|Dynamic Chiropractic – June 20, 2006, Vol. 24, Issue 13|
By James P. Meschino, DC, MS
|Editor's note: This is part one of a two-part article on vitamin A. Part one discusses general features, functions and health benefits of vitamin A; part two, which will appear in the July 16 issue, discusses toxicity, supplementation, drug-nutrient reactions and potential contraindications.Part two includes a complete list of references spanning both parts of the article.|
Vitamin A is a vital, but often-ignored nutrient for health optimization and disease prevention. It is important in the prevention of cancer, night blindness and infection. More than 50 percent of Americans do not meet the daily requirement of vitamin A intake, while other Americans ingest excess vitamin A from supplements, which increases the risk of vitamin A toxicity, birth defects and osteoporosis. The daily ingestion of vitamin A is a delicate balancing act between taking in enough of the vitamin to derive its beneficial effects, while at the same time avoiding problems mitigated by excess consumption. This article highlights important physiological and clinical aspects of vitamin A that health practitioners should be aware of when making recommendations to their patients about optimizing vitamin A status and using supplements containing vitamin A.
Preformed Vitamin A is a fat-soluble group of related compounds. The most common preformed version present in food is retinol; others are retinal (retinaldehyde) and retinoic acid. Retinol can be reversibly oxidized to retinal, which is required for night vision. Oxidation of retinal produces retinoic acid, which does not participate in the visual cycle and cannot be converted back to the aldehyde form (retinal). However, retinoic acid does support growth and normal differentiation of epithelial tissue, but does not support reproductive function, as do other forms of vitamin A.
In general, vitamin A serves at least five major functions in the body:
Absorption and Metabolism
Preformed vitamin A is absorbed in the gastrointestinal tract, enters the lymphatic system within chylomicrons and then enters into general circulation, which ultimately delivers vitamin A to the liver, the main storage site (90 percent) for vitamin A (which also is stored to a lesser degree in the kidneys, adipose tissue, and adrenal glands). It is released from the liver in the form of retinol, and bound to retinol-binding protein (RBP).
Between 80 percent and 90 percent of vitamin A typically is absorbed from the gut, demonstrating excellent bioavailability. Retinoic acid from food is absorbed from the gut and transported in the blood bound to albumin. It normally does not accumulate within the liver or other tissues in any real appreciable amounts.
Once delivered to the cells via the bloodstream, vitamin A is extracted from the bloodstream and binds to intracellular proteins known as CRBP (cellular retinal-binding protein) and CRABP (cellular retinoic acid-binding protein). Within the cells of the body, vitamin A modulates many biochemical reactions that promote growth, replication, differentiation, and provide additional antioxidant protection.
Retinol Equivalents (RE)
In addition to preformed vitamin A, which is present in animal foods, orange-yellow fruits and vegetables and dark green vegetables contain precursors to vitamin A synthesis, which occurs in the body (e.g., beta-carotene). In North America, approximately 50 percent of vitamin A is derived from vitamin A precursors via the consumption of fruits and vegetables. The following chart outlines the retinal equivalent values of various carotenes as well as preformed vitamin A.
1 retinol equivalent (RE):
= 1 ug retinal
= 6 ug beta-carotene
= 12 ug other provitamin A carotenoids
= 3.33 IU vitamin A activity from retinol
= 10 IU vitamin A activity from beta-carotene
Recommended Daily Allowance
|Adult males||1,000 retinol equivalents (RE)|
|Adult women||800 RE or 4,000 IU|
|Pregnancy||1,000 RE (5,000 IU)|
|Lactation||1,200 RE (2,000-5,000 IU)|
|Children||400-1,000 RE (2,000-5,000 IU); amount increases from infancy to 14 years.|
Overt Deficiency of Vitamin A
Join the conversation
Comments are encouraged, but you must follow our User Agreement
Keep it civil and stay on topic. No profanity, vulgar, racist or hateful comments or personal attacks. Anyone who chooses to exercise poor judgement will be blocked. By posting your comment, you agree to allow MPA Media the right to republish your name and comment in additional MPA Media publications without any notification or payment.
To report inappropriate ads,