Carotenoids

Carotenoids are fat-soluble plant pigments, some of which are important to human health. The most common carotenoids in the diet of North Americans are alpha-carotene, beta-carotene, beta-cryptoxanthin, lutein, zeaxanthin, and lycopene.

The role carotenoids play in human health is not well understood. Carotenoids are antioxidants that react with free radicals. Molecules, called free radicals, form during normal cell metabolism and with exposure to ultraviolet light or to toxins, such as cigarette smoke. Free radicals cause damage by reacting with fats and proteins in cell membranes and genetic material. This process is called oxidation. Antioxidants are compounds that attach themselves to free radicals so that it is impossible for the free radical to react with, or oxidize, other molecules. In this way, antioxidants may protect cells from damage. Although carotenoids have antioxidant activity in the laboratory, it is not clear how much they function as antioxidants in the body. Claims that carotenoids can protect against cancer and cardiovascular disease are primarily based on their antioxidant properties.

One subgroup of carotenoids—which includes alphacarotene, beta-carotene, and beta-cryptoxanthin—is converted into vitamin A (retinol) by the body. Vitamin A is important for maintaining good vision, a healthy immune system, and strong bones. Vitamin A also helps turn on and off certain genes (gene expression) during cell division and differentiation. The degree to which this group of carotenoids is converted into vitamin A appears to depend on whether or not the body is getting enough vitamin A in other forms. Only 10% of all carotenoids can be converted into vitamin A.

Carotenoids

The United States Institute of Medicine (IOM) of the National Academy of Sciences develops values called Dietary Reference Intakes (DRIs) for vitamins and minerals. The DRIs define the amount of a nutrient a person needs to consume daily and the largest daily amount from food or dietary supplements that can be taken without harm. The IOM has not developed any DRIs for carotenoids because not enough scientific information is available, and because no diseases have been identified as being caused by inadequate intake of carotenoids. The IOM, the American Cancer Society, and the American Heart Association all recommend that people get all their antioxidants, including carotenoids, from a diet high in fruits, vegetables, and whole grains rather than from dietary supplements.

Many health claims for carotenoids are based on laboratory and animal studies. Results from human studies are often inconsistent and confusing. One difficulty in evaluating these studies comes from the variety of ways in which they are conducted. When increased carotenoid intake comes from eating foods high in carotenoids, it is hard to separate the effects of the carotenoids from the effects of other vitamins and minerals in the food. When a dietary supplement is given to increase the level of a specific carotenoid, the outcomes often differ from those that occur in a diet of carotenoid-rich vegetables. In addition, the fact that some carotenoids are converted into vitamin A blurs the line between their effects and that of vitamin A from other sources. Many clinical trials are underway to determine safety and effectiveness of different carotenoids, both alone and in combination with other drugs and supplements.

BETA-CAROTENE. Beta-carotene is a yellow-orange provitamin A carotenoid. A provitamin is a substance that is converted into a vitamin in the body of an organism. Good sources of beta-carotene include carrots, sweet potatoes, winter squash, pumpkins, spinach, kale, and broccoli. When vitamin A stores are low, the body can convert beta-carotene into vitamin A to prevent symptoms of vitamin A deficiency. It takes 12 micrograms (mcg) of beta-carotene to make 1 mcg of retinol, the active form of vitamin A. Therefore, vitamin A deficiency is usually more effectively treated by eating more foods high in vitamin A and/or taking a vitamin A supplement than by increasing beta-carotene intake.

The only use for beta-carotene dietary supplements, proven in well-controlled clinical trials, is to treat a rare genetic disorder called erythropoietic protoporphyria, also called porphyria. This disorder causes the skin to be painfully sensitive to sunlight and causes the development of gallstones and problems with liver function. Symptoms are sometimes relieved by giving beta-carotene supplements under the supervision of a physician.

A diet high in vegetables rich in beta-carotene appears to reduce the risk of developing certain cancers. However, in a large study of 29,000 Finnish men (the Alpha-Tocopherol, Beta-Carotene Cancer Prevention [ATBC] Trial), when a beta-carotene dietary supplement was taken by men who smoked, they developed lung cancer at a rate of 18% higher and died at a rate of 8% higher than men who did not take the supplement. The study was halted in 1994, but side effects continued into the 2000s. A similar U.S. study (Carotene and Retinol Efficacy Trial [CARET]) that gave men dietary supplements of beta-carotene and vitamin A was also stopped when researchers found the men receiving the beta-carotene had a 46% greater chance of dying from lung cancer than those who did not take it. The official position of the IOM is that “beta-carotene supplements are not advisable for the general population.

ALPHA-CAROTENE. Alpha-carotene is the lesser known cousin of beta-carotene. It also is a provitamin A carotenoid, but it takes 24 mcg of alpha-carotene to make 1 mcg of retinol. Good sources of alpha-carotene include pumpkins, carrots, winter squash, collard greens, raw tomatoes, tangerines, and peas. Less research has been done on alpha-carotene than beta-carotene, but it is not recommended as a dietary supplement.

BETA-CRYPTOXANTHIN. Beta-cryptoxanthin is also a provitamin A carotenoid. It takes 24 mcg of beta-cryptoxanthin to make 1 mcg of retinol. Good sources of beta-cryptoxanthin include pumpkins, red bell peppers, papayas, tangerines, nectarines, oranges and orange juice, carrots, yellow corn, and watermelons.

LUTEIN AND ZEAXANTHIN. Lutein and zeaxanthin do not have vitamin A activity. They are the only carotenoids found in the human eye. It has been proposed, but not proven, that they may help slow the development of cataracts. Cataracts are changes in the lens of the eye that result in clouding and vision loss. These carotenoids are also found in the retina. They absorb light in the blue wavelength range. Some researchers theorize that they can help slow or prevent age-related breakdown of the retina (age-related macular degeneration), a common cause of vision loss in the elderly. Good sources of lutein and zeaxanthin include spinach, kale, turnips, collard and mustard greens, summer squash, peas, broccoli, Brussels sprouts, and yellow corn.

LYCOPENE. Lycopene is the carotenoid that gives tomatoes, watermelons, and guavas their reddish color. In the American diet, almost all dietary lycopene comes from tomato products.

The relationship between dietary intake of lycopene and the risk of men developing prostate cancer is of great interest to researchers. Results have been conflicting. A large study of 58,000 Dutch men found no relationship between the two, but a 2004 analysis of 21 observational studies examining the relationship between dietary lycopene intake and prostate cancer found that men with the highest dietary intake of lycopene were slightly less likely to develop prostate cancer. Studies conducted in 2007 on almost 30,000 found results similar to the original 1990s Dutch study, with no significant relationship.

The relationship between lung cancer and beta-carotene strongly suggests that all carotenoids should be obtained through diet and not through dietary supplements. There is also no information on the safety of carotenoid dietary supplements in children or women who are either pregnant or breastfeeding.

Interactions of specific carotenoids with drugs, herbs, and dietary supplements have not been well studied. In general, cholesterol-lowering drugs, the weight-loss drug orlistat (Xenical or Alli), and mineral oil reduce the absorption of carotenoids from the intestine, but it is not known whether this has an effect on health.

There are no identified complications from carotenoid deficiency.

Beta-carotene supplements of 30 mg per day or more or excessive consumption of carrots and other beta-carotene rich food can cause the skin to become yellow, a condition called carotenodermia. Carotenodermia is not associated with any health problems and disappears when beta-carotene intake is reduced.

Lycopene supplements, or excessive intake of tomatoes and tomato products, can cause the skin to turn orange, a condition called lycopenodermia. This condition disappears when lycopene intake is reduced.

No recommendations have been set about the maximum daily intake of carotenoids from diet, but dietary supplements of carotenoids are not recommended by the IOM, the American Heart Association, or the American Cancer Society.

Parents should encourage their children to eat a healthy and varied diet high in fruits, vegetables, and whole grains. There is no need to give children dietary supplements of carotenoids. The safety of these supplements in children has not been studied.

See also ADHD diet ; Antioxidants ; Cancer ; Cancer diet ; Cancer-fighting foods ; Dietary reference intakes (DRIs) ; Dietary supplements ; Metabolism ; Phytonutrients ; Whole grains .