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

References:

  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. 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.