Carotenoids are 40-carbon compounds that are found throughout nature. Animals do not produce carotenoids, thus any found in animals came from consumed plants or microorganisms. There are more than 600 natural carotenoids. However, the 6 main ones found in the diet and in the body are1:
Many carotenoids are pigments, meaning they are colored. The table below gives the color of some of these carotenoids, as well as some food sources.
|Beta-carotene||Orange||Carrots, Sweet Potatoes, Leafy Greens|
|Lycopene||Red||Tomatoes, Watermelon, Pink Grapefruit|
|Lutein/Zeaxanthin||Yellow||Kale, Corn, Egg Yolks, Spinach|
Carotenoids can be further classified as provitamin A or non-provitamin A. Provitamin A carotenoids are those that can be cleaved to form retinal, while the non-provitamin A carotenoids cannot. The structure and classification of the 6 major carotenoids are shown below.
Figure 12.611 Structure and classification of the 6 major carotenoids
After provitamin A carotenoids are taken up into the enterocyte, some are cleaved to form retinal. In the case of symmetrical beta-carotene, it is cleaved in the center to form 2 retinal molecules as shown below.
Figure 12.612 Cleavage of beta-carotene 2 to retinal molecules2
Alpha-carotene and beta-cryptoxanthin are asymmetrical, thus they can be used to form only 1 retinal.
To help account for the fact that retinol can be made from carotenoids, the DRI committee made retinol activity equivalents (RAE) that take into account the bioavailability and bioconversion of the provitamin A carotenoids.
1 ug RAE
- = 1 ug of retinol
- = 2 ug of supplemental beta-carotene
- = 12 ug of dietary beta-carotene
- = 24 ug of alpha-carotene or beta-cryptoxanthin3
References & Links
- Lindshield BL, Erdman JW. (2006) Carotenoids. In: Bowman BA, Russell RM, editors. Present Knowledge in Nutrition. Washington, D.C.: International Life Sciences Institute. pp. 184-197.r
- Anonymous. (2001) Dietary reference intakes for vitamin A, vitamin K, arsenic, boron, chromium, copper, iodine, iron, manganese, molybdenum, nickel, silicon, vanadium, and zinc. Washington, D.C.: National Academies Press.