VITAMIN A/Retinol/Carotene (Digestion/absorption/storage/transport/functions/recommended intakes/common food items/ Can you die from excess/toxicity

Vitamin A is the generic descriptor for compounds with vitamin A activity, namely all-trans retinol and its derivatives, retinal and retinoic acid, which are the naturally occurring vitamers. During the past few years a great deal of research into production of synthetic compounds with vitamin a activity has given rise to the term “retinoids” which is a generic term that includes both naturally occurring compounds with vitamin A activity, as well as synthetic analogues of retinol, with or without biological activity.

The dominant feature of the molecules of the vitamin a family is the conjugated double bond system which gives the molecule a high extinction coefficient. The basic structure also makes it extremely hydrophobic, a property that presents a major problem to animals that have to obtain it from the diet and transport it in an aqueous medium to target tissue.

Structure of the retinol

Digestion and absorption of Vitamin A/Retinol/Carotene

Vitamin a exists as an ester in the food and it is hydrolyzed in the duodenum in the presence of bile salts. Since both retinol and carotenes are highly unsaturated they tend to oxidized.

Absorption of both retinols and carotenes are facilitated by the presence of fat in the diet, being associated with micelles formed during fat digestion. Utilization of both retinol and beta-carotene is also enhanced by dietary protein and zinc. About 90% of dietary retinol is absorbed and 60% to 70% of beta-carotene is also absorbed.

Excessive intake of either retinol or beta-carotene decreases vitamin E status. Vitamin E, a powerful anti-oxidant, protects retinol and beta-carotene from oxidation.

Storage of Vitamin A/Retinol/Carotene

Hepatic vitamin A (95% as esters of long chain acids, chiefly palmitate) represents 90% of the body reserves of the Vitamin A. Mammalian liver has an exceptional capacity to store retinol over a wide range of dietary intakes. However, the plasma level remains remarkably constant.

Transport of Vitamin A/Retinol/Carotene

Retinol is mobilized from liver and transported in plasma to target cells as a complex with retinol binding protein (RBP), which is a single polypeptide of molecular weight about 21,000 and one binding site. Free retinol-RBP complex disappears from circulation quickly, being lost in the urine.

Recommended intakes of vitamin A/Retinol/Carotene

Infants are born with small stores and depend on breast milk to lay down their stores in the liver. A well nourished mother secretes about 60µg retinol per deciliter of milk, providing 50µg/kg body weight of infant.To provide an average secretion of milk of 650ml the recommended daily allowance (RDA) for lactating women is 850µg retinol/day. Poorly fed mothers provide only 30 to 40µg/kg infant weight in India and less than 20µg/kg in Jordan.

Age Group	Basal need (µg)	Safe limit (µg)
Infants	180	350
1-6 years	200	400
6-15 years	270-350	400-600
Adult male	300-400	500-600
Adult female	270-330	500
pregnancy	Normal+100	Normal+100
lactating	Normal+180	Normal+350

Recommended dietary allowance of vitamin A in retinol equivalents (RE)

Function of Vitamin A/Retinol/Carotene

Vitamin A exists in three forms; retinol: retinal (aldehyde): retinoic acid. It has three main functions;

1. Visual function; this is the first to be deranged in deficiency.
2. systemic function; promoting growth and maintenance of tissues
3. Reproductive function

Vitamin A toxicity (hypervitaminosis)

Infants and young children are more vulnerable for Vitamin A toxicity. Bulging of fontanelles, irritability, severe headache and vomiting are features of acute toxicity of Vitamin A. Painful extremities, dry skin, sparse hair and hydrocephalus are some of the features of chronic Vitamin a toxicity. Therefore Infants and young children can die from Vitamin a toxicity. Adults can also be affected, but less frequently than that of youngest.

Common sources of Vitamin A/Retinol/Carotene

The dietary sources of vitamin a fall into two categories;

1. Vitamin A (all-trans retinol or preformed vitamin A)
2. Provitamin A (carotenoid precursors)

Common food items are as follows;

Chicken eggs

Green vegetables

Organ meat

1. preformed vitamin A ( milk and milk products, eggs, shell fish, liver and organ meats)
2. Provitamin A ( dark green leafy vegetables, red and yellow vegetables and fruits)