Definition: 

The term " vitamin A " represents several compounds which have similar properties in humans. The most abundant and active compound is retinol.

Vitamin A comes from to different sources :

• retinol itself that is naturally present in foods like liver, eggs, butter, milk

• the transformation of pro-vitamins A into retinol. Pro-vitamins A are mainly alpha- and beta carotene, and cryptoxanthins that belong to the carotenoid family Carotenoids are found in vegetable and fruits. This second source of vitamin A normally account for the main intake of vitamin A

Because of the diversity of vitamin A compounds and provitaminic activities of certain carotenoids, vitamin A dosages are expressed as retinol equivalent (RE) or International Units (UI). One RE equals 1µg retinol or 6 µg beta-carotene or 12 µg other provitaminic carotenoids. However, equivalence between vitamin A and carotenoids is currently controversial. Actually 1 RE would be provided by 10 µg beta-carotene and 20 µg other provitaminic carotenoids.

Metabolism

After intestinal absorption, or after carotenoid transformation into retinol in gut, vitamin A reaches the liver by lymphatic vessels.  Intestinal absorption of retinol is more efficient than that of carotenoids (80% vs. 35%), moreover absorption is dependant of sufficient amounts of lipids (oil, butter). Carotenoid conversion into retinol is depressed or even stopped by nutritional deficiencies.

Liver is the main storage organ in the body, it stores about 50 – 80% of total body vitamin A. The main part of newly absorbed vitamin A is uptaken by the liver that releases it later when tissues need. Vitamin A is released in blood by liver, it is bound to a small protein: the retinol binding protein (RBP). Vitamin A concentrations in plasma are much influenced by RBP levels. As RBP levels depend from many other factors than vitamin A status, plasma concentrations of vitamin A are not good markers of its status. Storage must be very low to lower plasma vitamin A, indeed in normal conditions plasma levels of retinol are regulated and stay at a stable level: this is homeostasis.

However, expert consider that when plasma retinol concentrations drop below 300 µg/l (1.05 µmol/l) there is a real risk of vitamin A deficiency.

Vitamin A roles

Sight

Vitamin A plays an important role in sight. In retina rods, retinal, a derivative of retinol, combines with rodopsin to form a complex that is highly sensitive to light. When might hits this complex, it dissociates releasing an amount of energy which will stimulate nerve termination in retina depth. This captor will transmit an image to the brain by optic nerves. One of the earliest sign of vitamin A deficiency is night-impaired sight.

Growth

Many studies have shown that vitamin A is essential to normal growth. Deficiency induces, in a few weeks, a loss of appetite, a loss of weight and stuns growth.

Skin and membrane development

In case of vitamin A deficiency cells store keratin, a rigid protein, and progressively lose their normal capacity, being unable to play their normal role in tissues. Such alterations are seen in all tissues, but eyes lesions appear first. Lesions become irreversible if intake stay too low as symptoms aggravate. The effect of vitamin A on cell division is probably explained by transformation of retinol into retinoids which act at the molecular level on cell functions.

Skin and membrane alteration partly explain increased sensitivity to infection during vitamin A deficiency. Even marginal deficits may induce modifications of lung membranes.

Deficiencies

Vitamin A deficiency is frequent in developing countries. It is rare in industrialised countries but still occurs in low-income families. Causes of deficiency are inadequate intake or vitamin losses due to diseases or their treatment.

In developing countries, vitamin A deficiency is the first cause of blindness, it is also responsible for high mortality rates by infection (mainly measles) in infants and children.

In industrialised countries, deficiency states are less severe and signs are less apparent, these are: bad night sight, skin dryness, increased susceptibility to virus and bacteria infections.

European community recommends the following vitamin A daily intakes :

Simply multiply these amounts by 3.33 to convert into IU

Excessive vitamin A intakes may have toxic effects. Supplements should therefore be controlled to avoid any vitamin A overloading. Toxicity is rarely the consequence of absorption of vitamin A-rich foods.

Intakes as high as 30 000 RE in adults, 3600 RE in children may induce toxicity in 6 – 8 months. Signs of toxicity are : loss of appetite, itchy and dry skin, hair losses, bone thickening, big liver, nausea, and vomiting. During pregnancy vitamin A is believed to be teratogenic, however this effect has not yet been formally demonstrated. The American Teratology Society recommends daily intake of vitamin A lower than 3000 RE in pregnant women.