FOOD NUTRITION: PROTEIN QUALITY
Diagram
Proteins that carry all nine essential amino acids, and proteins that lack one
Complete protein
A protein that supplies all nine essential amino acids in the amounts the body needs. Most animal foods give complete protein.
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Incomplete protein
A protein that lacks one or more essential amino acids, or has too little of one. Most plant foods give incomplete protein.
Figure 1: Complete protein supplies all nine essential amino acids, while incomplete protein lacks one or more of them. Drawn in our own style from the protein quality discussion in the Encyclopedia of Human Nutrition and in Gropper et al., Advanced Nutrition and Human Metabolism.
Introduction
Complete protein and incomplete protein are both proteins, but they differ in the amino acids they supply. The body needs nine essential amino acids, which it cannot make, so they must come from food. A complete protein supplies all nine essential amino acids in the amounts the body needs, so it can build and repair body protein on its own. An incomplete protein lacks one or more essential amino acids, or has too little of one, so on its own it cannot support body building. Most animal foods give complete protein, and most plant foods give incomplete protein, but eating different plant foods together can cover the missing amino acids.
Quick comparison
Table 1: Complete Protein compared with Incomplete Protein
| Aspect | Complete Protein | Incomplete Protein |
|---|---|---|
| Essential amino acids | All nine, in the amounts needed | One or more missing, or too little of one |
| Limiting amino acid | None | Yes, the amino acid that is shortest decides the quality |
| Main sources | Meat, fish, eggs, milk and other animal foods | Most grains, pulses, nuts and vegetables |
| Can it build body protein alone | Yes | No, not on its own |
| Protein quality | High | Lower, by the missing amino acid |
| Combining foods | Not needed | Eating two plant foods that cover each other, such as grains with pulses, gives all nine |
| Digestibility | Usually well digested | Often less well digested, because of fiber and plant cell walls |
| Amount needed | A smaller amount meets the need | A larger amount, or a mix of foods, is needed to meet the need |
| Example | Egg, milk, meat, fish | Wheat, rice, beans, lentils, nuts |
| For a vegetarian diet | Eggs and milk give complete protein | Grains with pulses, such as rice with beans, cover the missing amino acids |
Scientific explanation
Definition
A complete protein is a protein that supplies all nine essential amino acids in the proportions the body needs. An incomplete protein is a protein that lacks one or more essential amino acids, or supplies too little of one, so its limiting amino acid decides its quality.
Working principle
The body builds its own proteins from amino acids, and nine of them are essential because the body cannot make them. When a food protein is eaten, it is digested into amino acids, and the body uses them to build muscle, enzymes and other body proteins. If one essential amino acid is missing, the body cannot finish building, and the other amino acids cannot be used for building, so the protein is used for energy instead. That is why the limiting amino acid, the one present in the smallest amount compared with the need, decides the quality of a protein. Eating two incomplete proteins that lack different amino acids, such as a grain and a pulse, covers the gap, because each supplies what the other lacks.
Apparatus
The books used here name no laboratory apparatus for measuring or comparing either term, so no apparatus list is given.
Measurement procedure
The books used here give no standard laboratory procedure with volumes and steps for either term, so no procedure is invented here.
Factors affecting readings
For both kinds, the food source decides the amino acid mix: animal foods carry all nine, while grains are short of lysine and pulses are short of methionine. Cooking and processing matter: heat can damage amino acids, and the fiber and cell walls of plants lower digestibility. The books used here give no amino acid scores with numbers, so none are stated.
Formulas and calculations
The books used here give no formula for either term, so no formula is given here.
Industrial applications
- Complete protein: meat, fish and eggs, used as the protein core of a meal.
- Complete protein: milk and dairy foods, used in diets that need high quality protein.
- Incomplete protein: grains such as wheat and rice, used as the staple food in most diets.
- Incomplete protein: pulses such as beans and lentils, used with grains to cover the missing amino acids.
- Both kinds: mixed meals, where grains with pulses give all nine essential amino acids.
Advantages and limitations
- Complete protein, advantage: it supplies all nine essential amino acids on its own, so it builds body protein without combining foods.
- Complete protein, limitation: most complete proteins come from animals, so a diet without animal foods must combine plant proteins instead.
- Incomplete protein, advantage: plant proteins come with fiber and cost less, and combining two plant foods covers the missing amino acids.
- Incomplete protein, limitation: on its own it cannot support body building, because one or more essential amino acids are missing.
FAQ
What is the difference between complete and incomplete protein?
Complete protein supplies all nine essential amino acids in the amounts needed, while incomplete protein lacks one or more of them.
Which foods give complete protein?
Meat, fish, eggs, milk and other animal foods.
Which foods give incomplete protein?
Most plant foods, such as grains, pulses, nuts and vegetables.
Can a vegetarian diet give complete protein?
Yes. Eating grains with pulses, such as rice with beans, covers the missing amino acids, and eggs and milk give complete protein on their own.
What is a limiting amino acid?
The essential amino acid present in the smallest amount compared with the need. It decides the quality of the protein.
References and further reading
- Encyclopedia of Human Nutrition. 3rd ed., Academic Press, 2012, sections Protein Digestion and Bioavailability, and Quality and Sources.
- Gropper, Sareen S., Jack L. Smith and Timothy P. Carr. Advanced Nutrition and Human Metabolism. 7th ed., Cengage, 2016.
- Google Books pages read for this article:
- Gropper, Sareen S., Jack L. Smith and Timothy P. Carr. Advanced Nutrition and Human Metabolism. 7th ed., Cengage, 2016 (Google Books page read)
