Pomegranates, blackberries, raspberries and cranberries also performed well in the tests that measured cellular antioxidant activity (CAA) in 25 fruits. But superfruits such as noni, acai and goji did not come under the scientists’ antioxidant microscope.
In addition to CAA, the researchers also measured total phenolic content, and oxygen radical absorbance capacity (ORAC) values. These are more typical ‘in-the-fruit’ readings but the researchers wanted to test the levels of antioxidants in fruits in a more biologically representative manner, hence their application of the CAA method which measures antioxidant activity ‘in-the-cell’.
The research, published in the Journal of Agricultural and Food Chemistry, found wild blueberries notched the highest antioxidant levels under these measures also.
Melons and bananas had the lowest antioxidant activity and cultivated blueberries demonstrated less antioxidant activity than their wild cousins.
Because of the volume at which they are consumed, apples contributed most fruit phenolics to the American diet, with apples and strawberries the biggest suppliers of antioxidant activity.
Free radicals react with and attack lipids, proteins, carbohydrates, and DNA and have been linked to a number of oxidative stress conditions including some cancers.
Cellular antioxidant activity
CAA, first developed at Cornell last year, seeks to “move beyond the test tube” to determine how antioxidant compounds perform in cells – in this case human liver cells.
“The CAA assay is a more biologically relevant method than the chemistry antioxidant activity assays as it accounts for uptake, metabolism, distribution and activity of antioxidant compounds within cells versus solely looking at antioxidant value,” said lead scientist Rui Hai Liu, PhD.
Both foods and food supplements were tested.
Antioxidant activity of selected compounds is typically measured using a range of lab-based assays, including the ferric reducing ability of plasma (FRAP) assay, the oxygen radical absorbance capacity (ORAC) and Trolox equivalent antioxidant capacity (TEAC).
CAA centres on dichlorofluorescin, a probe molecule trapped within cells that can be easily oxidised to produce fluorescence. The test uses 2,2'-azobis(2-amidinopropane) dihydrochloride (ABAP)-generated peroxyl radicals to oxidise dichlorofluorescin, and the ability of antioxidant compounds to inhibit this process.
Fruits like berries are naturally high in antioxidants because of their anthocyanin content, which not only give many fruits their colour, but are high in antioxidants.
“While further testing is needed to confirm how dietary antioxidants are absorbed by and go to work in the human body to prevent cancer and other chronic diseases, we’re encouraged by the response in this initial screening measure,” Dr Liu concluded.
Source: Journal of Agricultural and Food Chemistry
Volume 56, Number 18, Pages 8418–8426
"Cellular Antioxidant Activity of Common Fruits"
Authors: K.L. Wolfe, X. Kang, X. He, M. Dong, Q. Zhang, R.H. Liu