Proteins are large, complex molecules composed of amino acids linked by peptide bonds. They provide 4 kcal per gram and serve crucial structural, functional, and regulatory roles in the body. Proteins are the main structural components of cells and tissues, form enzymes and hormones, transport molecules, and support immune function.
Amino Acids
Twenty standard amino acids are used to build human proteins. Nine are essential (must be obtained from the diet): histidine, isoleucine, leucine, lysine, methionine, phenylalanine, threonine, tryptophan, and valine. Eleven are non-essential (can be synthesized by the body): alanine, arginine, asparagine, aspartic acid, cysteine, glutamic acid, glutamine, glycine, proline, serine, and tyrosine. Conditionally essential amino acids — such as arginine, cysteine, glutamine, glycine, proline, and tyrosine — become essential under certain conditions (stress, disease, rapid growth, prematurity).
Leucine is particularly important because it potently stimulates muscle protein synthesis through activation of the mTOR pathway, making it a key signal for anabolism.
Protein Quality
Protein quality is determined by digestibility and amino acid profile. Complete proteins contain all nine essential amino acids in adequate proportions — animal sources (meat, poultry, fish, eggs, milk, cheese, yogurt) and soy are complete. Incomplete proteins are low in one or more essential amino acids — most plant proteins (grains, legumes, nuts, seeds) are incomplete. The limiting amino acid is the one in shortest supply relative to requirements.
Complementary proteins combine incomplete plant proteins to provide all essential amino acids. Examples: rice and beans (grains + legumes), hummus and pita (legumes + grains), and peanut butter on whole wheat bread. However, the concept of complementing proteins at every meal is outdated — consuming a variety of plant proteins over the course of a day ensures adequate essential amino acid intake.
Protein digestibility-corrected amino acid score (PDCAAS) is the standard measure of protein quality. It considers the amino acid profile relative to human requirements and the true digestibility of the protein. Scores range from 0 to 1.0. Casein, soy, egg white, and whey have scores of 1.0 (the highest). Most plant proteins have scores of 0.5–0.9.
Digestion and Absorption
Protein digestion begins in the stomach, where hydrochloric acid denatures proteins and activates pepsinogen to pepsin, which hydrolyzes proteins into smaller peptides. In the small intestine, pancreatic proteases (trypsin, chymotrypsin, carboxypeptidase, elastase) continue protein breakdown into free amino acids and small peptides. Brush border peptidases at the enterocyte surface complete digestion. Amino acids, dipeptides, and tripeptides are absorbed via specific transporters and enter the portal circulation.
Protein Metabolism
The body maintains a dynamic equilibrium of protein turnover — continuous synthesis and breakdown of body proteins. The amino acid pool in blood and cells supplies substrates for protein synthesis and can be used for energy when needed. Excess amino acids are deaminated (nitrogen removed as urea) and the carbon skeletons are used for energy, glucose synthesis (gluconeogenesis), or fatty acid synthesis.
Nitrogen balance is the difference between nitrogen intake (from dietary protein) and nitrogen excretion (primarily urea in urine). Positive nitrogen balance (intake > excretion) occurs during growth, pregnancy, and recovery from illness. Negative nitrogen balance (excretion > intake) occurs during starvation, illness, trauma, and with inadequate protein intake.
Dietary Recommendations
The Recommended Dietary Allowance (RDA) for protein is 0.8 g per kg of body weight per day for healthy adults. For a 70 kg (154 lb) adult, this is 56 g per day. Athletes and active individuals require more: 1.2–1.7 g/kg/day depending on training intensity and type. Older adults (age 65+) benefit from 1.2–1.5 g/kg/day to counteract sarcopenia (age-related muscle loss). Growth, pregnancy, and lactation also increase requirements. The Acceptable Macronutrient Distribution Range (AMDR) for protein is 10–35% of total daily calories.
Food Sources
Animal sources. Meat (beef, pork, lamb), poultry (chicken, turkey), fish and seafood, eggs, milk, and dairy products. Animal proteins are complete, highly digestible, and contain all essential amino acids in favorable proportions.
Plant sources. Legumes (beans, lentils, chickpeas, peas), soy products (tofu, tempeh, edamame, soy milk), nuts and seeds, whole grains (quinoa is a complete protein, others are incomplete), and seitan (wheat gluten). Plant-based diets can meet all protein needs when a variety of foods is consumed and energy intake is adequate.
Health Effects
Adequate protein intake is necessary for maintaining muscle mass, bone health, immune function, and satiety. Higher protein diets (25–30% of calories) may help with weight management by increasing fullness and preserving lean mass during calorie restriction. In older adults, adequate protein combined with exercise is essential for preventing sarcopenia.
Very high protein intake (above 35% of calories, or >2 g/kg/day) is not harmful for healthy individuals but may be inadvisable in those with kidney disease, as it increases glomerular filtration rate. There is no evidence that high protein intake damages healthy kidneys.
Summary
Proteins are essential for virtually all biological functions. The nine essential amino acids must be obtained from the diet. Animal proteins are complete; plant proteins can be combined to meet requirements. The RDA is 0.8 g/kg/day for healthy adults, with higher needs for athletes, older adults, and during growth and illness. Protein-energy malnutrition remains a global health concern. A balanced diet including a variety of protein sources supports optimal health.