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 oxygen radical absorbance capacity (ORAC) measurement. Other than for dietary antioxidant vitamins – vitamin A, vitamin C and vitamin E – no food compounds have been proved to be antioxidants 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.[1][2]
Physiological context
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 antioxidantsin vitro, antioxidant effects in vivo are probably negligible or absent.[3][4][5] By non-antioxidant mechanisms still undefined, polyphenols may affect mechanisms of cardiovascular disease or cancer.[6]
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.[7][8] 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."[8] Another mechanism may be the increase in activities of paraoxonases by dietary antioxidants which can reduce oxidative stress.[9]
Alpha-carotene - found in carrots, winter squash, tomatoes, green beans, cilantro, Swiss chard
Astaxanthin - found naturally in red algae and animals higher in the marine food chain. It is a red pigment familiarly recognized in crustacean shells and salmon flesh/roe.
Beta-carotene - found in high concentrations in butternut squash, carrots, orange bell peppers, pumpkins, kale, peaches, apricots, mango, turnip greens, broccoli, spinach, and sweet potatoes.
Lutein - found in high concentration in spinach, kale, Swiss chard, collard greens, beet and mustard greens, endive, red pepper and okra
Lycopene - found in high concentration in cooked red tomato products like canned tomatoes, tomato sauce, tomato juice and garden cocktails, guava and watermelons.
Zeaxanthin - best sources are kale, collard greens, spinach, turnip greens, Swiss chard, mustard and beet greens, corn, and broccoli
Natural phenols are a class of molecules found in abundance in plants. Many common foods contain rich sources of polyphenols which have antioxidant properties only in test tube studies. As interpreted by the Linus Pauling Institute, dietary polyphenols have little or no direct antioxidant food value following digestion.[7] Not like controlled test tube conditions, the fate of flavones or polyphenols in vivo shows they are poorly absorbed and poorly conserved (less than 5%), so that most of what is absorbed exists as metabolites modified during digestion, destined for rapid excretion.[8]
Spices, herbs, and essential oils are rich in polyphenols in the plant itself and shown with antioxidant potential in vitro. 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, and other fruits like figs, cherries, guava, oranges, mango, grape juice and pomegranate juice also have significant polyphenol content.[10]
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 excreted.[7][8]
Chicoric acid - another caffeic acid derivative, is found in chicory and Echinacea.
Chlorogenic acid - found in high concentration in coffee (more concentrated in robusta than arabica beans), blueberries and tomatoes. Produced from esterification of caffeic acid.
Cinnamic acid and its derivatives, such as ferulic acid - found in seeds of plants such as in brown rice, whole wheat and oats, as well as in coffee, apple, artichoke, peanut, orange and pineapple.
Ellagic acid - found in high concentration in raspberry and strawberry, and in ester form in barrel-aged alcohol such as red wine and whisky.
Ellagitannins - hydrolysable tannin polymer formed when ellagic acid, a polyphenol monomer, esterifies and binds with the hydroxyl group of a polyol carbohydrate such as glucose.
Gallic acid - found in gallnuts, sumac, witch hazel, tea leaves, oak bark, and many other plants.
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) or by heat.[11]
Flavonolignans - e.g. silymarin - a mixture of flavonolignans extracted from milk thistle.
^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 Technologies. 5 (4): 567–76. doi:10.1089/adt.2007.064. PMID17767425.