Learn about carotenoids, precursors to vitamin A that can be found in yellow and orange fruits and vegetables.

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Carotenoids, the colorful plant pigments some of which the body can turn into vitamin A, are powerful antioxidants that can help prevent some forms of cancer and heart disease, and act to enhance your immune response to infections.

These precursors to vitamin A are sometimes called provitamin A. Bright-orange beta-carotene is the most important carotenoid for adequate vitamin A intake because it yields more vitamin A than alpha- or gamma-carotene.

Some carotenoids, such as lycopene, do not convert to vitamin A at all. Lycopene, the orange-red pigment found in tomatoes and watermelon, is still of value, however, because it's an antioxidant even more potent than beta-carotene. The other carotenoids are also valuable antioxidants. Antioxidants help the body reduce the inflammatory action of singlet or free-radical oxygen. Oxygen atoms like to combine into pairs. Singlet oxygen atoms are unstable and interact with the lipids found in cell walls causing inflammation and damage. Sometimes, your own body uses these free radicals to fight infections and abnormal cells. Most of the time, these free radicals cause inflammation and damage to cells, such as those that line your arteries.

Orange and yellow fruits and vegetables have high vitamin A activity because of the carotenoids they contain. Generally, the deeper the color of the fruit or vegetable is an indication of a higher concentration of carotenoids. Carrots, for example, are especially good sources of beta-carotene. Green, leafy vegetables such as spinach, asparagus, and broccoli also contain large amounts of carotenoids, but their intense green pigment, courtesy of chlorophyll, masks the tell-tale orange-yellow color. (See the table on page 194 for a list of good food sources of vitamin A.)

Most other carotenoids, such as alpha- and gamma-carotene, plus cryptoxanthin and beta-zeacarotene have less vitamin A activity than beta-carotene, but offer ample cancer prevention. Some carotenoids, such as lycopene, zeaxanthin, lutein, capsanthin, and canthaxanthin are not converted into vitamin A in the body. But again, they are powerful cancer fighters, prevalent in fruits and vegetables. There is abundant evidence that lycopene in particular helps reduce the risk for prostate cancer.

Carotenes are valuable preventive medicines, too. Research shows that people who eat a lot of foods rich in beta-carotene -- the carotenoid with the greatest vitamin A value -- are less likely to develop lung cancer. Even among smokers, lung cancer is less likely to occur in those people who eat a diet that includes lots of vegetables and fruits containing beta-carotene. Taking a beta-carotene supplement in pill form does not always have the same effect, however. Perhaps this is because in these foods there may be other substances that offer protection as well. In three studies involving 69,000 participants, many of them smokers, beta-carotene supplements increased the rate of lung cancer. Lutein/zeaxanthin, lycopene and alpha-carotene show evidence of being significantly more protective against lung and some other cancers.

Many experts now believe that the protective effect of carotenoids depends on the timing of when you take them. If you take betacarotene before your cells have undergone any pre-cancerous changes, the antioxidant action of the carotenoids can help reduce the likelihood that any mutations will take place. The carotenoids at this point can prevent free radicals from damaging cells and the DNA inside of cells, both of which can start cancerous growth. But if you take supplemental betacarotene AFTER cells have already mutated, the beta-carotene may protect the mutated cells from being destroyed by your own immune system.

Some of the most potent cancer fighting cells in your body utilize free radicals to fight infections and to destroy precancerous cells. So eating foods high in carotenoids or taking supplements helps with what is called primary protection from cancer - cancer never gets started. But after you have a growing colony of cancerous cells in your system, betacarotene supplements may prevent your own system from fighting the cancer, making carotenoid supplementation significantly less safe for what is called secondary prevention - stopping a recurrence of cancer.

