The term carotene is used for several related substances having the formula C40H56. Carotene is an orange photosynthetic pigment important for photosynthesis. It is responsible for the orange colour of the carrot and many other fruits and vegetables. It contributes to photosynthesis by transmitting the light energy it absorbs to chlorophyll.
Chemically, carotene is a terpene, synthesized biochemically from eight isoprene units. It comes in two primary forms designated by characters from the Greek alphabet: alpha-carotene (α-carotene) and beta-carotene (β-carotene). Gamma, delta and epsilon (γ, δ and ε-carotene) also exist. As hydrocarbons, carotenes are fat-soluble and insoluble in water. Beta-carotene is composed of two retinyl groups, and is broken down in the mucosa of the small intestine by beta-carotene dioxygenase to retinal, a form of vitamin A. Carotene can be stored in the liver and converted to vitamin A as needed, thus making it a provitamin.
The following foods are particularly rich in carotenes (see Vitamin A article for amounts):
Absorption from these foods is enhanced if eaten with fats, as carotenes are fat soluble, and if the food is cooked for a few minutes until the plant cell wall splits and the colour is released into any liquid.
β-Carotene is the more common form and can be found in yellow, orange, and green leafy fruits and vegetables. As a rule of thumb, the greater the intensity of the orange colour of the fruit or vegetable, the more β-carotene it contains.
Carotene protects plant cells against the destructive effects of ultraviolet light. β-Carotene is an anti-oxidant.
An article on the American Cancer Society says that The Cancer Research Campaign has called for warning labels on beta carotene supplements to caution smokers that such supplements may increase the risk of lung cancer.
The New England Journal of Medicine published an article in 1994 about a trial which examined the relationship between daily supplementation of beta carotene and vitamin E (alpha-tocopherol) and the incidence of lung cancer. The study was done using supplements and researchers were aware of the epidemiological correlation between carotenoid-rich fruits and vegetables and lower lung cancer rates. The research concluded that no reduction in lung cancer was found in the participants using these supplements (beta-carotene), and furthermore, these supplements may, in fact, have harmful effects.
The Journal of the National Cancer Institute published an article in 1996 about a trial that was conducted to determine if vitamin A (in the form of retinyl palmitate) and beta carotene had any beneficial effects to prevent cancer. The results indicate an increased risk of lung cancer for the participants who consumed the beta-carotene supplement.
A randomised trial into the use of β-carotene and vitamin A for prevention of lung cancer had to be stopped early due to the apparent increase in the incidence of lung cancer in those with lung irritation from smoking or asbestos exposure.
A review of all randomized controlled trials in the scientific literature by the Cochrane Collaboration published in JAMA in 2007 found that beta carotene increased mortality by something between 1 and 8% (Relative Risk 1.05, 95% confidence interval 1.01-1.08). However, this mea-analysis included two large studies of smokers, so it is not clear that the results apply to the general population.
Carotene is also found in palm oil, corn, and in the milk of Guernsey dairy cows, causing their milk to turn yellow. It is also found in some species of termites.
The two ends of the β-carotene molecule are structurally identical, and are called β-rings. Specifically, the group of nine carbon atoms at each end form a β-ring.
The α-carotene molecule has a β-ring at one end; the other end is called an ε-ring. There is no such thing as an "α-ring".
These and similar names for the ends of the carotenoid molecules form the basis of a systematic naming scheme, according to which:
6 μg of dietary β-carotene supplies the equivalent of 1 μg of retinol, or 1 RE (Retinol Equivalent). This is equivalent to 3⅓ IU of vitamin A.