Essential amino acid
Essential amino acids are a group of nine amino acids that the human body cannot synthesize de novo at a sufficient rate to meet physiological demands, necessitating their acquisition through dietary sources. These include histidine, isoleucine, leucine, lysine, methionine, phenylalanine, threonine, tryptophan, and valine.[1] They serve as fundamental building blocks for proteins and play critical roles in metabolic processes, such as hormone regulation, enzyme function, and tissue repair.[2] The classification of amino acids as essential stems from the absence of specific biosynthetic pathways in humans for these compounds, distinguishing them from non-essential amino acids like alanine and glutamine, which the body can produce from other precursors.[3] For instance, tryptophan cannot be synthesized de novo because humans lack the enzymes of the shikimate pathway, while lysine requires external supply because humans lack the enzymes for its synthesis from aspartate.[1] Infants and individuals with certain metabolic disorders may have additional requirements, such as conditionally essential amino acids like arginine under stress conditions.[4] Dietary proteins are categorized as complete if they contain all essential amino acids in adequate proportions, typically found in animal sources like meat, eggs, and dairy, whereas plant-based proteins are often incomplete and require complementary combinations, such as grains with legumes, to provide a full profile.[5] The recommended daily intake varies by amino acid and life stage; for example, adults require approximately 30 mg/kg body weight of lysine and 5 mg/kg of tryptophan to support protein synthesis and prevent deficiencies.[2] Supplementation with essential amino acids can enhance muscle protein synthesis, particularly in older adults or those with malnutrition, but must be balanced to avoid imbalances that impair absorption of other nutrients.[6] Deficiencies in essential amino acids can lead to impaired growth, reduced immune response, and conditions like kwashiorkor in severe protein-energy malnutrition, underscoring their importance in maintaining overall health.[3] Research emphasizes the need for diverse diets to ensure adequate intake, with global health organizations recommending 0.8–1.0 grams of protein per kilogram of body weight daily for adults to cover essential amino acid needs.[5]Fundamentals
Definition
Amino acids are the fundamental building blocks of proteins, with 20 standard amino acids commonly incorporated into proteins in humans.[1] These include alanine, arginine, asparagine, aspartic acid, cysteine, glutamic acid, glutamine, glycine, histidine, isoleucine, leucine, lysine, methionine, phenylalanine, proline, serine, threonine, tryptophan, tyrosine, and valine.[1] Essential amino acids are defined as those that an organism cannot synthesize de novo at a rate sufficient to meet its metabolic needs, requiring dietary intake to maintain health and support protein synthesis.[1] In contrast, non-essential amino acids are those that the body can produce endogenously from other metabolic intermediates, such as through transamination or other biosynthetic processes.[7] Some amino acids, known as conditionally essential, are typically non-essential but become necessary from the diet under specific physiological conditions, such as illness, stress, or rapid growth, when endogenous synthesis cannot keep pace with demand; for example, arginine may fall into this category during periods of trauma or in neonates.[1] In adult humans, the nine essential amino acids are histidine, isoleucine, leucine, lysine, methionine, phenylalanine, threonine, tryptophan, and valine.[7]Historical Discovery
The concept of essential amino acids emerged in the early 20th century through studies on protein nutrition in animals. In 1901, British biochemist Frederick Gowland Hopkins isolated tryptophan from casein. In 1906–07, he demonstrated its indispensability by feeding mice diets deficient in this amino acid, resulting in stunted growth and poor health that could be reversed upon supplementation. This work, building on earlier observations of dietary deficiencies, highlighted that not all protein components were interchangeable and laid the groundwork for recognizing specific amino acids as vital nutrients. Hopkins's experiments, published in the Journal of Physiology, influenced subsequent research by emphasizing the need for complete protein hydrolysis products in diets.[8] In the 1930s, American biochemist William C. Rose advanced this field through rigorous experiments on young rats at the University of Illinois. Rose developed synthetic diets composed of purified amino acids derived from hydrolyzed proteins, systematically testing mixtures to identify those required for normal growth and nitrogen retention. His studies revealed ten indispensable amino acids for rats: arginine, histidine, isoleucine, leucine, lysine, methionine, phenylalanine, threonine (which he discovered and isolated in 1935), tryptophan, and valine; omission of any led to growth failure, while supplementation restored it. These findings, detailed in a series of papers in the Journal of Biological Chemistry from 1931 to 1938, established a quantitative framework for essentiality based on animal models and shifted nutritional science toward precise amino acid requirements.74369-3/fulltext) The 1940s and 1950s saw confirmation of essential amino acids in humans via nitrogen balance studies led by Rose. Adult male volunteers were maintained on protein-free basal diets supplemented with mixtures of pure L-amino acids, with individual omissions tested over periods of 1-2 weeks to assess nitrogen equilibrium. Rose's team found that eight amino acids—isoleucine, leucine, lysine, methionine, phenylalanine, threonine, tryptophan, and valine—were required to prevent negative nitrogen balance, while others like histidine could be spared in these short-term trials. This culminated in the 1957 identification of these eight as essential for human adults, as summarized in Rose's comprehensive review, marking a milestone in translating animal data to human nutrition.[9] Understanding evolved further in the late 20th century, particularly regarding histidine. Initially classified as non-essential for human adults based on Rose's studies, histidine was reclassified as essential for infants following 1978 World Health Organization evaluations of growth data from histidine-deficient formulas, which showed impaired development reversible by supplementation. This adjustment, informed by pediatric nitrogen balance trials, underscored age-specific needs and prompted ongoing research into essentiality variations, such as in aging populations.[10]Essentiality in Humans
List of Essential Amino Acids
Humans require nine essential amino acids that cannot be synthesized endogenously in sufficient quantities and must be obtained from the diet: histidine (His), isoleucine (Ile), leucine (Leu), lysine (Lys), methionine (Met), phenylalanine (Phe), threonine (Thr), tryptophan (Trp), and valine (Val).[1] These amino acids, with leucine being the most prevalent at around 10% and tryptophan the least abundant at about 1.3%.[11] The essential amino acids can be categorized based on their side-chain structures. The branched-chain amino acids (BCAAs)—isoleucine, leucine, and valine—feature non-linear aliphatic hydrocarbon chains that confer hydrophobic properties, aiding in protein folding and stability.[1] The aromatic amino acids phenylalanine and tryptophan contain benzene or indole rings, respectively, which contribute to ultraviolet absorption and participate in electron transfer processes within proteins.[1] Methionine is distinguished by its thioether side chain, providing a source of sulfur for methylation reactions and antioxidant defense.[1] Histidine possesses an imidazole ring capable of buffering protons at physiological pH, while lysine has a basic ε-amino group on a long alkyl chain, enabling electrostatic interactions in protein structures.[1] Threonine includes a β-hydroxyl group, making it polar and involved in phosphorylation sites.[1] Beyond their universal role in protein biosynthesis, each essential amino acid supports specific physiological functions. Histidine serves as a precursor for histamine, which regulates immune responses and gastric acid secretion.[1] Isoleucine contributes to hemoglobin formation and energy regulation in muscles.[1] Leucine stimulates muscle protein synthesis via the mTOR pathway and helps regulate blood glucose levels.[1] Lysine is vital for collagen and carnitine production, supporting tissue repair and fatty acid metabolism.[1] Methionine acts as a methyl donor in one-carbon metabolism and is a precursor for cysteine and taurine.[1] Phenylalanine is a precursor for tyrosine, which is used in synthesizing catecholamines and thyroid hormones.[1] Threonine supports mucin production for gut integrity and immune function.[1] Tryptophan is the sole precursor for serotonin (a neurotransmitter) and melatonin (a sleep regulator).[1] Valine, like other BCAAs, provides energy during prolonged exercise and maintains nitrogen balance.[1]Biochemical Basis for Essentiality
The essentiality of amino acids in humans stems from the absence of specific enzymatic machinery required for their de novo biosynthesis from precursors in central metabolic pathways, such as glycolysis and the tricarboxylic acid cycle. Unlike non-essential amino acids, which can be produced through relatively simple transamination or amidation reactions using ubiquitous intermediates like glucose-derived pyruvate or glutamate, essential amino acids demand multi-step pathways involving specialized enzymes that humans lack. For example, the nine essential amino acids—histidine, isoleucine, leucine, lysine, methionine, phenylalanine, threonine, tryptophan, and valine—cannot be assembled because mammalian genomes do not encode the necessary catalysts, such as those for ring formation in aromatic amino acids or chain elongation in lysine.[1][12] Specific enzymatic deficiencies highlight these barriers. Branched-chain amino acids (isoleucine, leucine, and valine) require acetolactate synthase and ketol-acid reductoisomerase to condense pyruvate and α-ketobutyrate derivatives, enzymes absent in humans and other metazoans, rendering these amino acids dependent on dietary sources rather than derivable from central metabolism.[13] Similarly, methionine synthesis is impossible due to the lack of enzymes for incorporating inorganic sulfur into its thioether structure; while humans possess the transsulfuration pathway to convert methionine to cysteine via cystathionine β-synthase and γ-lyase, the reverse process from cysteine to methionine does not occur, making methionine irreplaceable endogenously.[14] For aromatic amino acids, phenylalanine and tryptophan necessitate complex polyketide-like assemblies absent in mammals—phenylalanine's benzene ring cannot be formed from simple precursors, though it can be hydroxylated to tyrosine via the tetrahydrobiopterin-dependent phenylalanine hydroxylase; however, this conversion is insufficient if phenylalanine intake is low, underscoring phenylalanine's essential status.[15] Lysine and threonine further exemplify this, as their biosynthesis in bacteria involves the diaminopimelate and aspartate-derived pathways, respectively, with key reductases and dehydratases missing in human metabolism.[16] From an evolutionary standpoint, the loss of these biosynthetic genes in humans and other animals reflects an adaptation to heterotrophic lifestyles, where nutrient acquisition from prey or diet became more efficient than maintaining energy-intensive synthetic pathways present in autotrophs like plants and microorganisms. Comparative genomics reveals that mammalian lineages shed operons for essential amino acid production—such as the ilv operon for branched-chains or trp operon for tryptophan—over billions of years, prioritizing catabolic and regulatory functions instead.[16] This genomic streamlining conserved metabolic resources but imposed strict dietary requirements.[12] Although the gut microbiota harbor bacteria capable of synthesizing essential amino acids through complete pathways, their net contribution to human supply remains limited, as microbial products are primarily utilized locally or poorly absorbed across the intestinal epithelium, with dietary proteins serving as the dominant source. Evidence from animal models and human studies indicates variable microbial contributions to specific essential amino acids, such as approximately 19-22% to leucine under moderate protein intake conditions, but this is generally insufficient to meet basal needs during fasting or deficiency, emphasizing the reliance on exogenous intake.[17][18][19]Biosynthetic Limitations
Synthesis of Non-Essential Amino Acids
Non-essential amino acids are synthesized endogenously in human cells through dedicated biosynthetic pathways that utilize intermediates from central metabolic routes such as the citric acid cycle and glycolysis, ensuring a steady supply for protein synthesis and other cellular functions. These pathways primarily involve transamination reactions, where an amino group from glutamate is transferred to a keto acid precursor, or direct amination and reduction steps, with glutamate often serving as the primary nitrogen donor. The synthesis is compartmentalized, occurring mainly in the liver, kidney, and muscle tissues, and is tightly regulated to match physiological demands.[20] A key family of non-essential amino acids derives from citric acid cycle intermediates, particularly α-ketoglutarate and oxaloacetate. Glutamate is produced via the reversible reductive amination of α-ketoglutarate, catalyzed by glutamate dehydrogenase (GLUD1 or GLUD2), which incorporates ammonia and uses NADPH or NADH as cofactors: \alpha\text{-ketoglutarate} + \text{NH}_4^+ + \text{NAD(P)H} + \text{H}^+ \rightleftharpoons \text{L-glutamate} + \text{NAD(P)}^+ + \text{H}_2\text{O} This reaction links nitrogen assimilation to the tricarboxylic acid (TCA) cycle and is a central hub for amino acid biosynthesis.[21] Aspartate, another TCA-derived amino acid, is formed by transamination of oxaloacetate using aspartate aminotransferase (AST), transferring the amino group from glutamate: \text{oxaloacetate} + \text{L-glutamate} \rightleftharpoons \text{L-aspartate} + \alpha\text{-ketoglutarate} This equilibrium reaction supports aspartate's role in urea cycle function and nucleotide synthesis.[22] Synthesis from glycolytic intermediates provides another major route, exemplified by alanine and serine. Alanine is generated through transamination of pyruvate, the end product of glycolysis, by alanine aminotransferase (ALT), again utilizing glutamate as the amino donor: \text{pyruvate} + \text{L-glutamate} \rightleftharpoons \text{L-alanine} + \alpha\text{-ketoglutarate} This pathway is prominent in skeletal muscle during exercise, facilitating the transport of nitrogen to the liver via the glucose-alanine cycle.[23] Serine biosynthesis follows the phosphorylated pathway, diverging from glycolysis at 3-phosphoglycerate (3-PG). The process involves three sequential steps: oxidation by 3-phosphoglycerate dehydrogenase (PHGDH) to 3-phosphohydroxypyruvate, transamination by phosphoserine aminotransferase (PSAT1) to 3-phosphoserine, and dephosphorylation by phosphoserine phosphatase (PSPH) to L-serine. This cytosolic pathway is the primary source of serine in non-photosynthetic tissues.[24] Additional non-essential amino acids are derived from these core precursors. Glutamine is synthesized from glutamate by glutamine synthetase (GS), an ATP-dependent enzyme that amidates glutamate with ammonia, primarily in the brain and liver to detoxify excess ammonia and support nucleotide biosynthesis: \text{L-glutamate} + \text{NH}_3 + \text{ATP} \rightarrow \text{L-glutamine} + \text{ADP} + \text{P}_i Glycine is formed from serine via serine hydroxymethyltransferase (SHMT), which transfers a hydroxymethyl group to tetrahydrofolate (THF), linking amino acid metabolism to one-carbon pool generation: \text{L-serine} + \text{THF} \rightleftharpoons \text{glycine} + \text{5,10-methylene-THF} This reversible reaction occurs in mitochondria and cytosol, aiding purine synthesis.[25] These biosynthetic pathways are regulated primarily through feedback inhibition to prevent overaccumulation and maintain metabolic balance. For instance, in the serine pathway, PHGDH is allosterically inhibited by high levels of serine, reducing flux when product is abundant. Similarly, glutamine synthetase activity is modulated by cumulative feedback from glutamine and other end products, while transaminases like AST and ALT are influenced by substrate availability and cofactor ratios. Such mechanisms ensure efficient resource allocation in response to nutritional and physiological states.[26][27]Barriers to Synthesizing Essential Amino Acids
The inability of humans to synthesize essential amino acids stems primarily from the evolutionary loss of multi-step biosynthetic pathways, as evidenced by comparative genomic analyses across species. These pathways, present in bacteria, plants, and some fungi, require coordinated enzymatic cascades that are absent in the mammalian genome. For instance, genes encoding enzymes like AroA (3-phosphoshikimate 1-carboxyvinyltransferase) for aromatic amino acid production are missing in humans, rendering de novo synthesis impossible without dietary input.[28] A key barrier is the complete absence of the shikimate pathway, a seven-enzyme sequence that converts phosphoenolpyruvate and erythrose-4-phosphate into chorismate, the precursor for phenylalanine and tryptophan. This pathway, conserved in microorganisms and plants, is not encoded in the human genome, forcing reliance on external sources for these aromatic essential amino acids. Similarly, lysine biosynthesis depends on the diaminopimelate (DAP) pathway, which branches from aspartate semialdehyde and involves multiple transamination and decarboxylation steps absent in mammals; humans lack the necessary genes for meso-DAP production and subsequent conversion to L-lysine.[29][30] For threonine, an additional limitation arises from irreversible catabolic commitments and cofactor constraints. While threonine can be degraded via pyridoxal 5'-phosphate (PLP)-dependent enzymes like threonine dehydrogenase or serine/threonine dehydratase, the reverse biosynthetic route from aspartate is not feasible due to the absence of dedicated synthases such as threonine synthase. Comparative genomics confirms the loss of threonine synthase homologs, a PLP-dependent enzyme essential in plants and bacteria.[31] Although the gut microbiome harbors bacteria capable of producing some amino acids, the net contribution to human essential amino acid requirements is generally limited, as microbial products are largely catabolized or inaccessible to the host due to absorption primarily occurring in the small intestine. This underscores the prohibitive nature of endogenous barriers, with microbial synthesis insufficient to fully meet nutritional demands.[32]Nutritional Guidelines
Recommended Daily Intakes
The recommended daily intakes for essential amino acids in humans are determined by expert consultations from the World Health Organization (WHO), Food and Agriculture Organization of the United Nations (FAO), and United Nations University (UNU), primarily through the 2007 joint report on protein and amino acid requirements. These intakes are expressed as average requirements (AR) in milligrams per kilogram of body weight per day (mg/kg/day), representing the level needed to meet the needs of half the healthy population, and safe levels that cover nearly all individuals (typically 1.63 times the AR to account for variability). The values are derived using methods such as nitrogen balance, indicator amino acid oxidation, and factorial analysis, with the adult protein AR set at 0.66 g/kg/day as the reference for scaling amino acid needs via scoring patterns (requirement in mg/kg/day divided by protein AR in g/kg/day to yield mg/g protein). This approach prioritizes maintaining nitrogen equilibrium and supporting metabolic functions without excess. For healthy adults (aged 19 years and older), the AR values ensure adequate synthesis of proteins for maintenance and repair. Representative examples include leucine at 39 mg/kg/day and lysine at 30 mg/kg/day, with the total essential amino acid AR summing to approximately 189 mg/kg/day (equivalent to 10-13 g/day for a 70 kg individual). The safe intake level for total protein is 0.83 g/kg/day, implying proportionally higher amino acid needs (around 15-20 g/day total essential amino acids for a 70 kg adult) to achieve population-wide adequacy. These figures assume high-quality, digestible protein sources adjusted for 100% digestibility in the reference pattern.| Essential Amino Acid | Average Requirement (mg/kg/day) | Scoring Pattern (mg/g protein) |
|---|---|---|
| Histidine | 10 | 15 |
| Isoleucine | 20 | 30 |
| Leucine | 39 | 59 |
| Lysine | 30 | 45 |
| Methionine + Cystine | 15 | 23 |
| Phenylalanine + Tyrosine | 30 | 45 |
| Threonine | 15 | 23 |
| Tryptophan | 4 | 6 |
| Valine | 26 | 39 |
| Total Essential | 189 | 285 |
Dietary Sources and Absorption
Essential amino acids are primarily obtained from dietary proteins, with animal-derived sources such as meat, poultry, fish, eggs, and dairy products serving as complete proteins that supply all nine essential amino acids in adequate proportions.[33] These foods are efficient for meeting nutritional needs because they contain high levels of bioavailable essential amino acids, including branched-chain types like leucine, isoleucine, and valine.[34] Plant-based sources typically provide incomplete proteins, lacking sufficient amounts of one or more essential amino acids, but combining complementary foods—such as rice with beans or grains with legumes—can achieve a balanced profile mimicking complete proteins.[34] Certain plants like quinoa, soy, and buckwheat are exceptions, offering all essential amino acids in a single source.[35] For vegan diets, recent 2020s research highlights potential shortfalls in specific essential amino acids like lysine and leucine despite adequate total protein intake, underscoring the role of fortified foods such as protein-enriched plant milks, cereals, or meat analogs to enhance essential amino acid availability.[36][37] In the gastrointestinal tract, essential amino acids from dietary proteins are absorbed primarily in the small intestine after enzymatic digestion into free amino acids, dipeptides, and tripeptides.[38] Peptide transporters like PEPT1 facilitate the uptake of di- and tripeptides across the apical membrane of enterocytes, while specific amino acid transporters, such as LAT1 for large neutral and branched-chain amino acids, handle free amino acid transport, often in exchange with other amino acids.[39][40] This process ensures efficient delivery to the bloodstream, with absorption largely completing by the end of the jejunum and ileum.[41] Bioavailability of essential amino acids is influenced by protein digestibility, which is generally higher for animal sources—often reaching 95% or more—compared to plant proteins, averaging around 85% due to factors like fiber content and anti-nutritional compounds.[42] These differences affect the net absorption of essential amino acids, with animal proteins providing superior utilization in meeting daily requirements.[43]Protein Evaluation
Protein Quality Metrics
Protein quality metrics provide standardized methods to evaluate the nutritional value of proteins based on their essential amino acid composition and digestibility, ensuring they meet human requirements for growth, maintenance, and health. These metrics help identify how well a protein source supplies the nine indispensable amino acids in proportions that align with physiological needs, accounting for both the amino acid profile and the extent to which the body can absorb them from the diet.[44] The Protein Digestibility-Corrected Amino Acid Score (PDCAAS) was adopted by the Food and Agriculture Organization (FAO) and World Health Organization (WHO) in 1991 as the primary method for assessing protein quality in human nutrition. It calculates the score by first determining the amino acid score, which is the ratio of the content of each essential amino acid in the test protein to its content in a reference pattern, expressed per gram of protein; the lowest such ratio identifies the limiting amino acid. This score is then multiplied by the protein's true fecal digestibility percentage, which measures the proportion of nitrogen absorbed in the intestines. The formula is: \text{PDCAAS} = \min\left(\frac{\text{mg of essential AA in 1 g test protein}}{\text{mg of same AA in 1 g reference protein}}\right) \times \text{true fecal digestibility (\%)} \times 100 PDCAAS values range from 0 to 100 (or higher, though truncated at 100 for labeling), with scores above 100 indicating surplus quality relative to needs. This method emphasizes overall protein utilization but has been critiqued for overestimating quality in cases of high fecal nitrogen loss unrelated to amino acids.[44] In 2013, the FAO proposed the Digestible Indispensable Amino Acid Score (DIAAS) as an advancement over PDCAAS to better reflect true bioavailability, particularly by using ileal digestibility—the proportion of amino acids absorbed before the large intestine—rather than fecal measurements, which can include microbial contributions. DIAAS focuses solely on the nine indispensable amino acids and calculates the score based on the first limiting indispensable amino acid's digestible content relative to the reference pattern. The formula is: \text{DIAAS} = \left(\frac{\text{mg digestible indispensable AA in 1 g test protein}}{\text{mg of same AA in 1 g reference protein}}\right) \times 100 where digestible content is determined from ileal measurements in human or animal models. Unlike PDCAAS, DIAAS does not truncate scores at 100 and is recommended for individual foods and diets, providing a more precise indicator of amino acid adequacy for vulnerable populations like infants. It has gained adoption in nutritional guidelines for its accuracy in addressing post-absorptive losses.[44][45] The limiting amino acid in a protein is the essential amino acid present in the smallest amount relative to the reference pattern, which determines the overall quality score in both PDCAAS and DIAAS; for example, lysine often limits cereal grain proteins, while methionine (or sulfur-containing amino acids) limits legume proteins. These limitations highlight the need for dietary complementarity, such as combining grains and legumes to balance profiles.[44][46] The reference pattern used in these metrics is the WHO/FAO ideal amino acid composition for preschool children aged 6 months to 3 years, representing a high-quality protein benchmark with the following mg/g protein requirements: histidine 20, isoleucine 32, leucine 66, lysine 57, methionine + cysteine 27, phenylalanine + tyrosine 52, threonine 31, tryptophan 8.5, and valine 43. This pattern prioritizes needs during rapid growth and is applied across age groups unless specified otherwise, ensuring metrics align with the most demanding physiological requirements.[44]Amino Acid Profiles in Foods
Animal proteins generally exhibit balanced profiles of essential amino acids, providing high concentrations relative to human requirements, which contributes to their superior nutritional quality. For instance, whole raw egg contains approximately 12.6 g of protein per 100 g, with essential amino acids including histidine (310 mg), isoleucine (670 mg), leucine (1,090 mg), lysine (910 mg), methionine (380 mg), phenylalanine (680 mg), threonine (560 mg), tryptophan (170 mg), and valine (860 mg). This composition results in a protein digestibility-corrected amino acid score (PDCAAS) of 1.0, indicating it meets or exceeds requirements for all essential amino acids.[47] In contrast, plant-based proteins often display imbalanced profiles, with deficiencies in one or more essential amino acids. Whole wheat flour, for example, provides about 15.0 g of protein per 100 g, but its essential amino acid content includes histidine (428 mg), isoleucine (532 mg), leucine (1,078 mg), lysine (431 mg), methionine (274 mg), phenylalanine (814 mg), threonine (441 mg), tryptophan (239 mg), and valine (612 mg); notably, lysine is limited at around 2.8% of total protein, leading to a lower PDCAAS of approximately 0.4. Corn (yellow, raw) similarly offers 3.3 g of protein per 100 g, with histidine (89 mg), isoleucine (129 mg), leucine (348 mg), lysine (137 mg), methionine (66 mg), phenylalanine (163 mg), threonine (139 mg), tryptophan (23 mg), and valine (174 mg), showing deficiencies in lysine and tryptophan. Kidney beans (raw) contain 23.6 g of protein per 100 g, featuring histidine (659 mg), isoleucine (1,008 mg), leucine (1,940 mg), lysine (1,664 mg), methionine (365 mg), phenylalanine (1,311 mg), threonine (815 mg), tryptophan (263 mg), and valine (1,233 mg), which are relatively high in lysine but lower in sulfur-containing amino acids like methionine.[48][49][50] To address these imbalances in plant-based diets, complementary pairing of foods can achieve a more complete essential amino acid profile. Corn, deficient in tryptophan and lysine, pairs effectively with beans, which are rich in these amino acids; for example, combining corn and kidney beans provides adequate levels of all essential amino acids when consumed in appropriate ratios, approximating the quality of animal proteins.[51] Processing methods, such as heating, can alter amino acid availability in foods through reactions like the Maillard reaction, where reducing sugars react with amino groups, particularly lysine, forming unavailable compounds. This reduces lysine bioavailability in processed items like baked goods or sterilized milk by up to 50% under high-temperature conditions, potentially lowering overall protein quality.[52]| Food Source | Total Protein (g/100g) | Limiting EAA Example | Key Profile Notes |
|---|---|---|---|
| Egg (raw, whole) | 12.6 | None (balanced) | High in leucine (1,090 mg) and valine (860 mg); PDCAAS = 1.0. |
| Wheat flour (whole-grain) | 15.0 | Lysine (431 mg) | Lysine ~2.8% of protein; adequate methionine (274 mg). |
| Corn (yellow, raw) | 3.3 | Tryptophan (23 mg), Lysine (137 mg) | Low sulfur AAs; leucine prominent (348 mg). |
| Kidney beans (raw) | 23.6 | Methionine (365 mg) | High lysine (1,664 mg); good for complementing grains. |