VITAMIN A

'Vitamin A' actually refers to a family of related compounds. These are divided into two groups: preformed Vitamin A and provitamin A.

Retinoids and Carotenoids

The former group, also called retinoids, contains compounds that are already in their active form, while the second group, also called carotenoids, can be converted into the active forms within the body.

Although there are over 600 recognised carotenoids, only about 10 % of these actually behave as provitamin A – the others play different important roles.

What are the functions of the retinoids?

VISION

As the name ‘retinoids’ suggests, these compounds play a role in vision where the retina plays a vital part.

Light coming into the eye is received by the retina where it stimulates the production of a compound called rhodopsin. This causes a nerve impulse that travels to the brain to be interpreted as sight.

Rhodopsin contains a protein (called opsin) as well as 11-cis-retinal which is derived from Vitamin A. This makes Vitamin A fundamental to good vision.

Light changes the 11-cis-retinal to all-trans retinal. This causes rhodopsin to break down and form metarhodopsin II which is responsible for the electrical impulses that travel to the brain via the optic nerve.

Then the metarhodopsin converts to scotopsin and the all-trans retinal is converted to 11-cis-retinal and the cycle continues.

Vitamin A deficiency and night blindness

One of the first signs of a Vitamin A deficiency is called ‘night blindness’ (or nyctalopia). This means that your ability to see at night or in poor light is severely impaired, while daytime vision appears to be normal.

The reason is that the retinal part of the rhodopsin molecule is present in inadequate amounts and the light available at night is insufficient to stimulate the vision reactions.

Vitamin A deficiency is not the only cause of night blindness, so you need to visit your ‘eye doctor’ for a diagnosis. But if a vitamin deficiency is the cause, it is easily remedied.

IMMUNITY

Vitamin A plays a protective role against infections because it is involved in the immune system in two ways:

* It is important as a first line of defense because it maintains the integrity of the skin and mucosal cells (lining the digestive tract, the urinary tract, the vaginal tract and the lungs).

This means that bacteria and other infectious agents and toxins cannot enter the body easily. Not surprisingly, some symptoms of Vitamin A deficiency include a rough, scaly skin, sinus and throat infections and ear and mouth abscesses.

* Secondly it is essential for the formation of white blood cells including lymphocytes. These play a role once noxious agents have gained entry into the body.

REPRODUCTION, GROWTH AND DEVELOPMENT

Vitamin A is necessary for reproduction in humans. A deficiency halts the formation of sperm in males and causes an interruption in the oestrus (menstrual cycle) of women.

Once fertilization has taken place it plays a role in foetal development, notably the formation of the limbs, the heart, eyes and ears and nervous tissue.

Then, once the child is born, ongoing deficiency results in growth retardation (possibly due to faulty human growth hormone (HGH) production), impaired bone and tooth formation, and a weakened immune response.

Consequently these children are very susceptible to infections including respiratory diseases and diarhhoea.

Of note is that excess Vitamin A is also likely to cause birth defects, so pregnant women need to exercise caution.

RED BLOOD CELL PRODUCTION

All blood cells come from the bone marrow which produces precursor cells called stem cells. Under different stimuli the stem cells differentiate into the wide variety of blood cells.

Vitamin A is essential for red cell differentiation. In addition, it appears to be responsible for bringing iron to the developing red cell so that haemoglobin can be made.

Without the haemoglobin the red cell cannot perform its function which is to carry oxygen from the lings to all parts of the body.

The influence of Vitamin A in red cell production is one example of many where it plays a role in cellular differentiation i.e. the specialization of cells for specific roles.

What are the functions of the carotenoids?

The roles of beta-carotene

The main carotenoid that is a provitamin A is called beta-carotene. In addition to being a source of active Vitamin A, it functions as an antioxidant.

This makes it an effective anti-cancer nutrient and it inhibits the deposition of plaque on the blood vessel walls thereby playing a preventative role in heart disease.

A real advantage of carotenoids as a source of Vitamin A is that the conversion from the provitamin only takes place when needed. Toxicity is therefore highly unlikely.

Can one take too much? Yes, but the symptom is a harmless yellowing of the skin which goes away when you stop taking it for a while.

Dietary sources of Vitamin A and Carotenoids

Liver and cod liver oil are rich in Vitamin A.

Beta-carotene comes from yellow, orange and green leafy vegetables e.g. carrots, sweet potatoes, spinach and pumpkin and yellow fruits such as papaya, peaches and cantaloupe.

The upper safe limit for Vitamin A supplementation has been set at 10 000 IU per day. Many multivitamins supply 5000 IU per dose – clearly well within these limits.

Beta-carotene is not known to be toxic, and the release of Vitamin A from this nutrient within the body is determined by need.


Return from Vitamin A to the base page on Vitamins.