Biochemistry · Topic
Protein quality and essential amino acids !
A protein is good quality when it supplies all the essential amino acids in the right amounts and is easy to digest. Mixing cereals and legumes fixes the gaps.
Updated 2026-11-01
Key facts
- Nine amino acids are essential in adults: histidine, isoleucine, leucine, lysine, methionine, phenylalanine, threonine, tryptophan and valine; arginine is semi-essential in growth.
- The limiting amino acid is the essential one in shortest supply: lysine in cereals (rice, maize, wheat), methionine (sulfur amino acids) in legumes (beans, lentils).
- Mutual supplementation: cereals + legumes (for example rice with beans, or arepa with beans) together give a complete protein mixture.
- Measures: biological value (BV, % of absorbed nitrogen retained), net protein utilisation (NPU = BV × digestibility), chemical score and PDCAAS/DIAAS (amino acid score corrected for digestibility).
- Egg and milk proteins are reference proteins (BV close to 100 and 90); gelatin lacks tryptophan and is the classic example of a poor protein.
In detail
- 1. Body protein cannot be made if one essential amino acid is missing: synthesis stops at the first missing residue (the 'all or none' rule).
- 2. The chemical score compares the content of each essential amino acid with a reference protein (egg); the lowest ratio is the score and names the limiting amino acid.
- 3. Digestibility matters: plant proteins inside cell walls and with anti-nutrients (trypsin inhibitors, phytates) are digested less well; cooking improves this.
- 4. Biological value counts how much absorbed nitrogen is kept in the body; NPU also counts what was not absorbed.
- 5. Adults need about 0.8 g of good-quality protein per kg per day; children, pregnant and breastfeeding women and people recovering from illness need more.
- 6. Maize lacks lysine and tryptophan; in populations living mainly on untreated maize, low tryptophan contributes to pellagra (niacin is made from tryptophan).
Memory aid
PVT TIM HaLL: Phenylalanine, Valine, Threonine, Tryptophan, Isoleucine, Methionine, Histidine, Leucine, Lysine.
- Also called
- Biological value, Limiting amino acid, Mutual supplementation
Clinical notes
- Diets based on a single staple (only cereal or only cassava) can give enough energy but poor protein, contributing to kwashiorkor in children.
- Vegetarian and vegan diets meet protein needs by combining cereals, legumes, nuts and seeds through the day.
- Mnemonic for the nine essential amino acids is useful in exams; arginine and histidine matter most during growth.
Related conditions
Related
- Nitrogen balance
- Macronutrients and the energy value of food
- Balanced diet
- Amino acids: structure, charge and pKa
- Glucogenic and ketogenic amino acids
- Tryptophan
- Phenylalanine
Test yourself
Think of the answer, then tap the question to see it.
Which is the limiting amino acid of cereals and which of legumes?
Lysine in cereals and methionine (the sulfur amino acids) in legumes; eaten together they complement each other.
What is net protein utilisation?
The fraction of eaten nitrogen kept in the body: biological value multiplied by digestibility.
Why is gelatin a poor-quality protein?
It lacks tryptophan (and is low in other essential amino acids), so it cannot support protein synthesis on its own.
Which proteins are used as reference proteins of high biological value?
Egg (biological value close to 100) and milk proteins.
How does a maize-based diet contribute to pellagra?
Maize is low in tryptophan, which the body uses to make niacin, and its niacin is poorly available unless treated with alkali.
Educational information, not medical advice. For any symptom, see a health professional.
Sources: Protein in diet: MedlinePlus Medical Encyclopedia (MedlinePlus, US National Library of Medicine, 2026); Dietary Proteins: MedlinePlus (MedlinePlus, US National Library of Medicine, 2026); Healthy diet (World Health Organization, 2026); Textbook of Biochemistry for Medical Students, 6th ed. (Jaypee Brothers Medical Publishers (Vasudevan DM, Sreekumari S, Vaidyanathan K), 2011).
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Representative model, for educational purposes only.