Title: The amino acid content of protein sources

Key words: plant proteins, animal proteins, classification, amino acids, lysine, tryptophan, methionine

Date: July 2000

Category: 4. Food Data

Type: Article

Author: Dr M Draper

 

The Amino Acid Content Of Protein Sources

The Extent And Nutritional Importance Of Variations

Introduction

About one-third of the protein in the average UK diet comes from plant sources and two thirds from animal sources (1). The World Health Organisation (2) estimates we need 4.5% of our calories from protein.

Proteins consist of chains of hundreds or thousands of amino acids units. There are only about 20 different amino acids but the way the sequences can be arranged is almost infinite. It is the specific and unique sequence that gives each protein its characteristic structural and enzymatic properties.

Classification of Amino Acids

There are eight essential amino acids. In rapidly growing infants, histidine and taurine are also essential. These essential amino acids cannot be made in the body and must be present in the food or breast milk.

The non essential or dispensable amino acids can be made from excess of certain other amino acids in the diet.

 

Animal and Vegetable Proteins - Amino Acid Composition

The overall proportions of amino acid content in any single vegetable food (cereals, nuts and seeds, potatoes or legumes such as peas or beans) differ from those needed by humans. The levels of three amino acids are plotted on Table 1 and it can be seen that wheat and rice proteins, for example, are comparatively low in lysine, legumes such as lentils are low in tryptophan and methionine. These proteins are said to have lower biological values because the quality of a protein depends on its ability to provide the essential amino acids. It must however be remembered that foods are rarely consumed in isolation.

Combining foods complements their individual amino acid content so that the usable protein is increased. Most animal proteins (meat, fish, milk, cheese and eggs ) have a high biological value for protein. The animals have selected from their dietary intake of protein sources and effectively refined the amino acids, burning the excess to produce energy.

Varying your diet to include protein from various foods is advisable. However the best protein sources (eg animal) may not necessarily be the best when assessing the impact on the diet of the other constituents such as saturated fat content.

A vegetarian diet can provide sufficient amino acids but may be lacking in vitamin B12 or iron (especially iron, as haem-iron is more easily absorbed). Studies of the amino acid content of the British diet in 1977 indicated that the average household consumed a diet which supplied adequate amino acids (4). Apparently our protein intakes have increased since 1979, to daily averages of 84 gms for men and 64 gms for women (5) which has caused concern that we may now be eating too much protein (6). Care should be taken in the consumption of specific amino acid supplements since it is desirable that dietary amino acids are in the correct balance.

Table 1. Content of specific amino acids in food sources.

Food source

Amino acid content (g/100g protein)

 

Lysine

Methionine

Tryptophan

Cow's milk

8.4

2.9

1.5

Eggs

6.2

3.1

1.8

Beef

9.1

2.7

1.3

Fish, cod

9.8

2.9

1.1

Lentils

6.7

0.7

0.9

Peanuts

4.0

1.3

1.3

Wheat flour*

2.7

1.8

1.3

Rice

3.3

1.8

1.2

* The content of some amino acids can be increased by genetic breeding.References

  1. Ministry Of Agriculture, Fisheries & Food - Manual of Nutrition 1995; p.22 - London: The Stationery Office
  2. World Health Organisation Energy and protein requirements. Report of a Joint FAO/WHO/UNU Meeting. 1985; Geneva WHO Technical Report Series; 724.
  3. Holford, P. The Optimum Nutrition Bible 1997; p.34-44 Piatkus
  4. Buss, D.H., Ruck, N.F. The amino-acid pattern of the British diet - J Hum Nutr; 1977; 31:165-169
  5. Gregory, J., Foster, K. et al. The Dietary and Nutritional Survey of British Adults - 1990; London:HMSO
  6. Department Of Health - Dietary Reference Values for Food Energy and Nutrients for the United Kingdom. 1991; p.82 London HMSO.