Discuss the different qualities of proteins

Complete proteins
These are proteins that contain all essential amino acids in proportions capable of
promoting growth when they are the sole sources of proteins in the diet. They are also
referred to as good quality proteins, or proteins of high Biological Value (BV). All
animal proteins except gelatin which has limited amounts of both tryptophan and
lysine are complete proteins. Foods containing all the essential amino acids include
eggs, meat and milk.

Incomplete proteins
They are also known as poor-quality proteins or proteins of low biological value.
They lack or have limited amounts of one or more essential amino acids. If used as
the sole source of proteins in the diet they cannot support growth. Most vegetable
proteins except nuts and soybeans are incomplete.

Limiting amino acid
Some proteins contain all the essential amino acids but a relatively small amount of
one of them has sufficient amino acids to repair body tissues but not enough to
promote growth. The amino acid present in the smallest amount relative to the mount
required for growth is the called the limiting amino acid. For example methionine is
the limiting amino acid in legumes while lysine is the limiting amino acid in cereals.
An understanding of the concept of limiting amino acid helps us to understand why it
is important to have a combination of different vegetable proteins rather than just one
in a meal. By combining two proteins that are limited in different amino acids it is
possible to simulate a complete protein. In this way vegetarians can have a diet as
adequate in protein as that of people who eat both animal and vegetable proteins.

For example:
- Wheat which has a lot of methionine but lacks lysine can be combined with
soybeans which have ample lysine but are limited in methionine
- Milk has all the essential amino acids and can be combined with wheat cereal
which lacks lysine.
- Mixtures of 2 vegetable proteins or small amounts of animal proteins with
vegetable protein can provide high quality protein

In this way vegetarians can have a diet as adequate in protein as that of people who
eat both animal and vegetable proteins.
For example:
- Wheat which has a lot of methionine but lacks lysine can be combined with
soybeans which have ample lysine but are limited in methionine
- Milk has all the essential amino acids and can be combined with wheat cereal
which lacks lysine.
- Mixtures of 2 vegetable proteins or small amounts of animal proteins with
vegetable protein can provide high quality protein

Complementary proteins
Refers to two or more proteins whose amino acid assortment complements each other
in such a way that the essential amino acid missing from one is supplied by the other.

Measurement of protein quality
Protein quality can be done using several methods which include:

Chemical scoring
This is a measure of evaluating protein quality by comparing a test proteins amino
acid pattern with that of a reference protein. It evaluates a foods protein quality by
determining its amino acids composition in the laboratory and comparing it with that
of egg protein which is the reference protein. The amino acid in a protein that falls
shortest compared to the standard is the limiting amino acid.

Biological Value (BV)
Biological value of a protein measures its efficiency in supporting the body’s needs.
Protein synthesis stops when an essential amino acid is missing therefore the
remaining amino acid cannot be stored and are dismantled. Scientists can test the
protein quality of a given food by feeding it to experimental animals as the role
protein in diet and measure its retention and loss of nitrogen. The higher the amount
retained the higher the quality of the protein. Biological value can also be referred to
as the amount of protein nitrogen that is retained for growth and maintenance
expressed as a percentage of the protein nitrogen that has been digested and absorbed.
The biological value of some foods is as follows: eggs-100%, milk- 93%, beef and
fish- 75% and corn- 72%.

Net Protein Utilization
Net protein utilization (NPU) measures nitrogen retention. However instead of
measuring retention of absorbed nitrogen as in biological value, net protein utilization
measures retention of food nitrogen

Protein Efficiency Ratio (PER)
Is a measure of protein quality expressed as the grams of weight gained by a growing
animal per gram of protein fed.

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Titany answered the question on November 5, 2021 at 09:44