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 [1].

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 [2].

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 [3]:

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


  1. Groff JL. Advanced Nutrition and Human Metabolism. 2nd ed. St. Paul: West Publishing; 1995.
  2. Weber D, Grune T. The contribution of β-carotene to vitamin A supply of humans. Mol Nutr Food Res. 2012;56(2):251-258.
  3. 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.

For further information on the research of mixed-carotene beyond pro-vitamin A function, please go to the RESEARCH section of this website.