Vitamin A, Retinol

Vitamin A (Retinol) is a fat-soluble vitamin essential for the production of adrenal and thyroid hormones, proper function of nerve cells, immune system and cell growth. The word "retinoids" is a generic term that includes both naturally occurring compounds with vitamin A activity and synthetic analogs of retinol. Different retinoids can display some, but not necessarily all, of the biological activities of the vitamin A (retinol). Retinol is derived from animal foods, the main sources being liver, dairy products and vitamin supplements.

Carotenoids are a class of natural fat-soluble pigments found principally in plants, algae, and photosynthetic bacteria, where they play a critical role in the photosynthetic process and can be a source for vitamin A activity. Beta-carotene, a vitamin precursor that is converted into vitamin A, is derived mostly from vegetables, such as carrots and green vegetables. Usually, vitamin A is absorbed from the circulation and stored in the liver. Retinol is toxic in high doses, but beta-carotene is not. At present, there more than 2,000 retinoids or synthetic retinol analogs. Recent studies have confirmed that carotenoids prevent several types of cancer, including breast and reproductive organs cancers.

Vitamin A plays an important role in maintaining healthy vision. Night blindness or poor dark adaptation is an early consequence of vitamin A shortage. Vitamin A deficiency can result in xerophthalmia, a severe drying of the eye surface caused by a malfunction of the tear glands, and permanent blindness. Vitamin A is also necessary for proper growth and development, and is particularly important in maintaining the health and structure of the skin. Many skin disorders, including acne and psoriasis, are often successfully treated with topical retinoids or vitamin A derivatives.

Vitamin A (Retinol) Deficiency and Toxicity

The recommended Dietary Allowance (RDA) for vitamin A is now 1,000 RE (Retinol Equivalents), or 5,000 IU (International Units). Children need 400 to 1,000 RE (2,000 to 5,000 IU), with the dosage increasing from infancy to fourteen years.

Rich Food Sources of Vitamin A

Rich food sources of vitamin A include liver, whole milk, and fortified skim milk. Vitamin A can also be formed from beta-carotenes and other carotenes, or carotenoids -- molecules that are widespread in nature and are typically seen as pigments in fruits, flowers, birds and crustacea. The major sources of provitamin A carotenes are dark green leafy vegetables, such as collards, spinach, and yellow-orange vegetables, such as carrots, sweet potatoes, yams, and squash. Over recent years there has been considerable interest in dietary carotenoids with respect to their potential in alleviating age-related diseases in humans.

Vitamin A Quick Calculator

To compensate a possible low level of vitamin A, make sure your food includes healthy portions of dark green leafy and yellow fruits and vegetables. Here is a selection of some foods containing vitamin A:

  • Liver, beef (3.5 oz) - 43,900 IU
  • Liver, calf (3.5 oz) - 22,500 IU
  • Chili peppers (3.5 oz) - 21,600 IU
  • Chicken liver (3.5 oz) - 12,100 IU
  • Carrots (3.5 oz) - 11,000 IU
  • Apricots, dried (3.5 oz) - 10,900 IU
  • Collard greens (3.5 oz) - 9,300 IU
  • Kale (3.5 oz) - 8,900 IU
  • Sweet Potatoes (3.5 oz) - 8,800 IU
  • Spinach (3.5 oz) - 8,100 IU
  • Mangoes (3.5 oz) - 4,800 IU
  • Broccoli (3.5 oz) - 2,500 IU
  • Whole milk (1 cup) - 227 IU

Bioavailability of Vitamin A

Bioavailability is the percentage of a nutrient absorbed from the food. Nutrients consumed but not released during the digestive process for absorption have no nutritional value. The vitamin A from animal tissue is more bioavailable to humans than the provitamin A in green plants. Provitamin A carotenes achieve the vitamin A efficacy only in the presence of oils. This is why carrots sauteed in oil will have more carotenoids available for production of vitamin A than boiled carrots. Red palm fruit oil (RPO) with its high content of natural carotenoids, was shown to improve the vitamin A levels of nursing mothers and their infants. Consumption of RPO included in a sweet snack or biscuits significantly improved blood vitamin A concentrations in children with vitamin A deficiency.

There is evidence that if only 35-50% of the recommended daily intake for vitamin A were to be provided by RPO, it may be sufficient to prevent vitamin A deficiency (hypovitaminosis A). Red palm oil has a highly bioconvertible form of alpha- and beta-carotene, and a long shelf life. Red palm oil is safe and cannot produce hypervitaminosis A: our bodies convert them into vitamin A as and when required. What's more important is that the amount of vitamin A absorbed from red palm oil is much greater than from any other vegetables.

High Dosages of Vitamin A Can Be Toxic

Osteoporosis and hip fracture have been reported in people who consume food with vitamin A intakes that are only twice the current RDA. Chronic toxic effects of high intakes of vitamin A include weight loss, low-grade fever, dryness and peeling of the skin, hair loss, hypercalcemia (higher than normal level of calcium in the blood with potential kidney failure), dermatitis that resembles contact dermatitis or pellagra, swelling of the hands, feet and face, rash, bone pain and tenderness, and liver enlargement. Acute hypervitaminosis A may occur after ingestion of greater than or equal to 500,000 IU (over 100 times the RDA).

Numerous vitamin supplements are available over-the-counter to the general public. Some such supplements are available as candy-like chewable preparations to encourage consumption by children. Parents should be warned about the dangers of excessive vitamin consumption which may lead to complications of vitamin A overdose. Water-miscible (specially formulated to improve absorption), emulsified, and solid preparations of retinol are approximately 10 times as toxic as are oil-based retinol preparations, so be careful while selecting a particular vitamin formulation. Young people taking topical retinoids (tretinoin, retinol, retinyl palmitate, adapalene and tazarotene) as part of acne treatment may develop high intracranial hypertension due to chronic A-hypervitaminosis, which usually returns to normal after discontinuing a medication. Prolonged and continuous consumption of doses in the low "therapeutic" range can result in life-threatening liver damage.