Protein — Functions, Quality, and Requirements

Protein is the macronutrient built from amino acids, of which there are 20; 9 are essential (EAAs) — they cannot be made by the body and must come from food. Protein provides 4 kcal per gram (see Macronutrients as Energy Sources). Beyond energy, protein's roles are structural and functional: it builds and repairs muscle and connective tissue, forms enzymes, hormones, antibodies, and transport molecules, and maintains fluid and acid-base balance.

Protein quality reflects how well a food's amino-acid profile and digestibility match human needs. Complete proteins (animal sources — meat, fish, eggs, dairy — plus soy) contain all nine EAAs in adequate amounts. Incomplete proteins (most individual plant sources) are low in one or more EAAs, but combining varied plant foods across the day (legumes, grains, nuts, seeds) supplies the full set; intentional "protein combining" at a single meal is not required.

Leucine is the EAA that most strongly triggers muscle protein synthesis (MPS). The ISSN notes that an effective protein dose should contain roughly 700–3000 mg of leucine alongside a balanced array of the other EAAs.

Requirements (per kg body weight per day):

• Sedentary adult RDA: 0.8 g/kg/day (Institute of Medicine Dietary Reference Intakes, 2002/2005, still current: about 56 g/day for the reference man and 46 g/day for the reference woman; a floor to prevent deficiency, not an optimum for active people). The DRIs' acceptable range for protein is 10–35% of calories for adults.
• Federal population goal (Dietary Guidelines for Americans 2025–2030): 1.2–1.6 g/kg/day, adjusted to individual calorie needs (see Dietary Guidelines for Americans — Overview).
• General exercising/active individuals: 1.4–2.0 g/kg/day (ISSN) to build and maintain muscle.
• Resistance-trained people in a caloric deficit: 2.3–3.1 g/kg/day to preserve lean mass (see Protein-Sparing in a Caloric Deficit).
• Endurance athletes: roughly 1.2–2.0 g/kg/day (ACSM/AND/DC).

Per-dose: about 0.25–0.4 g/kg (roughly 20–40 g) of high-quality protein per meal is a practical target. It is not a ceiling. The old idea that the body can only use ~20–30 g at a sitting, with the rest "wasted", doesn't hold: after a workout, 100 g produced a bigger and longer-lasting (≥12 h) muscle-building response than 25 g, with negligible extra amino-acid burning (Trommelen et al. 2023, in untrained young men). Larger meals don't hit a wall; they take longer to process. Total daily protein is the main driver (gains plateau around ~1.6 g/kg/day; Morton et al. 2018). Spreading it across ~4 meals is a sensible default (Schoenfeld & Aragon 2018: ~0.4 g/kg/meal × ≥4 meals) — see Protein Distribution Across the Day.

• Healthy adults who train for muscle, strength, endurance or fat loss.
• People in a calorie deficit who want to keep muscle: the higher targets apply (see Protein-Sparing in a Caloric Deficit).
• Older adults: the 2025–2030 Dietary Guidelines note older adults need equal or greater protein despite needing fewer calories.

• Kidney disease or a prescribed protein restriction: follow your clinician or renal dietitian. These targets are for healthy people.
• Pregnancy, breastfeeding, under-18s: needs differ; get individual advice.
• Higher body fat: g/kg of total body weight overestimates needs. A practical workaround is to use a goal or adjusted body weight (an editorial convention, not a guideline figure).
• The ~1.6 g/kg plateau is a group average (Morton 2018 reported a 95% confidence range up to about 2.2 g/kg), so some people benefit from more.
• Trommelen 2023 measured short-term muscle protein synthesis in young untrained men, not long-term growth.

Daniel, 80 kg, lifting 4×/week, maintaining weight. Target ~1.6 g/kg → ~130 g/day.

Four feedings of ~30–45 g sit inside the practical ~0.4 g/kg/meal pattern. If Daniel starts a cut, he can move toward 2.3 g/kg (~185 g) by enlarging two meals. Food protein values are approximate; check labels or a food database.

Next lesson: Protein Distribution Across the Day. It covers how to spread that daily total across meals and around training.

• 4 kcal/g; built from 20 amino acids, 9 essential (EAAs).
• Complete proteins: animal sources + soy; plant sources are usually incomplete but complement over a day.
• Leucine is the primary MPS trigger; aim ~700–3000 mg leucine per dose.
• RDA 0.8 g/kg/day (sedentary floor); DGA 2025–2030 general goal 1.2–1.6; active 1.4–2.0; cutting/trained 2.3–3.1; endurance ~1.2–2.0.
• Per-meal target ~0.25–0.4 g/kg (≈20–40 g) is practical, not a ceiling: 100 g gave a bigger, ≥12 h response than 25 g (Trommelen 2023). Daily total (~1.6 g/kg/day plateau) matters most.