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As vitamin sources - food or supplements?

With evidence growing that higher intakes of the antioxidant vitamins C and E may help protect us against cancer and heart disease, Agricultural Research Service and National Cancer Institute (NCI) scientists collaborated to see how well food sources stack up against supplements in raising blood levels of these vitamins.

According to Gladys Block, who began the study while at NCI, about 86 percent of vitamin C in the average U.S. diet comes from fruits and vegetables. However, about 25 percent nericans also take vitamin C supplements-alone, or in a multivitamin-- according to Block, who is now at the University of California, Berkeley.

But until now, the general assumption that vitamin C from fruits, vegetables, and supplements is equally absorbed by the body had not been tested thoroughly, says Reed Mangels, who was formerly with ARS' Beltsville Human Nutrition Research Center and now does nutrition consulting. So she organized a study of 68 men to determine the relative bioavailability of vitamin C from oranges, orange juice, and broccoli--both cooked and raw.

Mangels says she compared the men's response to orange juice vs. orange segments because orange juice is a major contributor of vitamin C in the U.S. diet. Based on one nationwide survey, she notes, orange juice provides more than one-quarter of the vitamin C consumed by adults.

With the exception of raw broccoli, both the food sources and the tablet were equal at restoring plasma vitamin C levels after the men had been fed very low vitamin C diets for 1 month to deplete their levels. Raw broccoli was at least 20 percent less effective at raising plasma levels than the other foods, says MangeIs.

But citrus fruits and broccoli are not the only rich sources of vitamin C. Between one and two times the Recommended Dietary Allowance is contained in a cup of cooked cauliflower, kohlrabi, brussels sprouts, or edible pea pods. One green or red pepper, one kiwifruit, a cup of strawberries, or half a cantalope also provide more than the RDA. Many other fruits and vegetables also contain significant amounts.

Unlike vitamin C, it's much harder to get extra vitamin E from foods. "It's virtually impossible to get more than 25 International Units (I.U.) per day through the diet," says vitamin E expert Orville A. Levander of the Beltsville center. The RDA is 15 I.U. for men and 12 I.U. for women.

Levander says the normal average daily intake is about 10 to 15 I.U. And because vitamin E is fat soluble, the richest sources are vegetable oils and high-fat products made from them, such as margarine.

Mangels and Levander collaborated with Rashmi Sinha, Block, and others at the NCI to compare how diets and supplements contributed to plasma levels of vitamin E in 65 of the men in the study.

They found that the differences in dietary intake of the vitamin were too small to produce any significant difference in the men's levels. The average daily intake in this group was 10 to 15

of less I.U., and all g than 20 I.U. from dietary sources. Those who took a multivitamin at least every other day got an extra 15 to 60 I.U. daily. And a third group that took daily vitamin E capsules got at least 100 I.U. above their dietary intake, says Sinha, a nutritional epidemiologist.

Compared to the group that did not take supplements regularly, plasma vitamin E levels averaged 14 percent higher in the group that took multivitamin supplements, she says. But the levels were more than twice as high in the group that took vitamin E capsules on a daily basis.

"If people want to substantially increase their plasma levels," says Sinha, "it has to be done with a vitamin E supplement." This puts the nutrient in the category of a pharmaceutical, such as aspirin when it's used to reduce risk of heart attack from blood clotting.

The problem is that science hasn't yet established the levels of vitamin E, either in the diet or blood plasma, that may help prevent heart disease and certain cancers, notes Levander. But there are hundreds of other antioxidant compounds in foods that may, together with vitamin E, provide all the antioxidant protection needed, says Jacqueline Dupont, who oversees ARS human nutrition research.

Orville A. Levander is at the USDAARS Beltsville Human Nutrition Research Center, Bldg. 307, 10300 Baltimore Ave., Beltsville, MD 207052350; phone (301) 504-8504, fax

(301) 504-9062.
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Author:McBride, Judy
Publication:Agricultural Research
Date:Dec 1, 1993
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