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[Biochemistry](https://durgical.com/en/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

- [Coeliac disease](https://durgical.com/en/conditions/coeliac-disease/)

## Related

- [Nitrogen balance](https://durgical.com/en/biochemistry/topics/nitrogen-balance/)
- [Macronutrients and the energy value of food](https://durgical.com/en/biochemistry/topics/macronutrients-and-the-energy-value-of-food/)
- [Balanced diet](https://durgical.com/en/biochemistry/topics/balanced-diet/)
- [Amino acids: structure, charge and pKa](https://durgical.com/en/biochemistry/topics/amino-acids-structure-charge-and-pka/)
- [Glucogenic and ketogenic amino acids](https://durgical.com/en/biochemistry/topics/glucogenic-and-ketogenic-amino-acids/)
- [Tryptophan](https://durgical.com/en/biochemistry/molecules/tryptophan/)
- [Phenylalanine](https://durgical.com/en/biochemistry/molecules/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.

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Educational information, not medical advice. For any symptom, see a health professional.

Sources: [Protein in diet: MedlinePlus Medical Encyclopedia](https://medlineplus.gov/ency/article/002467.htm) (MedlinePlus, US National Library of Medicine, 2026); [Dietary Proteins: MedlinePlus](https://medlineplus.gov/dietaryproteins.html) (MedlinePlus, US National Library of Medicine, 2026); [Healthy diet](https://www.who.int/news-room/fact-sheets/detail/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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