Protein is essential for tissue repair, muscle growth, and immune function, with most sports nutrition guidelines recommending 1.2–2.0 g per kg of body weight per day for athletes. Endurance athletes typically need 1.2–1.4 g/kg to repair tissue and support immune function, while strength athletes require 1.6–2.2 g/kg to support muscle hypertrophy and recovery. Both groups benefit from distributing protein evenly across four to five meals throughout the day. Training status also affects needs: beginners can build muscle even in a slight deficit through body recomposition, intermediate athletes need a small surplus for muscle gain, and advanced athletes require very precise nutrition to continue progressing.
A complete protein contains all nine essential amino acids in adequate amounts: histidine, isoleucine, leucine, lysine, methionine, phenylalanine, threonine, tryptophan, and valine. Animal sources like meat, fish, eggs, and dairy are complete, as are soy and quinoa. The biological value (BV) of a protein measures the proportion of absorbed protein that is retained and used by the body, with whole egg as the reference at 100, whey protein at about 104, casein at 77, and soy at 74. The newer Digestible Indispensable Amino Acid Score (DIAAS) evaluates protein quality based on the digestibility of individual essential amino acids, with scores above 100 considered excellent. Plant proteins are generally lower in leucine and may have lower digestibility, but combining sources such as rice and pea protein creates a complete amino acid profile.
Branched-chain amino acids (BCAAs) — leucine, isoleucine, and valine — make up about 35% of muscle protein, with leucine serving as the primary trigger for the mTOR signaling pathway that activates muscle protein synthesis. Research suggests that 20–40 g of high-quality protein per meal is needed to maximally stimulate this response, with a leucine threshold of approximately 2.5–3 g per meal being key. Whey protein is a fast-digesting, complete protein derived from milk, available as concentrate (70–80% protein, containing some fat and lactose), isolate (90%+ protein with minimal fat and lactose, ideal for the lactose-intolerant), and hydrolysate (pre-digested for fastest absorption, most expensive). Casein, by contrast, is a slow-digesting milk protein that forms a gel in the stomach, providing sustained amino acid release over 6–8 hours, and is often recommended before bed to support overnight muscle protein synthesis.