Macros is short for macronutrients: protein, carbohydrates, and fat, the three nutrient categories that provide calories (alcohol is a fourth, less commonly tracked source). Each gram of protein and carbohydrate provides 4 calories, while fat provides 9 calories per gram.
Tracking macros means setting individual gram targets for each, rather than just tracking total calories. This matters because two diets with identical calories can produce very different results depending on how those calories are split, particularly for protein, which is the macro most directly tied to muscle retention and growth.
Physiological roles of each macro
Calories are only part of the story; each macro also does specific jobs in the body that pure calorie counting ignores:
- Protein supplies the amino acids used to repair and build muscle tissue, enzymes, and hormones. It’s the macro most directly linked to muscle protein synthesis, and unlike carbs and fat, the body has no dedicated storage depot for it, so intake needs to be reasonably consistent day to day.
- Carbohydrates are the body’s preferred fuel for moderate-to-high intensity exercise, stored in muscle and liver as glycogen. Adequate carb intake supports training performance and glycogen replenishment between sessions, and it also spares protein from being burned for energy.
- Fat is essential for hormone production (including testosterone), the absorption of fat-soluble vitamins (A, D, E, K), and cell membrane structure. Going too low on fat for extended periods can impair hormonal health, which is a key reason fat has a practical floor even in an aggressive deficit.
The 80/20 tracking hierarchy
Not all macro precision matters equally. A practical hierarchy for where to spend tracking effort:
- Total calories: the single largest lever for both weight loss and weight gain; get this wrong and macro precision underneath it can’t compensate.
- Protein: hit your gram target consistently; this is the macro worth tracking most precisely because both too little and too much have real, distinct effects on body composition outcomes.
- Fat: keep it above the physiological floor (roughly 20% of calories, more if hormonal symptoms appear); beyond that floor, exact fat grams matter less.
- Carbohydrates: mostly a “fill the remaining calories” macro once protein and fat are set; the exact split relative to fat has limited practical impact for most non-endurance-sport goals.
This means it’s reasonable to track protein grams closely while eyeballing the carb/fat split day to day, as long as total calories stay on target, roughly the 80/20 rule of where precision actually changes outcomes.
Energy content vs. substrate utilization
Calories per gram (4/4/9) describes energy content, but it doesn’t fully describe how the body uses each macro. Protein has a notably higher thermic effect (see TEF), meaning a larger share of its calories are spent digesting it, so its “usable” calories are somewhat lower than its label value suggests. Carbohydrate and fat are metabolized more efficiently for storage or immediate energy, with carbs being preferentially used during higher-intensity training and fat during rest and lower-intensity activity. This is why two diets with identical total calories, but different macro splits, can produce measurably different results: the energy value on the label is the same, but how the body processes and prioritizes that energy is not.
A common starting point is 1.6 to 2.2g of protein per kg of bodyweight (see our protein calculator), 20 to 35% of calories from fat, and the remainder from carbohydrates, adjusted based on training volume and personal preference.