Beta-Carotene and the Carotenoids: Vitamin A’s First Cousins

With all the press this compound continues to receive, it’s hard to imagine that beta-carotene didn’t arrive on the scene until 1932. That’s when researchers figured out that vegetable foods could stand in for animal sources of vitamin A. Initially, betacarotene was overshadowed by vitamin A because the benefits of this colorful compound were thought to be related to its conversion to vitamin A. While vitamin A may confer a protective effect against epithelial cancers, current research suggests that carotene may protect against other types of cancer and heart disease, as well as boost the immune system.

The Carotenoids

The carotenoids are an eclectic array of compounds, and they seem to be intriguing to everyone. Scientists can’t seem to get their research results into print fast enough for a public fascinated by the disease-fighting properties of the carotenoids. Research interest dates back to the first mystery posed by these compounds. Although scientists had been aware that plant foods possessed vitamin A activity, they were stymied by the color of the vegetable tissues when compared to the colorless compound they extracted from liver and animal tissues. Shortly after, they determined that the compound in plant foods was actually a unique substance, quite different from active vitamin A, but one which the body could convert to a colorless form of vitamin A.

Closer analysis of plant tissues uncovered the class of compounds known collectively as carotenoids, a generic term for over 600 compounds (of which the liver can convert about 50 to active vitamin A). Only those carotenoids possessing biologic activity of vitamin A are called provitamin A. They all vary in their ability to convert to active vitamin A, ranging from 20 to 60 percent. Five main carotenoids show up in human blood: alpha-carotene, betacarotene, lutein, beta-cryptoxanthin, and lycopene. Food sources of the various carotenoids with some fruits and vegetables higher in specific carotenoids than others. They exist in two different chemical forms, like mirror images of each other, changing to the other forms when exposed to light, heat, or chemical reaction. One current area of study is whether the two different chemical forms work differently in the body.

The body isn’t too efficient in absorbing carotenoids, using only 10 to 30 percent, with most being excreted. As with vitamin A, carotenoids are absorbed along with and the same way as dietary fat. The amount of fat in the diet has a major impact on how much you absorb, as it does for other fat soluble nutrients. One recent study compared carotenoid absorption rate in a diet consisting of 40 percent fat to one with 20 percent fat. The lower-fat diet, a level not uncommon for health-conscious people, showed a significant reduction in absorption. Other factors that can lower carotenoid absorption include fiber intake, smoking, high body weight, and alcohol.

Once inside the intestinal cell, carotenoids can either be converted to vitamin A or be taken up by chylomicrons, the same carriers of the fat you eat, and transported in the bloodstream to the liver. From the liver, carotenoids get back into circulation via other lipoproteins, the same ones that carry cholesterol in the blood.

Lycopene, the main red pigment in some fruits and vegetables, is a form of carotene but, unlike its sister compounds, possesses no vitamin A activity. Scientists are excited, however, about its potential protection against cancer. Tomatoes and tomato products are the best sources of this potent antioxidant, although other fruits and vegetables, such as watermelon and pink grapefruit, can be good sources as well.

Ironically, not long after researchers first began studying the compound, they realized that people who frequently eat fast foods get more than their fair share oflycopene. The reason is that when tomatoes are processed for ketchup, tomato paste, pizza sauce, and taco sauce, the compound becomes more concentrated. Interestingly, a recent study suggests that not only does tomato paste contain more lycopene than fresh tomatoes, the compound is more available for human absorption from the paste.

But what do these colorful compounds, the cartenoids, do besides subsidize our vitamin A intake that has everyone so excited? Originally known only for serving as vitamin A precursors, current research of carotenoids focuses on other effects in the body. The most important effect is their potent antioxidant activity. Take a few minutes to complete the quiz on cutting your risk for a heart attack. The next sections highlight important new research that may offer clues as to how carotenoids may protect against diseases such as heart disease and cancer.

Beta-Carotene and Immune Function

When most people think about their immune function, they usually think about a body system that protects them against catching the latest flu or cold bug. While this is certainly true, in a similar way the immune system also takes center stage in the body’s fight against cancer. This defense plan involves a myriad of antitumor activities, and researchers had theorized that betacarotene might act as an immunity enhancer by stimulating some of these processes. Immunology experts and nutrition researchers have long known that the body’s ability to mount an effective immune response in warding off disease depends on nutritional status and, in some cases, specific nutrients such as zinc. So it wasn’t a big leap to consider that individual nutrients might be able to boost immunity.

The immune system is a complex of cells and compounds including lymphocytes, T cells and B cells, specialized liver cells, white blood cells, antibodies, and cells that act like garbage collectors in the blood, phagocytes. One important type of immune response, cell-mediated immunity (CMI), is involved in resistance to viral and bacterial infections, certain autoimmune diseases, tissue and organ transplant rejection, and some aspects of defense against cancer. The major players of CMI are T cell lymphocytes because they help make antibodies.

The presence of antigens, or foreign particles such as bacteria and viruses, stimulates T cells which initiate CM!. In order for this to occur, a group of compounds called MHC II proteins must be available. The MHC II proteins sit on the surface of some cells and help destroy any type of invader, even cancer cells. We see their importance from studies showing that the degree of immune response is proportional to the level of these MHC II proteins.

Recently, British researchers studied how beta-carotene supplementation affected the immune system. The study included twenty-five men who received either 15 mg of beta-carotene or a placebo for twenty-six days. The researchers measured blood levels of compounds that reflect immune function related to tumor development. Beta-carotene supplements caused increases in blood levels of the MHC II proteins. The beta-carotene supplements also increased the level of tumor necrosis factor, which seeks out cancer cells and helps kill them. And the beneficial effects lasted for several weeks after the subjects stopped taking the beta-carotene.

The researchers suggested that one way beta-carotene boosts immunity is its antioxidant activity, lowering the level of free radicals and protecting lipids in cell membranes from oxidation; they found higher levels of polyunsaturated fatty acids (PUFAs) after beta-carotene supplementation, which supports this theory. And earlier studies showed that free radicals reduce the MHC II protein. Another reason for beta-carotene’s immunity-boosting power might be related to its effect on prostaglandins, powerful compounds made by the body that regulate many processes including immune responses. Whichever is correct, it appears that moderate increases in dietary intake of beta-carotene can enhance Immune responses.

The only problem with the immune effect is that two studies of beta-carotene in the past few years have linked this carotenoid to an increased risk for lung cancer in smokers. In the first study, from Finland, researchers expected supplements to lower the risk of lung cancer, and their results shocked scientists and consumers alike. The current thinking is that other carotenoids may be responsible for the protective effect, with beta-carotene having the opposite effect in people who smoke. Watever the reason, the smart consumer will continue eating fruits and vegetables high in all the carotenoids and skip the beta-carotene supplements.

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