Protein belongs to a group of nutrients which are called macro- nutrients because the human body needs them in relatively higher quantities for its normal functioning and growth.
The other nutrients in this group are carbohydrate and fats. The human body needs protein for functions such as the repair of old cells and the building of new cells during growth.
In addition, in times of starvation when there is no carbohydrate and fat in the body to serve as a source of energy, the body falls on protein as a source of energy.
It is also used for the production of hormones such as insulin as well as for the production of enzymes such as ptyalin and trypsin which are needed for digestion. Common sources of protein in our diet include eggs, fish, meat, beans, milk, groundnuts and chicken.
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Protein, when completely digested or broken down, exists as amino acids (the basic unit of protein). Every protein, therefore, contains a certain number and sequence of amino acids. Amino acids can be categorised into two main groups; they are either essential or non-essential. Essential amino acids are those that cannot be produced by the body and so must be available in the food we eat. Examples are arginine, histidine and methionine. Non-essential amino acids are those that can be made by the body or from the breakdown of protein, whose examples include alanine and asparagine. Some amino acids may also become essential during stress or illness in the body. These are referred to as conditional amino acids and they include proline, glycine and serine.
Quality of protein
The quality of a protein depends on the type and amount of amino acid it contains. A high quality protein contains all the essential amino acids in quantities that are required and adequate for human use. Biological value (BV) is used to describe the proportion of protein which is absorbed by the body from the food we eat to be used in protein synthesis in the cells. Dietary protein sources that contain the essential amino acids in a proportion similar to what humans require is said to have a high biological value while those in which one or more essential amino acids are scarce or absent is said to have a low biological value.
Determinants of protein quality
Digestion and absorption are considered important predictors or determinants of protein quality. For protein to be classified as being of high biological value, the body must be able to digest and absorb it. For easy identification of proteins of high biological value, the Food and Agricultural Organisation of the United Nations/World Health Organisation (FAO/WHO) and US Food and Drug Administration (FDA) in 1993 adopted the Protein Digestibility Corrected Amino Acid Score (PDCAAS) method. This method, which is simple and scientifically sound, is used for the routine evaluation of the quality of protein. This method is used to evaluate the protein quality based on both the amino acid requirements of humans and their ability to digest it. The method is based on comparison of the concentration of the first limiting essential amino acid in the test protein with the concentration of that amino acid in a reference (scoring) pattern. The essential amino acid requirement of the pre-school child is what is used as the basis for which the scoring pattern is derived. Simply, this method measures protein quality as a ratio of its limiting essential amino acid level to the level of this same essential amino acid in the reference “ideal” protein, corrected for digestibility. The PDCAAS scores in this method range from 0.0-1.0 with 1.0 being the upper limit of protein quality and 0.0 the lowest limit of protein quality. some examples of dietary protein sources and their PDCAAS scores are provided in the table below this column.
A careful look will show that generally, protein from animal sources have higher scores than protein from plant sources. It is important to note, however, that having a high PDCAAS score is not the end of the matter. Some protein sources contain other dietary components which inhibit the digestion of protein or absorption of amino acid and, therefore, make the amino acids, though present, become unavailable for the body to use. These dietary components include high levels of insoluble dietary fiber which binds to protein and makes it difficult to digest.
Trypsin inhibitors in foods such as soybean and other anti-nutritive factors such as tannins and phytates in unrefined grains, cereals and legumes also limit their amino acid availability. These factors are mainly found in plant food sources. Protein from animal sources, therefore, tend to have higher biological value with little or no inhibitions to availability compared to protein from plant sources. People who may have health challenges relating to protein intake such as patients with kidney diseases are, therefore, often advised to eat more animal source proteins than plant sources in order to obtain maximum benefits of health.
The good news is that some traditional methods of food preparation such as cooking, soaking in water, fermentation and malting reduces the negative effects of anti-nutritive factors such as phytic acid, oxalic acid and polyphenols and this may increase the bioavailability of amino acids in foods containing these anti-nutritive factors. Mutual supplementation, the combination of proteins in which one protein source limited in one amino acid is cooked with another that is rich in the amino acid that is limited in the first protein source, can also be done to ensure the availability of amino acids, especially the essential amino acids in foods. An example is seen in the preparation of rice and beans (waakye), where rice, which is limited in lysine but rich in methionine, is combined with beans which is rich in lysine and limited in methionine.
Food PDCAAS Score Food PDCAAS Score
Casein 1.00 Whole wheat 0.42
Egg white 1.00 Chicken 1.00
Beef 0.92 Soy protein 0.96
Fruits 0.75 Beans 0.70
Cereals 0.59 Milk 1.00
Peanuts 0.52 Fish 0.96
It can, therefore, be concluded that, as a matter of fact, some dietary protein sources have higher biological value than others.
The writers are with the Department of Nutrition and Dietetics,
School of Biomedical and Allied Health Sciences,
University of Ghana.