In the following discussion, the term "antioxidant" refers mainly to non-nutrient compounds in foods, such as polyphenols, which have antioxidant capacityin vitro and so provide an artificial index of antioxidant strength – the ORAC measurement. Other than for dietary antioxidant vitamins – vitamin A, vitamin C and vitamin E – no food compounds have been proved with antioxidant efficacy in vivo. Accordingly, regulatory agencies like the Food and Drug Administration of the United States and the European Food Safety Authority (EFSA) have published guidance disallowing food product labels to claim an inferred antioxidant benefit when no such physiological evidence exists.
ORAC in foods
Many common foods are good sources of antioxidants. In the list of foods given below, "rich in antioxidants" usually means at least an ORAC rating of 1000 per 100 g. A typical apple or pear weighs around 200 g and hence 200 g can be considered as the serving size.
Dried fruits are a good source of antioxidants by weight/serving size as the water has been removed making the ratio of antioxidants higher. Typical dried fruits are pears, apples, plums, peaches, raisins, figs and dates. Dried raisins are high in polyphenol count. Red wine is high in total polyphenol count which supplies antioxidant quality which is unlikely to be conserved following digestion (see section below).
Deeply pigmented fruits like cranberries, blueberries, plums, blackberries, raspberries, strawberries, blackcurrants, figs, cherries, guava, oranges, mango, grape juice and pomegranate juice also have significant ORAC scores.
Sorghum bran, cocoa powder, and cinnamon are rich sources of procyanidins, which are large molecular weight compounds found in many fruits and some vegetables. Partly due to the large molecular weight (size) of these compounds, their amount actually absorbed in the body is low, an effect also resulting from the action of stomach acids, enzymes and bacteria in the gastrointestinal tract where smaller derivatives are metabolized and prepared for rapid excretion.
Despite the above discussion implying that ORAC-rich foods with polyphenols may provide antioxidant benefits when in the diet, there remains no physiological evidence that any polyphenols have such actions or that ORAC has any relevance in the human body.
On the contrary, research indicates that although polyphenols are good antioxidantsin vitro, antioxidant effects in vivo are probably negligible or absent. By non-antioxidant mechanisms still undefined, flavonoids and other polyphenols may reduce the risk of cardiovascular disease and cancer.
As interpreted by the Linus Pauling Institute and EFSA, dietary polyphenols have little or no direct antioxidant food value following digestion. Not like controlled test tube conditions, the fate of polyphenols in vivo shows they are poorly conserved (less than 5%), with most of what is absorbed existing as chemically-modified metabolites destined for rapid excretion.
The increase in antioxidant capacity of blood seen after the consumption of polyphenol-rich (ORAC-rich) foods is not caused directly by the polyphenols, but most likely results from increased uric acid levels derived from metabolism of flavonoids. According to Frei, "we can now follow the activity of flavonoids in the body, and one thing that is clear is that the body sees them as foreign compounds and is trying to get rid of them." Another mechanism may be the increase in activities of paraoxonases by dietary antioxidants which can reduce oxidative stress.
Salicylic acid - found in most vegetables, fruits, and herbs; but most abundantly in the bark of willow trees, from where it was extracted for use in the early manufacture of aspirin.
Other nonflavonoid phenolics
Curcumin - Curcumin has low bioavailability, because, much of it is excreted through glucuronidation. However, bioavailability is substantially enhanced by solubilization in a lipid (oil or lecithin), heat, addition of piperine, or through nanoparticularization.
Uric acid, in humans, accounts for roughly half the antioxidant capacity of plasma.Fructose, which is found abundantly in fruits, significantly elevates uric acid levels in humans, and thus indirectly increases antioxidant capacity. High levels of uric acid may be a protective factor against Parkinson's disease and possibly other diseases related to oxidative stress.
^ abEFSA Panel on Dietetic Products, Nutrition and Allergies (2010). "Scientific Opinion on the substantiation of health claims related to various food(s)/food constituent(s) and protection of cells from premature aging, antioxidant activity, antioxidant content and antioxidant properties, and protection of DNA, proteins and lipids from oxidative damage pursuant to Article 13(1) of Regulation (EC) No 1924/2006". EFSA Journal8 (2): 1489. doi:10.2903/j.efsa.2010.1489.
^ abcLotito, S; Frei, B (2006). "Consumption of flavonoid-rich foods and increased plasma antioxidant capacity in humans: Cause, consequence, or epiphenomenon?". Free Radical Biology and Medicine41 (12): 1727–46. doi:10.1016/j.freeradbiomed.2006.04.033. PMID17157175.
^ abAnand, Preetha; Kunnumakkara, Ajaikumar B.; Newman, Robert A.; Aggarwal, Bharat B. (2007). "Bioavailability of Curcumin: Problems and Promises". Molecular Pharmaceutics4 (6): 807–18. doi:10.1021/mp700113r. PMID17999464.
^Kurien, Biji T.; Singh, Anil; Matsumoto, Hiroyuki; Scofield, R. Hal (2007). "Improving the Solubility and Pharmacological Efficacy of Curcumin by Heat Treatment". ASSAY and Drug Development Technologies5 (4): 567–76. doi:10.1089/adt.2007.064. PMID17767425.