Some bacteria in the digestive tract synthesize some vitamins. The availability of this source of vitamins to the chicken is variable and usually not relied upon in compound feed formulation. Similarly, synthesis of vitamin D in the skin under intensive conditions is insignificant and not considered a reliable source. 

Vitamins are not changed by the digestive process in the gastrointestinal tract and are absorbed intact from the intestines. Following absorption, vitamins are available for immediate use or conversion into the biologically active form of metabolites. Absorption of vitamins is by both active and passive processes. Some vitamins can be stored in the body and other need to be supplied daily to meet all the physiological requirements.

Vitamins are essential for tissue integrity, reproduction, and normal physiological functions and to maintain health. Vitamins participate in many biochemical reactions and participate in the metabolism of nutrients derived from the digestion of proteins, fats and carbohydrates. Vitamins are needed for cellular functions and a single vitamin may influence a number of physiological functions.

Deficiency or depletion of vitamins from the tissues results in specific or non-specific signs or symptoms and productivity losses. The development of deficiency signs and its significance in chickens under practical conditions is discussed separately under each vitamin in this publication.

FAT SOLUBLE VITAMINS

For convenience or convention, vitamins are divided into two groups. The first group includes those, which are soluble in fats or normally found in association with fat in various feed ingredients. In animal this group of vitamins is stored in fatty tissues.

For practical application fat-soluble vitamins are completely soluble in water.

The availability of fat soluble vitamins from natural ingredients require digestion of fat by the actions of various enzymes and bile salt in the digestive tract.

Fat-soluble vitamins require the presence of fat since they are absorbed into the lymphatic system and then to the blood circulation to reach the liver and other body tissues. The lymphatic system in the chicken is not well developed; therefore fat-soluble vitamins are also absorbed directly into blood circulation.

Fat-soluble vitamin intake excess to the requirements is stored in the liver and to a limited extent in other tissues throughout the body.

Fat –soluble vitamins include:

• Vitamin A

• Vitamin E

• Vitamin D

• Vitamin K

For practical purposes this group of vitamins can be classified as vitamins for specific functions as opposed to being fat-soluble.


WATER SOLUBLE VITAMINS

This group includes nine vitamins that are readily soluble in water. In practical terms eight vitamins belong to, what are commonly known as, B group of vitamins and the ninth vitamin is ascorbic acid. This distinction is important since the B group of vitamins is predominantly involved in metabolism where ascorbic acid is essential for other specific functions as well.

All water-soluble vitamins are absorbed direct from the digestive tract into the blood circulation. The absorption from the intestine is by both active and passive processes. In the body all vitamins are distributed to every organ and tissues, and are present in minute quantities in every cell. In the cellular level this group of vitamins participate in essential metabolic functions and the release of energy from nutrients. 

Water-soluble vitamins are not stored in any of the tissues or in any organs in any appreciable amounts. Any excess to the requirement is excreted rapidly. These vitamins need to be present in the feed to meet the daily requirements.

Some synthesis of B group of vitamins in the intestine does occur as a result of microbial metabolism. The availability and the absorption of water –soluble vitamins from microbial synthesis are not regarded as a reliable source for practical purposes.

The vitamins included in this group are:

• Thiamine (B₁), Riboflavin (B₂)

• Nicotinic acid (Niacin), Pyridoxine (B₆)

• Biotin (H₂), Pantothenic acid

• Folic acid, Cobalamin (B₁₂)

• Ascorbic acid (Vit. C)