Stressed Plants Offer More Antioxidants

Jan. 31, 2007 — Flavonoids — the powerful, plant-based antioxidant compounds that have been linked to disease prevention — are found in a variety of fruits, nuts and vegetables. But, as a recent government report points out, the amount you get depends on the kind of plant you eat, and how much stress it has endured.

In its most comprehensive study yet on the healthy compounds, the U.S. Department of Agriculture not only suggest that certain varieties of fruits, nuts and veggies contain more antioxidants than others, but that flavonoid content varies widely between individual samples. For example, a market may display a tub full of red delicious apples, but flavonoid levels in each individual apple may vary.

The research showed that the difference may be due to stress, which appears to increase the healthful properties of produce. Nutritionist David Haytowitz, one of the study’s authors, explained to Discovery News that taxing conditions — such as those created by more natural farming methods — appear to force plants to release greater amounts of flavonoids and other beneficial compounds.

"Insect attacks and weather conditions can be stressful to plants," he explained. "Even harvest times are important, as early morning harvests are less stressful than those conducted around noon, since the midday sun adds to a plant’s stress."

Such factors may explain why one flavonoid varied from 31 to 114 milligrams in 3 ½-ounce samples of the same variety of sweet red cherry.

Another factor is color, which is chemically linked to many antioxidants. Raw blueberries, blackberries, cranberries and strawberries all ranked very high for several flavonoids that have been shown to reduce inflammation and to rid the body of damaging free radicals.

"We are interested, because they may help to prevent coronary vascular disease, cancer and other diseases," said Haytowitz, who added that color does not always indicate antioxidant content.

"Green tea and green leafy vegetables happen to be high in flavonoids, but that is not related to their color, which comes from chlorophyll," he explained.

Bananas ranked high for compounds called flavan-3-ols, even though the flesh of this fruit is creamy white.

Among other fruits, red delicious apples with peel, golden delicious apples with peel, Fuji apples and gala apples with peel all ranked high, as did other tested apples. Regular kiwi edged out golden kiwi, although both were found to contain antioxidants.

Nectarines and peaches were neck and neck in the listings, with both yielding high amounts of flavonoids.

Cooking appears to have a negative effect on many vegetables, as raw broccoli and tomatoes had more antioxidants than their cooked counterparts. Baking, however, still resulted in high measured levels for red potatoes, so long as their skin was left on.

All of the nuts ranked high, but almonds, pistachios, pecans and cashews had greater levels overall than Brazil nuts, peanuts, macadamias and other tested nuts.

Americans may not even be eating the best sources of antioxidants, suggests another study that has been accepted for publication in the Journal of Food Composition and Analysis.

Bharti Odhav, a researcher at South Africa’s Durban Institute of Technology and colleagues, analyzed antioxidants and other nutritional content within 20 leafy vegetables that require little cultivation and are traditional to early native South African diets. These included everything from a plant called "hairy beggarticks" (Bidens pilosa) to Asiatic pennywort (Centella asiatica). All were found to be very nutritious and high in antioxidants.

For now, however, Haytowitz advises to eat a wide variety of available produce, since each plant contains a different mixture of flavonoids. In the future he and his team hope to study more about how processing foods — such as through cooking, canning and freezing — affects their healthful properties.