Protein- how is it used in the body?

Proteins are needed in the body for:

1. Body growth and repair: building and maintenance muscles and other soft tissues

2. Regulation: proteins and amino acids are or are part of compounds that regulate or modulate the processes in the body, such as enzymes, hormones, antibodies and neurotransmitters.
3. Energy: to burn as fuel (i.e. to provide a source of energy).
Proteins are found in all active tissues of the body (e.g. muscle cells, the liver, glands, etc). Though one gram of protein can yield 4 calories, the most important functions of the protein are tissue building, and repair and provision of the ingredients for the formation of enzymes, hormones and antibodies.

Protein use depends on the following:
  • The all or none rule
  • An appropriate amount of all necessary amino acids must be present to make a particular protein, or the protein will not be made. Any essential amino acids not used to make proteins will soon be oxidised for energy, or converted to produce carbohydrates or fats.

Calorie intake adequacy

If a diet doesn't contain adequate carbohydrates or fats for calories in energy production as ATP, tissue proteins are then used for energy. (NB: ATP is a chemical involved in the process of storage and release of energy, allowing work and movement to occur in the body).

Body nitrogen balance

In healthy adults, the rate at which proteins are synthesised should be the same as the rate at which they are being broken down in the body. This balance is reflected by the nitrogen balance in the body based on the fact that all protein average approximately 16% nitrogen.

The body is considered in balance when the amount of nitrogen ingested in protein is equal to the amount excreted in urine. Under physical or emotional stress, the protein breakdown can be affected and exceed by the amount of protein being synthesised: this is called a "negative nitrogen balance".

Hormone activity
Anabolic hormones can cause acceleration of protein synthesis and growth. The effect of these hormones may continuously change (affected by things such as age, stress, growth rate etc), hence growth rates and protein synthesis rates can be continuously changing.


As a general guide: approximately 0.8-1g per kilogram of body weight per day. Typically a small serve of fish and an egg would be adequate for an average person, per day. Protein needs vary much among different people, depending on their metabolism, physical activity and health conditions. A sports person will need to rebuild muscles more often that a sedentary one. They will also have higher metabolic rates. So their protein consumption will need to adapt to higher requirements, and can be as high as 1.2-1.4g of protein per kg of body weight. People with kidney stones will have to be careful about their nitrogen intake, as in concentrated proteins sources, as a high intake will tend to stress the kidney in an attempt to increase urea elimination. According to the theories behind Metabolic Typing, some people may require more proteins in their diet, as much as 55%, and others may require less protein and more carbohydrates, as much as 70%. This is due to genetic differences in the way biochemistry cycles occur in our bodies, if people are fast or slow oxidisers.

Proteins sources are very varied, they can be supplied with plant or animal sources, and both of them lead to good health provided the essential amino acids are supplied and an adequate total quantity of protein is ingested. Too much protein is as bad too little. Moderation is the key. Too many proteins can lead to blood pH unbalances, which will put stress on other bodily systems in order to equilibrate the pH. Also, protein degradation will produce residues that will need to be eliminated from the body.

Animal sources of proteins are animal meats, poultry, fish, dairy and eggs. Plant sources of proteins include seeds, nuts and beans, grains, leafy greens, other vegetables and fruits.

Find out more about nutrition with Human Nutrition I.