Digestion is the process of breaking down large, complex molecules, as provided by the birds' feed, into smaller components that can be absorbed into the portal blood system.

Proteins are complex compounds made up of amino acids. Feed proteins are broken down into amino acids by digestion. They are then absorbed and transported by the blood to the cells, which assemble these amino acids into body proteins. Body proteins are used in the construction of body tissue. Tissues, which mainly consist of protein, are muscles, nerves, cartilage, skin, feathers and beak. The albumen (white) of the egg is also high in protein. The main sources of protein in poultry rations are animal proteins such as fishmeal, meat and bone meal, and plant proteins such as soybean meal, cottonseed meal, and corn gluten meal. There is no one source of protein that will provide all the amino acids in one feed ration. But when the proteins from different feedstuffs are used, the ration can be formulated to contain all the necessary amino acids.

A vitamin A deficiency can affect the ability of poultry to utilize protein. Meat proteins also provide the enzymes that aid in digestion and metabolism of proteins. Fishmeal is an excellent source of protein for poultry since it contains adequate quantities of all the essential amino acids required by chickens and is an especially good source of lysine and methionine. All feed should be formulated in such a way to provide balanced nutrition for appropriate age levels.

The digestion of proteins occurs predominantly in the duodenum and small intestine where most of the pH sensitive enzymes are produced. Enzymes are largely responsible for molecular degradation and are rarely a limiting factors to digestion; reduction in protein and amino acid digestibility is more commonly associated with the presence of anti nutritional factors like tannins and antitrypsins, commonly found in some ingredients.

There is no digestion of protein in the mouth or crop. There is, however, lubrication and softening of the feed by the mucus secreted by the crop wall and saliva from the mouth. The proventriculus represents the "glandular stomach" where digestive juices are secreted. The juices contain HCI and the enzyme precursor (zymogen) pepsinogen which is converted to the active enzyme pepsin.
In the gizzard, the ingesta is further mixed with the fluid secreted by the proventriculus. There is also mechanical grinding of the feed, however, protein digestibility of ground feed is little affected by the absence of a fully developed gizzard.

The pancreas produces fluids containing zymogens that are converted to their active enzyme forms at the sites of digestion. Trypsinogen is activated to trypsin in the duodenum by enterokinase, an enzyme secreted from the intestinal mucosa. Thus, trypsin is central in the development of full proteolysis activity. Trypsin, chymotrypsin and elastase catalyse the break down of proteins, peptones and peptides into smaller peptides and amino acids in the duodenum.

Feedstuffs commonly used in poultry diets vary widely in content and digestibility of protein. Peptide uptake is most rapid in the jejunum whereas amino acid uptake is most rapid in the ileum.

Dietary supplementation with synthetic amino acids or their analogues is a common practice in commercial poultry production. Results of many studies show that the digestibilities of such additives are much higher than for comparable amino acids fed as intact proteins, for example lysine has bioavailability of 93% for the synthetic form as opposed to 88% for the natural form of lysine.

a. Naturally Occurring Anti-Nutritional Compounds
Certain feedstuffs that are commonly used in poultry diets contain naturally occurring compounds that significantly reduce their nutritional value. In most cases this is due to reduced digestibility.

Proteins found in animal hide, scales, hair, feather, and bone are not easily digested and contain high concentrations of keratin and collagen.

b. Heat Processing.
Excess heat applied during processing may either destroy or render unavailable certain heat- sensitive amino acids thereby greatly reducing the nutritive value of such feedstuffs. This can happen due to either very high processing temperature, prolonged processing time or both. The effects of prolonged heating on digestibility observed in soybean meal was greatest for lysine where 90 minutes of heating reduced lysine digestibility by about 60%.

The liver cells – hepatocytes are involved in the synthesis of various useful proteins. It is the main site for the synthesis of lipoproteins, ceruloplasmin, transferrin, complement and glycoproteins. Hepatocytes manufacture their own structural proteins and intracellular enzymes. Synthesis of proteins is undertaken by the rough endoplasmic reticulum (RER), and both the rough and smooth endoplasmic reticulum (SER) are involved in secretion of the proteins formed.Some useful proteins are-