refers to retinol, which comes from carotenoids. There are more than 600 carotenoids, and about 50 of them have pro-vitamin A activities and can be metabolized to vitamin A. The two retinoids, retinaldehyde and retinoic acid, are closely related to retinol and are biologically active as well.
Functions of Vitamin A:
- It is required for normal vision. Retinaldehyde is the essential form of vitamin A that is necessary for synthesis of the “rhodopsin” pigment. Being famous as “visual purple”, rhodopsin enables the retina to detect small amounts of light.
- Retinoic acid is required for normal growth, development, and cell differentiation.
- It plays a role in iron utilization.
- It is highly important for normal, healthy immune system, and may bolster resistance against infection.
- It assists in the repair of body tissues and in maintaining healthy skin.
- It has an antioxidant activity.
- It may have anti – cancer property.
Food Sources and Absorption:
Vitamin A be can be found in foods in two forms. Retinol is the major vitamin A found in animal sources with cod liver oil and all kinds of liver containing the highest amounts. It is also found in dairy products and eggs as well. Carotenoids are found in green leafy vegetables, and yellow and orange - colored fruits and vegetables. Beta – carotene is the most abundant carotenoid in the food supply.
Foods high in carotenoids are dandelion greens, mustard greens, kale, Brussels sprouts, asparagus, sweet potatoes, carrots, broccoli, pumpkin, apricots, cantaloupe, mango and papaya (see “Carotenoids” under the section of “Phytonutrients”).
Vitamin A is absorbed largely from the small intestine and can be stored in the body with liver containing approximately 90% of the vitamin A. It is also stored in the fatty tissues, kidneys, lungs and eyes. Absorption of carotenoids is enhanced by some fat in a meal and is reduced by alcohol, excessive iron intake, vitamin E deficiency and exercise. Poor absorption of fat, infections, and measles could lead to vitamin A deficiency as well.
Vitamin A deficiency increases the risk of mortality from diarrhea, dysentery, measles, malaria, and respiratory diseases. It also poses an increased risk of maternal infection and mortality rate during pregnancy.
Benefits of Vitamin A:
The following conditions may benefit from vitamin A:
- Night blindness.
- Keratomalacia (softening of the cornea).
- Cystic fibrosis.
- Abnormal Pap smear.
- Compromised immune function.
- Low function thyroid.
- Diabetic retinopathy.
- Wound healing.
- HIV support.
- Peptic ulcer.
Dosage and Side Effects:
The RDA for vitamin A for adults is 900 mcg (3000 IU), and the tolerable upper limit for adults is 3000 mcg (10,000 IU) per day. Signs and symptoms of vitamin A deficiency would be evident when dietary daily intake drops to below 300 mcg (1000 IU).
High doses of vitamin A could lead to toxicity characterized by nausea, vomiting, headaches (due to increased pressure inside the skull), anorexia, dry skin, peeling of skin, irritability, blurry vision, hair loss, muscle soreness, dry mouth, inflammation of the mouth, bone demineralization and pain, hyperlipidemia, and fatigue.
Excessive doses of vitamin A during pregnancy could cause congenital malformations, abortion, craniofacial abnormalities, and valvular heart disease.
The following medications interfere with the absorption of vitamin A:
- Mineral oils.