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Proteins are composed of 20 amino acids; nine are essential (cannot be synthesized by the body) and must be obtained from diet. Protein quality is assessed by amino acid completeness and digestibility — animal proteins are generally complete, while most plant proteins are limiting in one or more essential amino acids. Nitrogen balance — the difference between nitrogen intake (protein consumption) and nitrogen excretion (urine, feces, sweat) — reflects whether the body is building, maintaining, or catabolizing lean tissue.
Compare the amino acid profiles of animal and plant protein sources. Calculate nitrogen balance scenarios for different physiological states (growth, illness, athletic training) to understand how protein requirements shift.
Proteins are built from 20 amino acids linked in sequence by peptide bonds. Of these 20, the body can synthesize 11 from common metabolic precursors — these are the nonessential (or dispensable) amino acids. The remaining nine — histidine, isoleucine, leucine, lysine, methionine, phenylalanine, threonine, tryptophan, and valine — are called essential because humans lack the enzymes to construct their carbon skeletons from scratch. They must come from food. When any one essential amino acid is absent or insufficient, protein synthesis in the body stalls — the whole chain cannot be assembled if even one link is missing.
Protein quality describes how well a dietary protein source supplies the essential amino acids relative to human requirements. Animal proteins (meat, dairy, eggs) are generally "complete" — they contain all nine essential amino acids in proportions close to human needs, with high digestibility. Most plant proteins are "limiting" in one or more essential amino acids: rice lacks lysine, legumes lack methionine, corn lacks tryptophan and lysine. Protein quality is now assessed using the Digestible Indispensable Amino Acid Score (DIAAS), which accounts for both amino acid content and digestibility. Well-planned plant-based diets can meet all essential amino acid requirements by combining complementary sources (e.g., rice + beans) or relying on soy protein, which has a complete amino acid profile.
Nitrogen balance is the clinical framework for assessing whether protein intake matches the body's needs. Since protein is approximately 16% nitrogen by mass, measuring nitrogen intake and excretion provides a proxy for net protein synthesis or breakdown. Positive nitrogen balance — intake exceeds excretion — means the body is accumulating protein, occurring normally during growth, pregnancy, and athletic training. Negative nitrogen balance — excretion exceeds intake — indicates net protein catabolism, seen in illness, injury, inadequate intake, or severe caloric restriction. Neutral balance reflects protein maintenance, which is the goal for healthy, non-growing adults.
Protein requirements shift substantially with physiological state. Healthy sedentary adults need roughly 0.8 g/kg body weight per day to maintain neutral nitrogen balance. Athletes, particularly those doing resistance training, benefit from 1.6–2.2 g/kg/day to support muscle protein synthesis. Illness, surgery, burns, and other catabolic states can increase requirements dramatically — hospitalized patients may need 1.5–2.5 g/kg/day. Infants and pregnant women have elevated requirements relative to body weight due to growth demands.
A key misconception is that eating more protein automatically builds more muscle. Protein synthesis is rate-limited: once the body's anabolic machinery is saturated (roughly 20–40 g of high-quality protein per meal for most adults), additional protein in that meal provides no additional muscle-building signal. Excess protein is not stored as protein; it is deaminated, and the carbon skeleton is oxidized for energy or converted to glucose or fat. The nitrogen component is excreted as urea. Protein is not a magical anabolic hormone — resistance training provides the stimulus; adequate protein (not maximal protein) provides the building material.