energy intake from fat, prioritising high-quality sources just as you would with protein. Monounsaturated and polyunsaturated fats are a good foundation, and there's solid evidence that omega-3 fatty acids support muscle growth (particularly in older adults).

Saturated fat is fine in modest amounts for most people (under 10% of energy intake, roughly 20g a day) when it comes from natural, minimally processed sources; trans fats should be avoided altogether. Too much fat (both dietary and body fat) can impair muscle growth. So keeping an eye on body weight, waistline, and total energy intake alongside food quality is worth doing.