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Many fruits, such as cherries, blueberries, and plums, contain high levels of antioxidants, but not all are absorbed in the same way into the blood, research shows.

Dr. Ronald Prior, a chemist with the U.S. Department of Agriculture in Arkansas, has been investigating how eating different fruits affects the antioxidant capacity in the human body. Antioxidants are considered important in the diet because they intercept free radicals and protect cells from oxidative damage that may lead to disease.

Prior and colleagues at the Arkansas Children’s Nutrition Center measured the blood antioxidant capacity of volunteers who had just eaten blueberries, cherries, dried plums, dried plum juice, grapes, kiwis, or strawberries. They found that despite the high antioxidant content of plums, eating plums did not result in higher antioxidant levels in the blood of the volunteers. One of the major phytochemicals in plums is chlorogenic acid, which is not readily absorbed by the body.

Wild blueberries are touted as a high-antioxidant fruit, but Prior found that a larger-than-average serving was needed to boost blood antioxidant levels. A noticeable increase wasn’t detected until volunteers consumed at least half a cup of berries. The antioxidants in cherries appear to be readily absorbed, and consumption of 280 grams (10 ounces) of cherries increased antioxidant levels, Prior reported.

Volunteers in the control group consumed a shake containing protein, carbohydrates, and fat, but no antioxidants. Their antioxidant levels dropped after the meal, increasing the risk of oxidative stress.

Prior said the metabolism of glucose and energy from food is a source of free radicals, and there’s a relationship between the amount of food consumed and the need for antioxidants.

"My take is that in any meal there need to be one or more sources of antioxidants," Prior said. "The worst case is a doughnut for breakfast and nothing else. It’s just energy and no antioxidant source."

Prior advocates eating a variety of foods, and said it makes sense to include those known to be high in antioxidants. "I think it’s still reasonable to start with those that are high in antioxidants," he said, "but what we measure in the test tube, so to speak, may not translate to the same ranking in vivo."

His study is a step towards establishing recommendations for antioxidants in the diet. Additional research is needed to determine if higher antioxidant capacity of the blood translates to a lower risk for chronic degenerative disease. Prior said oxidative stress is definitely involved in cardiovascular disease, but it’s not yet known if it’s a primary factor or secondary to some other mechanism that initiates the process.