The word protein comes from the ancient Greek word meaning “of prime importance.” It consists of a series of amino acids.

Protein sources:

Meat, poultry, fish, dairy products, legumes, seeds, eggs.

Functions of proteins and amino acids:
Building proteins for new body organs, such as fetal development, childhood growth, regenerated blood cells, wound healing, and hair and nail growth.
Replacing damaged cells leads to the formation of red blood cells (renewed every 3-4 months). Digestive system cells are renewed every 3 days.
Building enzymes and hormones.
Some amino acids have a special role, such as producing the vitamin niacin from the amino acid tryptophan.
2- Building antibodies:

Antibodies are part of the body’s immune system and distinguish between the body’s own proteins and foreign proteins that attack and destroy them. 3- Maintaining body fluid balance:
The concentration of proteins inside and outside cells maintains a constant volume of body fluids inside and outside cells.
4- Maintaining acid-base balance:

The concentration of proteins in the blood maintains a constant balance between acids and bases in the body.

5- Providing energy:

 

If the body does not have energy from carbohydrates and fats, the body breaks down proteins and extracts their potential energy, resulting in increased nitrogen excretion in the urine.
6- Other functions:
Transporting fats from the blood (lipoproteins).
Blood clotting.

Recommended daily intake (RDA):

10-15% of total calorie intake.
Adults need 0.8 grams of protein per gram of body weight. Pregnant women and children need higher amounts of protein in their daily diet.
Athletes also need higher amounts of protein in their daily diet.
Excessive protein intake leads to its conversion to fat and storage in adipose tissue (weight gain).
Excessive protein intake leads to heart fatigue.
Excessive protein intake leads to increased calcium excretion from the body, which may lead to osteoporosis.

Protein Digestion and Absorption:

In the stomach, hydrochloric acid (HCl) breaks down protein chains.
In the small intestine, protein-digesting enzymes convert proteins into their basic components (amino acids).
Amino acids are absorbed into the body and transported via the blood to the liver.
After the liver has consumed its supply of amino acids, the blood transports the amino acids released from the liver to the rest of the body.
In some cases, the body cannot digest proteins, and they are digested by bacteria in the digestive tract and then excreted in the feces.
Quality of Dietary Proteins:
Proteins of animal origin are easier to digest than proteins of plant origin.
The quality of dietary proteins depends on their amino acid content.
Essential amino acids are nine amino acids that the human body cannot synthesize from basic components and must be available in food.
Non-essential amino acids can be synthesized by the body from carbon, oxygen, hydrogen, and nitrogen consumed from other foods.
A complete protein is one that contains all the essential amino acids at the rate required by the human body.
Examples of complete protein from food sources include meat, fish, poultry, eggs, milk, cheese, and some soybean products.
Incomplete protein: Lacks one or more of the essential amino acids the human body needs.
Vegetable protein sources: Contain incomplete proteins, but with a variety of sources, the body can obtain all the essential amino acids. For example, eating foods made from whole grains, legumes, or rice.