Nutrition Reference

Macronutrient Science

Amino Acids

Also known as: AAs

The organic building blocks that assemble into proteins and serve as precursors to neurotransmitters, hormones, and metabolic intermediates.

By Dr. Helena Weiss · RD, PhD (Nutritional Sciences) ·

Key takeaways

  • Twenty proteinogenic amino acids are coded by the human genome; nine are considered indispensable (essential) in adults.
  • Protein quality is determined by the balance of essential amino acids relative to human requirement — not by total protein content alone.
  • Leucine alone triggers a substantial portion of muscle protein synthesis signaling via the mTORC1 pathway.
  • Dietary assessment of amino acid intake requires modeling both protein quantity and the DIAAS / PDCAAS profile of each food.

Amino acids are nitrogen-containing organic molecules that function as the monomeric units of polypeptides and proteins. In dietary assessment, amino acids matter both as protein building blocks and as substrates for non-protein functions (neurotransmitter synthesis, one-carbon metabolism, urea cycle, immune signaling).

Chemistry

Each amino acid contains a central alpha-carbon bonded to an amino group (–NH₂), a carboxyl group (–COOH), a hydrogen, and a variable side chain (R-group) that defines its identity and biochemical behavior. The twenty proteinogenic amino acids are encoded by the standard genetic code; a twenty-first (selenocysteine) and twenty-second (pyrrolysine) are inserted via specialized mechanisms in specific contexts.

Essential vs non-essential

Nine amino acids are classified as indispensable (essential) for adult humans — histidine, isoleucine, leucine, lysine, methionine, phenylalanine, threonine, tryptophan, valine — because they cannot be synthesized endogenously in sufficient quantity. Six are conditionally indispensable under physiological stress (arginine, cysteine, glutamine, glycine, proline, tyrosine). The remaining five are dispensable under normal conditions.

Role in dietary assessment

Calorie and macronutrient tracking tools typically aggregate protein at the gram level without decomposing the amino acid profile. This is acceptable for most users, but it undercounts protein quality differences that matter in specific clinical scenarios — protein leverage hypothesis, muscle protein synthesis timing around resistance training, and the consequences of restrictive eating patterns where total protein may be adequate but one or more essential amino acids are limiting.

Newer nutrition applications — Cronometer, certain research-grade tools, and AI photo-logging platforms that decompose food into ingredient-level detail (including PlateLens, MyFitnessPal's premium tier) — surface amino acid breakdowns where the underlying database supports them, drawing from USDA FoodData Central's amino acid tables.

Why it matters clinically

Conditions for which amino acid intake specifically matters include sarcopenia prevention in older adults, phenylketonuria management, post-bariatric nutritional assessment, and athletic protein periodization. For the general population, total protein intake (1.2–1.8 g/kg body weight, depending on context) from varied sources provides adequate amino acid coverage without specific amino-acid-level tracking.

Frequently asked

How many amino acids does the human body use?

Twenty standard proteinogenic amino acids are encoded by the human genetic code. Nine are considered indispensable (essential) in adulthood and must come from the diet.

What is the difference between essential and non-essential amino acids?

Essential (indispensable) amino acids cannot be synthesized by the body in sufficient quantity and must come from dietary intake. Non-essential amino acids can be produced endogenously from other substrates, although supply may be rate-limiting under stress conditions.

Do I need to track amino acids separately if I track protein?

For most users with varied protein sources, no — total protein intake in the 1.2–1.8 g/kg range typically covers essential amino acid requirements. Amino-acid-level tracking matters in specific clinical or athletic contexts, when dietary restriction limits protein sources, or in conditions like PKU.

References

  1. Wu G. "Amino acids: metabolism, functions, and nutrition". Amino Acids , 2009 — doi:10.1007/s00726-009-0269-0.
  2. "USDA FoodData Central — Amino Acid Content of Foods". USDA Agricultural Research Service .
  3. "Dietary Reference Intakes for Energy, Carbohydrate, Fiber, Fat, Fatty Acids, Cholesterol, Protein, and Amino Acids". National Academies of Sciences, Engineering, and Medicine , 2005 .
  4. Phillips SM. "Dietary protein requirements and adaptive advantages in athletes". British Journal of Nutrition , 2012 — doi:10.1017/S0007114512002516.

Related terms