Carotenoids: From (vitamin) A to zeaxanthin

The properties of many carotenoids are being studied, such as their pro-vitamin A and antioxidant activities; immune, endocrine and metabolic activities; and their role in cell cycle regulation, apoptosis and cell differentiation.

“I smell carrots a-cooking, and where there's carrots, there's rabbits.”

—Yosemite Sam 

Carotenoids belong to the category of lipid-soluble terpenes (as a tetraterpenoid) non-polar compounds. Of the more than 600 carotenoids found in nature, about 40 are present in a typical human diet. Of these carotenoids, only 14 and some of their metabolites have been identified in blood and tissues.More than 1,100 known carotenoid compounds2 are split into two chemical classes: carotenes (which are unoxygenated), such as alpha-carotene, beta-carotene and lycopene; and xanthophylls (which contain oxygen), such as astaxanthin, canthaxanthin, beta-cryptoxanthin, lutein and zeaxanthin. Both classes have at least nine conjugated double bonds, which absorb specific wavelengths of visible light during a plant’s photosynthesis process; and thus, provide carotenoids their characteristic colors.  Xanthophylls are more yellow, while carotenes are reddish-orange.3 

Carotenoids are found in high concentrations in plants, algae and microorganisms. Humans cannot synthesize them and therefore are required to ingest them in their diet. The carotenoids that have been .studied the most for human health are: astaxanthin, beta-carotene, lycopene, lutein and zeaxanthin.4

Carotenoids are found in plants, algae and photosynthetic bacteria, where they play a critical role in the photosynthetic process. They synthesize photosynthetic pigments (lipochromes), which are important for photosynthesis process, since they absorb ultraviolet (UV) light wavelengths. Carotenes typically appear blue, violet, orange, red and, in low concentrations, yellow. Xanthophylls appear yellow, orange and red. Carotenoids contribute to photosynthesis by transmitting the light energy they absorb to chlorophyll. They also protect plant tissues by acting as antioxidants, helping to absorb the energy from singlet oxygen.

The term carotene (from the Latin carota, "carrot," daucus carota L.) is used for many related unsaturated hydrocarbon compounds (with no oxygen atoms). Because they are hydrocarbons containing no oxygen, carotenes are lipophilic, meaning they are insoluble in water. Carotenes are terpenoid hydrocarbons found in plants with several isomers forms. The isomers are designated by characters from the Greek alphabet. The two primary isomers are alpha-carotene and beta-carotene, which are precursors of vitamin A. Gamma-carotene, delta-carotene, epsilon-carotene and zeta-carotene isomeric forms also exist.

Carotenes can be found in many dark green and yellow leafy vegetables, and appear as fat-soluble pigments, while beta-carotene can be found in yellow, orange and red colored fruits and vegetables. Beta-carotene is sold separately commercially and has several sources: plant (palm oil), algae (dunaliella salina), fungal (blakeslea trispora and mucor circinelloides), and synthetic.

Xanthophylls (originally called phylloxanthins) are yellow pigments that occur widely in nature. In 1837, the Swedish chemist Jöns Jacob Berzelius described the yellow pigments extracted from autumn leaves, which he named xanthophylls (from the Greek words “xanthos” [yellow] and “phyllon” [leaf]), due to their formation of the yellow, orange, red leaf pigments. They are also considered accessory pigments, along with anthocyanins, carotenes and sometimes phycobiliproteins.

The molecular structure of xanthophylls is like that of carotenes, but xanthophylls contain oxygen atoms (the oxidized form of carotenes). Xanthophylls contain their oxygen either as hydroxyl groups and/or as pairs of hydrogen atoms that are substituted by oxygen atoms (an epoxide). For this reason, they are more polar than the carotenes.

Commercial xanthophylls include: astaxanthin (algae), beta-cryptoxanthin (plant), canthxanthin (plant), fucoxanthin (algae), lutein (plant). and zeaxanthin (algae, plant). Other examples of xanthophylls are: capsanthin, neoxanthin, violaxanthin. and flavoxanthin.

Carotenoid’s health benefits

Carotenoids are fat-soluble micronutrients that play an important role in human health. Studies have shown several carotenoids can provide a multitude of health benefits, including provitamin A activities, a role as antioxidants, preventing age-related macular degeneration (AMD), reducing risk for cardiovascular disease and reducing the risk of some cancers.5

Astaxanthin—antioxidant, anti-inflammatory, anti-diabetic, cardiovascular benefits, cancer reduction, Immuno-modulation, etc.6

Beta-carotene—Vitamin A precursor,7 supports eye health, antioxidant,8 cardiovascular benefits,9 etc.

Lycopene—antioxidant, reduces benign prostatic hyperplasia (BPH), reduced risks of prostate, ovarian, gastric, and pancreatic cancers, prevents UV-induced sunburn, etc.10

Lutein/Zeaxanthin—reduces age-related macular degeneration (AMD), eye health/development, cognitive development, antioxidant, etc.11

Vitamin A precursor

Vitamin A consists of both retinol from animal sources, and provitamin A carotenoids from plant sources. In Western societies, the provitamin A carotenoids derived from plants provide less than 30 percent of daily vitamin A intake, whereas retinol vitamin A derived from animal products provides more than 70 percent daily vitamin A intake, according to the National Health and Nutrition Examination Survey.

Four provitamin A carotenoids can be claimed as vitamin A on product labels: alpha-carotene, beta-carotene, gamma-caroten. and beta-cryptoxanthin. Other carotenoids such as astaxanthin, lutein, lycopene, and zeaxanthin are not provitamin A carotenoids.

Currently, international units (IU) describe vitamin A activity. This is changing, starting in January 2020, to RAE (retinol activity equivalents), which will be used to compare the vitamin A activity of the different forms of vitamin A.  

Carotenoid conversion to vitamin A

The BCMO1 gene codes the BCMO1 protein, which cleaves provitamin A carotene into two molecules of retinal (a form of vitamin A that can be converted to and from retinol, which is the storage and transport form of vitamin A). This conversion happens mainly in the intestinal mucosa (gut barrier), but also occurs in the liver and other organs. Every person has two copies of the BCMO1 gene. But, about 45 percent of the population carries at least one gene variation that reduces BCMO1 enzyme activity, resulting in significantly impaired ability to convert beta-carotene into retinal. Depending on which combination of variants someone inherits, beta-carotene conversion can be nearly 70 percent lower than its normal efficiency, according to the National Institutes of Health (NIH).


Carotenoids are absorbed into the body as lipids and transported via the lymphatic system into the liver. With the exception of carotenoid epoxides, all of the carotenoids described earlier are absorbed by humans into the blood and tissues intact, including: pro-vitamin A carotenoids (such as alpha-carotene, beta-carotene, beta-cryptoxanthin and gamma-carotene) and non-vitamin A carotenoids.12 For the carotenoid fraction remaining in the body, little information is available on their further metabolism, but most likely many are degraded into smaller, more polar fragments via the formation of epoxides and carotenals (apocarotenoids).13

Historically, the ingesting of carotenoids has been known to have beneficial properties for human health. Scientists continue to study their biological role in the prevention and treatment of some human chronic diseases. The properties of many carotenoids are being studied, such as their provitamin A and antioxidant activities; immune, endocrine and metabolic activities; and their role in cell cycle regulation, apoptosis and cell differentiation.

Robin C. Koon, executive vice-president at Best Formulations, has more than 35 years of pharmaceutical experience in clinical pharmacy practice, in retail drug chain operations, in managed care, and in nutraceutical/pharmaceutical manufacturing.

Carotenoids are among many supplements that can support the needs of aging consumers. For tools, tips and tricks on how to effectively market to healthy aging consumers, join us for the "Healthy Aging: Lifelong Wellness" workshop on Wednesday, Nov. 7, at SupplySide West 2018. This workshop is underwritten by Lonza.


1.       Bendich A. “Biological functions of dietary carotenoids.” Acad Sci. 1993; 691:61–67.

2.       Yabuzaki J. "Carotenoids Database: structures, chemical fingerprints and distribution among organisms." 2017. DOI: 10.1093/database/bax004.

3.       Zaripheh S, Erdman JW. “Factors That Influence the Bioavailablity of Xanthophylls.” J Nutr. 2002;132(3):531S–534S.

4.       Wang XD. “Carotenoids.” In: Ross CA, Caballero B, Cousins RJ, Tucker KL, Ziegler TR, eds. Modern Nutrition in Health and Disease. 11th ed: Lippincott Williams & Wilkins; 2014:427-439.

5.       Krinsky NI, Johnson EJ. “Carotenoid actions and their relation to health and disease.” Mol Asp Med. 2005;26:459–516.

6.       Yuan JP et al. “Potential health-promoting effects of astaxanthin: a high-value carotenoid mostly from microalgae.” Mol Nutr Food Res. 2011 Jan;55(1):150-65.

7.       Eggersdorfer M, Wyss A. “Carotenoids in human nutrition and health.” Arch Biochem Biophys. 2018 Aug 15;652:18-26.

8.       Di Martino A et al. “Enhancement of the antioxidant activity and stability of β-carotene using amphiphilic chitosan/nucleic acid polyplexes.” Int J Biol Macromol. 2018 Jun 3;117:773-780.

9.       Csepanyi E et al. “Cardiovascular effects of low versus high-dose beta-carotene in a rat model.” Pharmacol Res. 2015 Oct;100:148-56.

10.   Story E et al. “An Update on the Health Effects of Tomato Lycopene.” Annu Rev Food Sci Technol. 2010;1:189-210.

11.   Giordano E, Quadro L. “Lutein, zeaxanthin and mammalian development: Metabolism, functions and implications for health.” Arch Biochem Biophys. 2018 Jun 1;647:33-40.

12.   Khachik F. “Distribution and metabolism of dietary carotenoids in humans as a criterion for development of nutritional supplements.” Pure Appl Chem. 2006;78:1551–1557.

13.   Bonnie P, Choo M. “Oxidation and thermal degradation of carotenoids.” J Oil Palm Res. 1999;2:62-78. 

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