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12.61: Carotenoids

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:

  • Beta-carotene
  • Alpha-carotene
  • Beta-cryptoxanthin
  • Lutein
  • Zeaxanthin
  • Lycopene

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.

Table 12.611 Carotenoids’ color and food sources
Carotenoid Color 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

  1. 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
  3. 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.