A group of phytochemicals called carotenoids play a vital role in bone growth, reproduction, and healthy immune, detoxification, and inflammatory systems. They help your skin and mucous membranes repel bacteria and viruses more effectively. They are essential to healthy vision, and may slow declining retinal function in people with retinitis pigmentosa.  They are also important for growth and development in infants and children and in red blood cell formation and sperm production.

Carotenoids represent one of the most widespread groups of naturally occurring pigments: There are over 600 known carotenoids that are largely responsible for the red, yellow, and orange color of fruits and vegetables, and are also found in many dark green vegetables. The most abundant carotenoids in the North American diet are:

• Carotenes are a large group of intense red and yellow pigments found in all plants that photosynthesize, which are vital for the process of photosynthesis and also protect the plant against damage from the free radicals produced during photosynthesis. They include:
  • Alpha-carotene and beta-carotene, found in sweet potatoes, carrots, kale, spinach, turnip greens, winter squash, collard greens, cilantro, fresh thyme, cantaloupe, Romaine lettuce, broccoli, tomatoes, green beans, chard, apples, and avocadoes, protect your cells from the damaging effects of free radicals, provide a source of vitamin A, enhance the functioning of your immune system, and help your reproductive system function properly. Beta-carotene is a more powerful anti-oxidant than retinoid vitamin A and not only protects epithelial cells (the cells that form the lining of numerous body structures including your blood vessels) from free radical damage, but also helps prevent free radicals from oxidizing cholesterol in your blood stream. Only after it has been oxidized does cholesterol build up in blood vessel walls, initiating the development of atherosclerosis, whose end result can be a heart attack or stroke. Free radical damage is a contributing factor in many other conditions as well, including asthma, osteoarthritis, and rheumatoid arthritis. Beta-carotene also helps to internally protect your skin from sun damage by both deflecting and repairing cell damage caused by excessive ultraviolet exposure, and helps prevent premature wrinkling, acne, dry skin, pigmentation, blemishes, and uneven skin tone.
  • Lycopene, found in tomatoes, pink grapefruit, watermelon, guava, and persimmons, protects your cells from the damaging effects of free radicals, and helps prevent the oxidation of cholesterol, thereby slowing the development of atherosclerosis.
• Xanthophylls, oxygenated forms of carotenes, form the yellow color in plant leaves. They include:
  • Lutein and zeaxanthin, found in raw and lightly steamed kale, spinach, turnip greens, collard greens, Romaine lettuce, broccoli, zucchini, corn, peas, squash, pumpkin, and Brussels sprouts, defend your cells from the damaging effects of free radicals, and protect your eyes from developing age-related macular degeneration and cataracts.
  • Astaxanthin, found in microalgae and yeast, suppresses the activation of certain T cells associated with asthma, improves “working memory,” may contribute to the prevention of dementia, significantly increases peripheral capillary blood flow, reduces both systolic and diastolic blood pressure, prevents cancers of the bladder, colon, and mouth, reduces the severity and duration of carpal tunnel syndrome pain, counteracts the free radicals resulting from high glucose levels in diabetics, may prevent the progression of diabetic kidney damage, may improve endurance and strength, may decrease the accumulation of fat, protects the macula at the center of the retina from degeneration from UV light, heading off Age-Related Macular Degeneration, the world’s leading cause of blindness, increases HDL (“good”) cholesterol production, protects LDL (“bad”) cholesterol from free radical damage, significantly reduces heartburn, especially in patients infected with H. pylori, an inflammatory bacteria that causes ulcers, raises levels of T and B cells, key cells of the immune system, may prevent DNA damage, improves sperm quality and function, may help maintain natural testosterone levels in aging men, and protects your skin from UV damage that would otherwise damage skin DNA.
  • Beta-crpytoxanthin, found in red bell peppers, papaya, cilantro, oranges, corn, watermelon, serrano pepper, avocados, and grapefruit, protects your cells from the damaging effects of free radicals, provides a source of vitamin A, and reduces your risk of lung cancer, rheumatoid arthritis, and inflammatory polyarthritis.
  • Canthaxanthin, found in chanterelle mushrooms, green algae, and paprika, inhibits the growth of melanoma and colon cancer cells, and induces their self-destruction.

Of the more than 600 known carotenoids in plants, and your body can convert about 50 (including alpha-carotene, beta-carotene, and beta-cryptoxanthin) into retinoids (retinoic acid, retinal, and retinol): a group of chemicals that we know as vitamin A. Dr. T. Colin Campbell doesn’t believe that vitamin A meets the definition of a vitamin, because it isn’t essential: your body can make all the retinoids it needs from carotenoids.

In comparison to alpha-carotene or beta-cryptoxanthin, it takes only half as much beta-carotene for your body to create the same amount of retinol. The corpus luteum in human ovaries contains a very high level of beta-carotene, suggesting that this nutrient plays an important role in the female reproductive processes. Beta-carotene can also improve the availability of two minerals—iron and zinc—from grains. Eating one medium-sized carrot (about 50 grams) along with each cup of cooked rice  results in a 50% increase in the availability of iron and increases the availability of zinc by about 35-40%. Beta-carotene may form a complex with the minerals to help maintain their solubility, and it also may help prevent their getting bound together with phytates in the rice that would otherwise be able to lower their absorption. Your retinas contain the xanthophyll carotenoids lutein and zeaxanthin. Vitamin A benefits related to eye health (for example, prevention of age-related macular degeneration) depend on foods that are rich in vitamin A, but more specifically, rich in these two specific carotenoid forms of the vitamin, such as spinach, kale, and chard.

Almost all carotenoids are potential anti-cancer and anti-aging compounds because of their antioxidant and anti-inflammatory properties, even the 90% that your body can’t convert into retinoids. Lycopene, for example, may prevent prostate cancer. Carotenoids also promote proper communication between cells, which may play a direct role in cancer prevention, because poor communication between cells may be one of the causes of cell overgrowth, which can lead to cancer.

You need retinoids for normal cell growth and development. Retinoic acid helps synthesize many glycoproteins, which control cell growth, cell differentiation, and the ability of cells to attach to one another. For example, your bone marrow uses retinoic acid to produce red blood cells.  Retinoids are also required for proper production of sperm, and for normal bone metabolism.

Your retinas contain four kinds of photopigments that store retinoid compounds. One of these pigments, called rhodopsin, is located in the rod cells of your retinas. Rhodopsin allows the rod cells to detect small amounts of light, and, therefore helps your eyes adapt to low-light conditions and provides you with night vision. Retinal, the aldehyde form of retinoid, helps your eyes synthesize rhodopsin, and helps with the series of chemical reactions that occur when light strikes your rod cells. The remaining three pigments, collectively known as iodopsins, are in the cone cells of your retina and are responsible for day vision.

Throughout your body, but particularly in your digestive tract, carotinoids and retinoids play key roles in support of immune, detoxification, and inflammatory functions. Your digestive tract can be exposed to unwanted substances (like pesticide residues), as well as unwanted microbes (like infectious bacteria). Your immune, detoxification, and inflammatory systems help prevent you from being harmed by these substances. For example, in order to neutralize harmful micro-organisms, your immune system makes and releases antibodies that can block their activity. Retinoids help synthesize the T cell and B cells of your immune system and activates your immune response. Whenever you experience an increase in whole-body inflammation, your cells also increase their conversion of retinol into retinoic acid. Carotenoids and retinoids may be equally important for turning off your immune and inflammatory systems, so that they don’t become over-reactive  Because allergies can be related to your immune system’s overreaction to foreign proteins, optimal intake of vitamin A may be important for lowering your risk of certain types of allergies.

Retinoids are fat-soluble, so your tissues store them. If you ingest pre-formed retinoids from animals or supplements, they can build up in your body and become toxic. Symptoms of toxicity include hair loss, confusion, liver damage, and bone loss. In addition, the effects of vitamin D deficiency may be worsened by taking high-dose vitamin A supplements in retinoid form. Excess retinoid-form vitamin A may also interfere with the metabolism of vitamin K, a fat-soluble vitamin necessary for blood clotting and bone health. Finally, taking supplements of vitamin A or beta-carotene may interfere with the absorption of other crucially important carotenoids, such as lutein and lycopene, thus potentially increasing cancer risk. The best way to get carotenoids is from unrefined plant-based food.

Retinoid deficiency is rare in developed nations, but is common in developing ones. Retinoid deficiency symptoms begin with night blindness, and if it progresses, it can lead to corneal ulcers, scarring, and blindness. Retinoid deficiency also reduces immunity, and allows infectious diseases such as pneumonia to become deadly. It is also associated with very low birth weights in infants of mothers with deficiency. 

Retinoid deficiencies may be caused by alcoholism, a diet that is extremely low in fat, or medical conditions that cause a reduction in the ability to absorb fat, such as pancreatic enzyme deficiency, Crohn’s disease, celiac sprue, cystic fibrosis, surgical removal of part or all of the stomach, gall bladder disease, and liver disease. In addition, chronic diarrhea caused by gastrointestinal infections or intestinal parasites may contribute to deficiency of carotenoids and retinoids. Viral infections, including the measles, can decrease retinoids, and exposure to certain toxic chemicals, such as dioxins, can enhance the breakdown of retinoids by your liver, and can lead to deficiency. You need to eat sufficient fat, protein, and zinc to absorb, transport, and use carotenoids and retinoids. Cooking vegetables and eating them along with some healthy fats should help increase absorption of carotenoids.

During World War II, the United States National Academy of Sciences (NAS) established a committee (later named the Food and Nutrition Board) to develop the Recommended Dietary Allowance (RDA) for nutrients. In 1974, the NAS set vitamin A requirements in terms of micrograms retinol equivalents (micrograms RE, or mcg RE). In 1993, the U.S. Food and Drug Administration (FDA) established Daily Values (DVs), which measured vitamin A requirements in terms of International Units (IUs). Food and supplement labels currently list nutrient content as a percentage of DV. In 2001, the NAS changed its measurement standard from mcg RE to mcg RAE (micrograms retinol activity equivalents): the degree to which carotenoid forms of vitamin A can be converted into retinoid forms. Today, you can find public health recommendations using all three units of measurement: RE, RAE, and IU. The Recommended Dietary Allowances (RDA) is the daily dietary intake level of a nutrient considered sufficient by the Food and Nutrition Board to meet the requirements of 97.5% of healthy individuals in each life-stage and gender group. The RDA for vitamin A is 900 RAE for men and 700 RAE for women.

Excellent Sources of Carotenoids

Eat green leafy vegetables and brightly orange- or red-colored vegetables like carrots, spinach, kale, butternut squash, tomatoes, sweet potatoes, pumpkins, and red bell peppers as part of your regular diet to ensure an adequate supply of carotenoids. Orange-, red-, or pink-colored fruits like watermelon, cantaloupe, papaya, mangoes, and pink grapefruit are also good sources of carotenoids.