Two forms of vitamin A can be found in human diet: Preformed vitamin A (retinol and retinyl ester) that can be found from animal sources or dairy products; and Provitamin A carotenoids that are commonly distributed in plant based foods such as palm oil .
Alpha-carotene, beta-carotene and beta-cryptoxanthin are the most recognized provitamin A carotenoids that contribute to daily vitamin A intake. Both provitamin A and preformed vitamin A must be metabolized or converted to active forms of vitamin A (ie: retinal and retinoic acid) in the body to provide essential biological functions of vitamin A such as preventing night blindness, promote cell growth and development and immune system function. The efficiency of vitamin A conversion highly depends on external factors such as food matrix properties, preparation of foods, amount of dietary fats and fiber consumed, and gastrointestinal tract health issues .
Large amounts of natural plant-derived beta-carotene and other pro-vitamin A carotenoids are not associated with major adverse effects.
In addition, the vitamin A activity for the same amount of provitamin A differs when one acquires provitamin A through supplementation or from daily diet. Vitamin A activity is measured using retinol activity equivalent (RAE) and the table below gives a general idea on the quantity of preformed or provitamin A consumed for biological active vitamin A compounds :
|Quantity Consumed||Preformed / Pro-vitamin A||Acquired through||Quantity Bio-converted to Retinol||RAE Ratio|
|1µg (microgram)||Preformed vitamin A||Diet / Supplementation||1µg of retinol||1:1|
|2µg||Beta-carotene||Supplementation||1µg of retinol||2:1|
|12µg||Beta-carotene||Diet||1µg of retinol||12:1|
|24µg||Alpha-carotene||Diet||1µg of retinol||24:1|
|24µg||Beta-cryptoxanthin||Diet||1µg of retinol||24:1|
- Groff JL. Advanced Nutrition and Human Metabolism. 2nd ed. St. Paul: West Publishing; 1995.
- Weber D, Grune T. The contribution of β-carotene to vitamin A supply of humans. Mol Nutr Food Res. 2012;56(2):251-258.
- Institute of Medicine, (2001). Dietary Reference Intakes for Vitamin A, Vitamin K, Arsenic, Boron, Chromium, Copper, Iodine, Iron, Manganese, Molybdenum, Nickel, Silicon, Vanadium and Zinc. The National Academic Press. https://www.nap.edu/read/10026/chapter/6#83
For further information on the research of mixed-carotene beyond pro-vitamin A function, please go to the RESEARCH section of this website.