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 macronutrient-energy). 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 (a floor to prevent deficiency, not an optimum for active people).
• 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-deficit).
• Endurance athletes: roughly 1.2–2.0 g/kg/day (ACSM/AND/DC).

Per-dose: about 0.25 g/kg or an absolute 20–40 g of high-quality protein maximally stimulates MPS at a sitting; larger amounts give diminishing returns for muscle building. Distributing intake every 3–4 hours is recommended (see protein-distribution).

• 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); active 1.4–2.0; cutting/trained 2.3–3.1; endurance ~1.2–2.0.
• Per-meal optimum ~0.25 g/kg or 20–40 g; distribute every 3–4 h.

• protein-distribution — how to spread the daily total across meals.
• muscle-gain-nutrition — protein within an energy surplus.
• protein-sparing-deficit — higher protein to protect muscle while cutting.
• muscle-hypertrophy — MPS is the cellular basis of muscle growth.
• protein-supplementation — whole food vs. powder for meeting the target.