Vitamin A

Vitamin A is an important essential nutrient for most animal species, and both deficiencies and excess dietary intake cause disease

Vitamin A in foods

The main dietary sources of vitamin A are found in liver, other animal tissues, and fish liver oils .

Vitamin A is not found in plant material - but carotene is. Carotene gives plants their yellowish colour and it can be converted into active vitamin A by animals which have an enzyme to do this in the wall of their intestine, including :
buffalo
dog
goat
pig
rabbit
rat
sheep

In other species eg humans, cattle, chicks and horses a large amount of the carotene in food is absorbed intact. Animals that absorb carotene have yellow body fat, whereas those that convert carotene to vitamin A before absorption have white body fat.

Carotene is also known as pro-vitamin A. Cats and mink are two species that are obligatory carnivores and they can not use carotene so they need to have vitamin A from animal sources in their ration. This is one of the reason why cats can not be fed a vegetarian diet.

Roles in the body

In the body vitamins control the rate of biochemical pathways - as enzymes, co-enzymes or as a precursor to enzymes. They are important for the control of many physiological processes in the body.

Many authors consider vitamin A to be the most important vitamin in the body because it is important for normal :
Growth
Protective mucous membranes
Reproduction
Sight

Vitamin A deficiency

Vitamin A is important for normal bone growth and development and deficiency has been associated with several abnormalities.

Animals deficient in vitamin A are more susceptible to infections and stress-related disorders, and shedding of cells into the urinary tract may lead to the formation of stones (called uroliths) which can cause urinary tract problems.

Vitamin A is a constituent of retinal which is found in the rods of the retina of the eye combined with a protein called "opsin" to form visual purple (also called rhodopsin), and vitamin A deficiency causes night blindness . Vitamin A is also responsible for maintaining a normal surface on the eye (the cornea) and deficiency leads to drying of the eye surface - a condition called xerophthalmia. This can lead to blue cloudiness of the eye followed by ulcer formation. In humans vitamin A deficiency is the commonest cause of blindness in young children.

Vitamin A toxicity

Taking excess vitamin A supplement, or eating too much liver are common causes of toxicity. 90% of the body's vitamin A is stored in the liver, so liver is an excellent source of vitamin A - on the other hand too much liver can cause vitamin A toxicity.

Too much vitamin A fed to pregnant dogs can cause an anatomical defect called cleft palate to occur in the offspring. Toxicity is common in cats fed fresh liver and this causes new bone to form around joints which is painful and causes lameness.

Vitamin A requirements

Vitamin A requirements increase during growth and pregnancy, and vary in different species. Poor storage conditions such as prolonged storage of food at high temperatures can reduce the amount of active vitamin A in a food.

Most prepared complete pet foods have adequate amounts of vitamin A added to the mix and additional supplementation is unnecessary and may be harmful so don't give supplements to your pet without taking veterinary advice first.