Lycopene is one of the lesser known carotenoids. Like beta-carotene, its more celebrated kin, this plant pigment contributes to the brilliant hue of many fruits and vegetables -- especially tomatoes. But a new respect and growing interest in lycopene traces not to its role in a tomatos eye appeal but to its superior ability to quench the oxidative radicals that play a role in aging and many degenerative diseases.
Initially cultivated by the Aztecs, tomatoes started out yellow. Through breeding efforts over the centuries, most now ripen into a deep, lycopene-rich red.
Indeed, recent studies have suggested that lycopenes antioxidant properties -- the highest among those of all the dietary carotenoids -- may explain its apparent ability to reduce an individuals risk of prostate and certain other cancers.
Lycopene is the predominant carotenoid in red tomatoes. But before you run out to the nearest farm stand for a half bushel of plump Beefsteaks or Better Boys, consider the findings of a new German study. Helmut Sies and his colleagues at Heinrich Heine University in Düsseldorf have just shown that the body extracts lycopene more effectively from tomato paste than it does from the fresh fruit.
His team recruited three women and two men to participate in an experiment. After eating a diet low in carotenoids -- forswearing tomato products and some other fruits and vegetables -- for 3 days, each volunteer came in for a breakfast consisting of 400 grams of fresh tomatoes together with 15 grams of corn oil and 100 grams of bread. The healthy 25- to 39-year-old participants received only water or black coffee to wash down the tomato-heavy meal. Two weeks later, each volunteer returned for a similar breakfast. This time, Sies team substituted 40 grams of tomato paste for the fresh fruit.
Each meal contained 23 milligrams of lycopene and included equivalent amounts of the all-trans form that predominates in tomatoes.
Over 6 hours following each meal, the German team drew blood from each volunteer and measured the lycopene present in their chylomicrons, a type of lipid-shuttling particle. Sies group focused on chylomicrons because these lipoproteins, which form in the intestines, better reflect the intestinal absorption of lycopene than do other bloodborne constituents.
In the July American Journal of Clinical Nutrition, the scientists now report that lycopene peaked at 1.9 to 2.5 times higher concentrations following the paste-based breakfast. Overall absorption differences were even more striking: During the 6 hours they were studied, these volunteers appeared to absorb nearly four times as much lycopene from the paste as from the fresh tomatoes.
"This lends experimental support to the epidemiologic observation regarding lycopene bioavailability," they say, referring to a study 2 years ago by researchers at the Harvard School of Public Health. It found that diets rich in foods made with tomato sauce -- and presumably based on a tomato paste --appeared to reduce risk of prostate cancer.
Milligram for milligram, why does eating paste deliver more lycopene into the body than consuming a fresh slice of tomato? Sies team suspects that the mechanical disruption of the plant cells, which will occur during the mashing of a tomato to make paste, may liberate more of the carotenoid so that it can dissolve into fats, like vegetable oil. Thats important, because carotenoids preferentially reside in fats. Once in the body, theyll ride those fats into the chylomicrons.
However, the heating of a tomato in the cooking up of a paste may also yield the same cell destruction and lycopene liberation as mashing, Sies team observes. Indeed, work 5 years ago by Sies and Wilhelm Stahl, another of the authors of the new study, showed that drinking tomato juice that had been heated in the presence of a small amount of corn oil doubled or tripled an individuals blood-serum concentrations of lycopene. No increase occurred when a volunteer consumed uncooked juice.
What about tomato juice as a source of lycopene? It possesses at least three times more lycopene per gram of edible product than does a slice of tomato, and about 50 percent more lycopene than an equivalent weight portion of paste. However, juice proved a relatively poor contributor to the blood concentration of the carotenoid, Sies team found -- which is consistent with the Harvard studys finding of no apparent anticancer benefit among individuals who regularly drank lots of the juice. However, the German scientists argue, this may merely reflect the fact that tomato juice is seldom consumed together with fat -- which might be necessary to aid its absorption.
Once lycopene is absorbed, however, it offers outstanding antioxidant protection. For instance, Sies team showed in earlier studies that lycopene possesses at least twice the potency of beta-carotene in quenching singlet oxygen, a source of biologically damaging reactions in the body.
Unlike most carotenoids, lycopene occurs few places in the diet besides tomatoes. Pink grapefruit, fresh papaya, raw guava, and watermelon all contain amounts of the nutrient comparable to whats found in fresh tomato slices. Dried apricots and pureed rosehips, too, contain relatively large amounts.
The rub: None of these appear as dietary staples in most households. And who would eat watermelon together with vegetable oil?
Moreover, as the new study points out, factors other than natural concentrations of the carotenoid in food determine how much of it will be absorbed by the body.
To date, no study has administered prophylactic doses of lycopene to demonstrate its potential therapeutic promise. However, in a paper published earlier this year, Helga Gerster of F. Hoffmann-La Roche Ltd. in Basel, Switzerland, notes that a host of epidemiologic studies point to some apparent lycopene benefits.
In one study that she reviewed, eating tomatoes at least weekly appeared to cut a mans risk of esophageal cancer by 40 percent. Lycopene also has emerged as an apparent protective dietary nutrient in several surveys of stomach cancer. More tentative associations offer the suggestion that it may diminish an individuals chances of developing cervical, bladder, and skin cancers.
Observes Gerster, "awareness in the nutritional and medical sciences of lycopene as a potentially beneficial carotenoid is only 6 to 8 years old." Yet over that short period, she notes, a number of features have emerged that appear to set it apart from the other carotenoids -- both in its potential benefits and mode of action. For instance, one recent Russian study showed that lycopene was twice as effective as beta-carotene at combating oxidant damage from exposures to gamma radiation. An Israeli study showed that the carotenoid was 10 times better than beta-carotene at inhibiting the growth -- at least in test tubes -- of cancer cells.
Gärtner, C., W. Stahl, and H. Sies. Lycopene is more bioavailable from tomato paste than from fresh tomatoes. American Journal of Clinical Nutrition 66(July):116.
Gerster, H. 1997. The potential role of lycopene for human health. Journal of the American College of Nutrition 16:109.
Giovannucci, E., et al. 1995. Intake of carotenoids and retinol in relation to risk of prostate cancer. New England Journal of Medicine 87:1767.
Kapitanov, A.B., et al. 1994. Radioprotective properties of lycopene (English translation available from Polyglot International). Radiatsionnaya Biologiya, Radioekologiya 34 (No. 3):439.
Levy, J., et al. 1995. Lycopene is a more potent inhibitor of human cancer cell proliferation than either alpha-carotene or beta-carotene. Nutrition and Cancer 24 (No. 3):259.
Lu, Y., et al. 1995. A new carotenoid, hydrogen peroxide oxidation products from lycopene. Bioscience, Biotechnology and Biochemistry 59:2153.
Raloff, J. 1996. Antioxidants: Confirming a heart-y role. Science News 150(July 6):6.
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This week's Food for Thought is prepared by Janet Raloff, senior editor of Science News.