Such a ranking has important implications for consumers seeking the most antioxidant-rich food, for scientists investigating the health effects of dietary antioxidants, and even for functional food formulators searching for the best antioxidant health benefits for their products.
"This ranked antioxidant food table provides a useful tool for investigations into the possible health benefits of dietary antioxidants," wrote lead author Nebte Halvorsen from the University of Oslo.
A vast body of epidemiological studies have linked an increase dietary intake of antioxidants from fruits and vegetables to reduced risks of a range of disease, including cancer, cardiovascular disease and diabetes.
However, when such antioxidants have been extracted and put into supplements the results, according to the randomized clinical trials (RCTs), the results have been conflicting and, at times, disappointing.
The discrepancies between the epidemiological and intervention studies was highlighted recently by the publication of a report in the New Scientist magazine that concluded that the benefits of antioxidant supplements, particularly vitamins C and E, and beta-carotene, was a "myth".
This discrepancy between epidemiology and intervention trial results was picked up by the scientists behind the antioxidant ranking, published in the July issue of the American Journal of Clinical Nutrition (Vol. 84, pp. 95-135) who set about testing and compiling a list of food typically consumed in the US diet according to their antioxidant ranking.
The researchers used the ferric reducing ability of plasma (FRAP) assay, ahead of other measures such as the oxygen radical absorbance capacity (ORAC) and Trolox equivalent antioxidant capacity (TEAC). This choice, said co-researcher Rume Blomhoff, is based on several reasons that make FRAP a better measure of total antioxidant acitivity.
In an article published in Current Opinion in Lipidology (2005, Vol. 16, pp. 47-54) Blomhoff notes that, unlike ORAC and TEAC, the FRAP assay, the measure is less specific, it allows for a quantitative determination of the total antioxidant content, and it does not measure levels of glutathione (GSH), the major cellular endogenous antioxidant.
According to this measure, Halvorsen and colleagues report that cloves have the highest antioxidant content (125.5 millimoles per 100 grams). The rest of the top five is also spices, with top-placed cloves followed by oregano leaf, ginger, cinnamon and turmeric.
In terms of the highest antioxidant content per serving, Halvorsen reports that blackberries top the list, with an antioxidant content of 5.75 millimoles per serving.
Walnuts, strawberries, artichokes, cranberries, coffee, raspberries, pecans, blueberries, and ground cloves completed the top 10 antioxidant foods.
The juice with the highest antioxidant content was grape juice, with cranberry juice coming a close second in this subset.
Bran Flakes had the highest antioxidant content amongst breakfast cereals, and canned chili with meat and beans was the top ranking processed food per serving.
"The present results of the analysis of 1120 food samples that were obtained from the USDA National Food and Nutrient Analysis Program represent by far the largest published systematic screening of antioxidants in dietary items," wrote the researchers.
"Our extensive total antioxidant food table should be useful for further testing of the antioxidant hypothesis," Halvorsen concluded.
Daniel Fabricant, Ph.D., vice president of scientific affairs for the industry association the Natural Products Association (NPA) welcomed the attempt to provide such data for both industry and consumers.
Dr. Fabricant told NutraIngredients-USA.com: "The study was a large effort that provides a good base on which further animal and hopefully clinical studies can be based on, which is great for those in the industry who are seeking to do such research.
"With the different approaches and methods to quantify antioxidant activity (i.e. ORAC, TRAP), any effort to normalize or standardize the language or evaluation criteria allows for good information to be passed along through the industry to the consumer who ultimately benefits the most."
However, Fabricant also responded to the references to the poor results from the clinical trials mentioned in this study, and pointed out that there are currently 101 U.S. government-funded clinical studies focusing on the beneficial properties and uses of antioxidant supplements.
"If the science is strong enough to support further clinical studies by the nation's leading medical scientists, then the authors may wish to reconsider that statement," he said.
According to the Institute of Food Technologists, sales of products carrying an antioxidant claim jumped nearly 20 percent last year. One of every four consumers says they eat fruits or vegetables to prevent disease, one in three eats them to feel healthy, and nearly nine of ten eat them to stay healthy.