Protein (4 kcal/g)
Protein is composed of amino acids — the building blocks of muscle, enzymes, hormones, and immune factors. It has the highest thermic effect of any macronutrient (20–35% of calories burned during digestion) and the strongest satiety effect per calorie.
Evidence-based targets: 1.6–2.2 g/kg body weight for active individuals; 0.8 g/kg minimum RDA for sedentary adults. Complete protein sources include meat, fish, eggs, dairy, and soy. Incomplete sources (legumes, grains) should be varied for all amino acids.