By Cliff Harvey
Vitamin A is a fat soluble, antioxidant vitamin. It helps to protect cells from free-radical damage and is involved with immunity, vision, reproduction and cellular communication.
Vitamin A is found as ‘pre-formed’ retinol and retinyl compounds in foods (mainly in fatty tissue from animals, or as a synthetic [vegan] option) and can also be created within the body from beta-carotene and other carotenoids (such as alpha-carotene and beta-cryptoxanthin) found in vegetable foods.
Pre-formed vitamin A is found in fatty tissue from animal sources such as liver, other organ meats, salmon and full-fat dairy. Beta-carotene is found in green leafy vegetables and other green, orange, and yellow vegetables, such as broccoli, carrots, and squash and in fruits, including cantaloupe, apricots, and mangos
It has been suggested that beta-carotene is the safest way to take vitamin A supplements, as pre-formed vitamin A is toxic at high doses whereas beta-carotene is not able to be converted in high enough doses to induce toxicity. However, there may be some evidence that high doses of beta-carotene may also offer some risk.
Because of this beta-carotene is still the most commonly used carotenoid in supplements even though retinyl (a pre-formed vitamin A compound) is much more readily available as usable vitamin A than beta-carotene, and conversion rates of beta-carotene to usable vitamin A differ by a factor of nine-fold (1). In whole food such as spirulina or types of rice, beta-carotene may be required in amounts around four times higher than taking a pre-formed vitamin A type (2, 3). Higher body-fat levels may also negatively affect beta-carotene conversion (1).
For this reason, moderate intakes amounts of both Vitamin A from a pre-formed (i.e. retinyl palmitate) form, along with mixed carotenoids are likely to be the most effective form for supplementation whilst also avoiding toxicity, especially for those who are overweight or suffer any type of malabsorption syndrome (Coelica Disease, Inflammatory Bowel Disease).
Vitamin A containing supplements or foods should be taken with at least 2.4g or more of fat to ensure adequate uptake. Nutrient Reference Values for New Zealand and Australia list a Recommended Daily Intake of around 700mcg of Vitamin A per day for women and 900mcg per day for men.
1. Tang, G., et al., Short-term (intestinal) and long-term (postintestinal) conversion of β-carotene to retinol in adults as assessed by a stable-isotope reference method. The American Journal of Clinical Nutrition, 2003. 78(2): p. 259-266.
2. Wang, J., et al., Vitamin A equivalence of spirulina β-carotene in Chinese adults as assessed by using a stable-isotope reference method. The American Journal of Clinical Nutrition, 2008. 87(6): p. 1730-1737.
3. Tang, G., et al., Golden Rice is an effective source of vitamin A. The American Journal of Clinical Nutrition, 2009. 89(6): p. 1776-1783.