Antioxidants

An antioxidant is a chemical that reduces the rate of particular oxidation reactions in a specific context, where oxidation reactions are chemical reactions that involve the transfer of electrons from a substance to an oxidising agent.

Antioxidants are particularly important in the context of organic chemistry and biology: all living cells contain complex systems of antioxidant chemicals and/or enzymes to prevent chemical damage to the cells' components by oxidation. The importance and complexity of antioxidants in biology is reflected in a medical literature of more than 142,000 scholarly articles.

A diet containing polyphenol antioxidants from plants is required for the health of most mammals, since plants are an important source of organic antioxidant chemicals. Antioxidants are widely used as ingredients in dietary supplements that are used for health purposes such as preventing cancer and heart disease. However, while many studies have suggested benefits for individual antioxidant supplements, several large clinical trials have failed to clearly demonstrate a benefit for the formulations tested, and excess supplementation may be harmful. It is logical to assume that a one dimensional approach to dietary supplementation with one specific antioxidant is not a panacea, since a broad diet rich in phytonutrients will yield thousands of different polyphenol antioxidants available for metabolism.

History
The term antioxidant (also "antioxygen") originally referred specifically to a chemical that prevented the consumption of molecular oxygen. In the 19th and early 20th century, antioxidants were the subject of extensive research in industrial processes such as the corrosion of metals, explosions, the vulcanization of rubber, and the knocking of fuels in internal combustion engines (Mattill 1947). Early nutrition researchers focused on the use of antioxidants for preventing the oxidation of unsaturated fats (which made them rancid). Antioxidant activity could be measured simply by placing the fat in a closed glass container with oxygen and observing the rate of oxygen consumption. However, it was the identification of vitamins A, C, and E as antioxidants that revolutionized the field and led to the realization of the importance of antioxidants in biology.

The possible mechanisms for the action of antioxidants was first explored thoroughly by Moreau and Dufraisse (1926), who recognized that a substance with anti-oxidative activity is likely to be one that is itself a target for oxidation. Research into how Vitamin E prevents the process of lipid peroxidation led to the current understanding of antioxidants as reducing agents that break oxidative chain reactions, often by scavenging reactive oxygen species before they can cause damage to the cells (Wolf 2005).

Antioxidants in biology
Living organisms all contain complex systems of antioxidant enzymes and chemicals. Some of these systems, like the thioredoxin system, are conserved through all of evolution and are required for life. Antioxidants in biological systems have multiple purposes, including defending against oxidative damage and participating in the major signaling pathways of the cells.

One major action of antioxidants in cells is to prevent damage due to the action of reactive oxygen species. Reactive oxygen species include hydrogen peroxide (H2O2), the superoxide anion (O2&minus;), and free radicals such as the hydroxyl radical (·OH). These molecules are unstable and highly reactive, and can damage cells by chemical chain reactions such as lipid peroxidation, or formation of DNA adducts that could cause cancer-promoting mutations or cell death. All cells therefore contain antioxidants that serve to reduce or prevent this damage.

Antioxidants may be further classified by the products they form on oxidation (these can be antioxidants themselves, inert, or pro-oxidant), by what happens to the oxidation products (the antioxidant may be regenerated by different antioxidants or, in the case of "sacrificial" antioxidants, its oxidised form may be broken down by the organism) and how effective the antioxidant is against specific free radicals.

Antioxidants are especially important in the mitochondria of eukaryotic cells, since the use of oxygen as part of the process for generating energy produces reactive oxygen species. The process of aerobic metabolism requires oxygen because oxygen serves as the final resting place for electrons generated by the oxidation steps of the citric acid cycle (i.e. oxygen is the final "electron acceptor" of the redox reactions). However, the superoxide anion is produced as a by-product of this reduction of oxygen in the electron transport chain. Specifically, the reduction of coenzyme Q in complex III is the major source of superoxide anion, since a highly reactive free radical is formed as an intermediate (Q·&minus;). This unstable radical can lead to electron "leakage": instead of moving along the well-controlled reactions of the electron transport chain, the electrons jump directly to molecular oxygen, forming the superoxide anion (Finkel and Holbrook 2000).

Important examples of the systems that cells have evolved to tightly regulate the redox state of the cell and to protect from damage by reactive oxygen species include:


 * The thioredoxin system, including thioredoxin and thioredoxin reductase. Thioredoxin is a 12-kDa protein that is present in all known living organisms except the bacteria that causes Whipple's disease.  The active site of thioredoxin consists of two neighboring cysteines, as part of a highly conserved CXXC motif, that can cycle between an active dithiol form (reduced) and an oxidized disulfide form. In its active state, thioredoxin acts as an efficient reducing agent, scavenging reactive oxygen species and maintaining other proteins in their reduced state. After being oxidized, the active thioredoxin is regenerated by the action of thioredoxin reductase, and thioredoxin reductase is in turn reduced by NADPH (Nordberg and Arner, 2001).


 * The glutathione system, including glutathione, glutathione reductase, and glutathione peroxidase. Glutathione peroxidase is an enzyme with four selenium-containing groups that catalyze the breakdown of hydrogen peroxide and protects lipids in cell walls from peroxidation.


 * Superoxide dismutase (SOD), a class of closely related proteins found in almost all living cells and in extracellular fluids. Each molecule of superoxide dismutase contains atoms of copper, zinc, manganese or iron. SOD that is formed in the mitochondria contains manganese (MnSOD). This SOD is formed in the matrix of the mitochondria. SOD that is formed in the cytoplasm of the cell contains copper and zinc (CuZnSOD). Superoxide dismutase protects cells by catalysing the breakdown of the highly reactive superoxide anion into oxygen and hydrogen peroxide.


 * Catalase, a widely occurring enzyme containing four iron atoms in a 500 amino acid protein. Catalase catalyses the conversion of hydrogen peroxide to water and oxygen at rates of up to 6,000,000 molecules per minute. Catalase has a secondary role oxidising toxins including formaldehyde, formic acid and alcohols.


 * Peroxiredoxins catalyse the reduction of hydroperoxides.


 * Uric Acid is the antioxidant in highest concentration in the extracellular fluids in humans and higher primates.  It may have partially-substituted for vitamin C in human evolution.

How antioxidants preserve health
Antioxidants are chemicals that reduce oxidative damage to cells and biochemicals. Researchers have found high correlation between oxidative damage and the occurrence of disease. For example, Low density lipoprotein(LDL)oxidation is associated with cardiovascular disease. The process leading to atherogenesis, artherosclerosis, and cardiovascular disease is complex, involving multiple chemical pathways and networks, but the precursor is LDL oxidation by free radicals, resulting in inflammation and formation of plaques.

Research suggests that consumption of antioxidant-rich foods reduces damage to cells and biochemicals from free radicals. This may slow down, prevent, or even reverse certain diseases that result from cellular damage, and perhaps even slow down the natural aging process (see free-radical theory of aging).

Some of the reactions in the body that produce free radicals involve metal ions. Furthermore, metal ions are themselves free radicals that can cause oxidation directly. Some antioxidants, such as the tannins in walnuts, chelate (wrap around) metal ions. This not only reduces the formation of ion-dependent free radicals, but also prevents the metal ions from oxidizing cells and biochemicals directly.

By destroying free radicals and reducing cellular damage, antioxidants, as a group, can: Any specific antioxidant may perform only a small fraction of these functions.
 * Promote eye health and prevent macular degeneration, cataracts, and other degenerative eye diseases. The benefits of antioxidants were examined during the Age-Related Eye Disease Study.
 * Keep the immune system in good shape, or boost the immune system when it has been compromised.
 * Prevent age-related neurodegeneration (decline of the brain and nervous system).
 * Prevent DNA damage and therefore have anticarcinogenic effects (that is, help prevent cancer).
 * Have antiatherogenic effects (that is, promote cardiovascular health and help prevent artherosclerosis, heart attacks, strokes, and other cardiovascular diseases). Antioxidants can decrease LDL and cholesterol, increase High density lipoprotein(HDL), and lower blood pressure.  The mechanisms behind these effects are not fully understood, and can occur even if the person has a diet high in saturated fat.

Dietary antioxidants are not the primary antioxidant inside the body, and there are still many questions as to how polyphenols and other dietary antioxidants protect cells and biochemicals from oxidation. Some antioxidants preserve, or even recycle, other antioxidants such as vitamin E. Some antioxidants have far-reaching effects, such as moderating insulin, that are not clearly understood.

How some antioxidants can harm health
Some of the plant based reducing acids, most notably oxalic and phytic, bind to needed dietary minerals, rendering them unabsorbable in the gastrointestinal tract. Some of the tannins also have this negative characteristic. Calcium and iron deficiencies are not uncommon in mideastern diets where there is high consumption of phytic acid present in beans and unleavened whole grain bread. Such antinutrients can sometimes result in deceptively high Oxygen Radical Absorbance Capacity (ORAC) ratings given to various "healthy" beverages and foods, particularly:
 * cocoa/chocolate, spinach, and berries - oxalic acid
 * whole grains, maize - phytic acid
 * tea - tannins

Other extremely powerful nonpolar antioxidants such as eugenol also happen to have toxicity limits that can easily be exceeded with the misuse of essential oils.

High levels of antioxidants can be powerful agents against tumours, but in some scenarios can interfere with the effects of other cancer treatments.

Recent laboratory studies suggest that at levels much higher than occur through normal diets, antioxidant vitamins such as A, E and C can have pro-oxidant effects, increasing the formation of free radicals. Natural antioxidants are always ingested together with a wide variety of flavonoids and other phytochemicals are also likely to play a part. Many supplement manufacturers supply products containing antioxidants in combination with these other natural chemicals. Another significant factor is that the mechanisms by which different antioxidants regenerate each other require balanced levels to work optimally. Newer liquid nutritional supplements using plant ionic compounds are believed to be more readily absorbed in the human body.

Calorie restriction and reduced oxidative stress
Virtually all studies of mammals have concluded that a restricted calorie diet (CR) extends median and maximum lifespan (CR is almost the only protocol to have achieved this). This benefit appears to be at least partly due to substantially reduced oxidative stress. Very large increases in lifespan (up to around 100%) have only been observed in short lived species and the effect in humans is expected to be far less dramatic. The best evidence from animal studies is likely to come from ongoing studies in primates where median life spans have already been shown to be increased and biomarkers of health significantly improved. Due to the long life span of primates, confirmation of maximum lifespan increase will not be available until around 2014. The striking results from animal experiments provide strong evidence that an excess of food reduces life expectancy, although the relationship is not a simple one. Other research suggests that being a little overweight is actually a healthier option in humans (New Scientist 26 November 2005), and a recent major study concluded that mortality rates were positively correlated with waist size, but for a fixed waist size mortality rates were negatively correlated with body mass index (particularly for underweight subjects). As food produces free radicals (oxidants) when metabolized, antioxidant-rich diets are thought to stave off the effects of aging significantly better than diets lacking in antioxidants.

Exercise and antioxidants
During exercise, oxygen consumption can temporarily increase by a factor of more than 10. This leads to a temporary large increase in the production of oxygen free radicals, resulting in increased cell damage contributing to muscular fatigue during and after exercise. The body uses antioxidants to reduce the amount of such damage. The inflammatory response that occurs after strenuous exercise is also associated with increased occurrence of free radicals, especially during the 24 hours after an exercise session. In this phase too, antioxidants in the body reduce the damage. The immune system response to damage done by exercise peaks 2 to 7 days after exercise, the period during which adaptation resulting in greater fitness is greatest. During this process, free radicals are used by neutrophils in the immune system to identify damaged tissue. As a result, excessive antioxidant levels have the potential to inhibit recovery and adaptation mechanisms.

There is a popular view that those who undertake vigorous exercise can benefit from increased consumption of antioxidants, but an examination of the literature finds support that this is the case only for certain antioxidants at certain levels, and some evidence that very large intake of some antioxidants may be detrimental to recovery from exercise. There is strong evidence that one of the adaptations that result from exercise is a strengthening of the body's antioxidant defenses, particularly the glutathione system, to deal with the increased oxidative stress. It is possible that this effect may be to some extent protective against diseases which are associated with oxidative stress, which would provide a partial explanation for the lower incidence of major diseases and better health of those who undertake regular exercise.

The antioxidant system that protects lipid membranes from free radicals includes vitamin E, beta-carotene, vitamin A, and coenzyme Q10. The system that scavenges free radicals in the water based cytoplasm includes vitamin C, glutathione peroxidase, superoxide dismutase, and catalase. The effect of each of the exogenous antioxidants needs to be examined separately, although they work in a co-operative manner.

The body of research suggests no benefits from supplementing with vitamin A above normally recommended levels. Recent well-designed studies suggest there are no ergogenic benefits from vitamin E (except for those who do exercise at high altitude) ,,(Mehdani et al, 1997), despite its key role in preventing lipid membrane peroxidation. For example, 6 weeks of vitamin E supplementation had no effect on muscle damage indicators in ultramarathon runners. Although selenium is essential to the glutathione antioxidant system which, as mentioned above, is upregulated by exercise, there is no evidence that supplementation with selenium above the RDA is of any ergogenic benefit. However, for vitamin C there is considerable evidence that vitamin C requirements are greater in those who do vigorous exercise, with plasma levels falling with intake of 100mg (well over the accepted RDA) and around 300mg per day being required to maintain blood plasma levels (Keith, 1997). There is some evidence that supplementation with vitamin C increased the amount of intense exercise that can be done, and lowered the heart rate while doing it (which is indicative of greater efficiency), and that vitamin C supplementation before strenuous exercise reduces the amount of muscle damage. However, some other studies found no such effects, and some research suggests that supplementation with amounts as high as 1000mg inhibits recovery, although the very short pre-exercise supplementation period in this study may have influenced the results. There is strong evidence that vitamin C supplementation reduces upper respiratory tract infections in ultra-endurance athletes.

In summary, a diet with at least 300mg of vitamin C is of benefit to those who undertake high intensity or high volume exercise, but it is not clear that normal requirements for vitamin A, vitamin E or selenium are increased.

Clinical trials of antioxidant supplements
Although some levels of antioxidant vitamins and minerals in the diet are required for good health, there is considerable doubt as to whether antioxidant supplementation is beneficial, and if so, which and what amount of antioxidant(s) are optimal.

One study of lung cancer patients found that those given beta-carotene supplements had worse prognoses. Two 1994 studies found an increased rate of lung cancer in smokers supplementing with beta carotene. This is believed to be due to antioxidant interference with the body's normal use of localised free radicals e.g. nitric oxide for cell signalling. Due to the complex nature of the interactions of antioxidants with the body, it is difficult to interpret the results of many experiments. In vitro testing (outside the body) has shown many natural antioxidants, in specific concentration, can halt the growth of or even kill cancerous cells.

In the early 1990s, it was hypothesized that oxidation of LDL cholesterol contributes to heart disease, and several observational studies found that people taking Vitamin E supplements had a lower risk of developing heart disease (Rimm 1993). Taken together, this led researchers to conduct at least seven large clinical trials testing the effects of antioxidant supplement with Vitamin E, in doses ranging from 50 to 600 mg per day. However, none of these trials found a statistically significant effect of Vitamin E on overall number of deaths or on deaths due to heart disease (Vivekananthan 2003).

While several trials have investigated supplements with high doses of antioxidants, the "Supplementation en Vitamines et Mineraux Antioxydants" (SU.VI.MAX) study tested the effect of supplementation with doses comparable to those in a healthy diet (Hercberg 2003). Over 12,500 French men and women took either low-dose antioxidants (120 mg of ascorbic acid, 30 mg of vitamin E, 6 mg of beta carotene, 100 $$\mu$$g of selenium, and 20 mg of zinc) or placebo pills for an average of 7.5 years. The investigators found there was no statistically significant effect of the antioxidants on overall survival, cancer, or heart disease. However, a subgroup analysis showed a 31% reduction in the risk of cancer in men, but not women. The authors interpreted these results as suggesting that "an adequate and well-balanced supplementation of antioxidant nutrients, at doses that might be reached with a healthy diet that includes a high consumption of fruits and vegetables, had protective effects against cancer in men."

Measurement of antioxidant capacity
Wikipedia contains a discussion of oxygen radical absorbance capacity or ORAC which has become the current industry standard for assessing antioxidant strength of whole foods, juices and food additives. References, see Wu et al., 2004 which reports ORAC for more than 100 common foods in the United States, and External Links, commercial assays for antioxidant capacity.

Antioxidants in food industry - food preservatives
Antioxidants used as food additives to help guard against food deterioration include:
 * Ascorbic acid (vitamin C)
 * Tocopherol-derived compounds
 * BHA, BHT, EDTA
 * Citric acid
 * Acetic acid - found in vinegar; used for pickling
 * Pectin
 * Rosmarinic acid - in the form of the herb rosemary and Italian seasoning mixtures in naturally or minimally processed foods, and pet foods

Nutritional antioxidants

 * See also List of phytochemicals and foods in which they are prominent

Since the discovery of vitamins, it has been recognized that antioxidants from the diet are essential for healthful lives in humans and many other mammals. More recently, a large body of evidence has accumulated that suggests supplementation of the diet with various kinds of antioxidants can improve health and extend life. Many nutraceutical and health food companies now sell formulations of antioxidants as dietary supplement. These supplements may include specific antioxidant chemicals, like resveratrol (from grape seeds), combinations of antioxidants, like the "ACES" products that contain beta carotene (provitamin A), vitamin C, vitamin E and Selenium, or specialty herbs that are known to contain antioxidants such as green tea and jiaogulan.

There are hundreds of different types of antioxidants. The following substances may have nutritional antioxidant effects:

Vitamins

 * Vitamin A (Retinol), also synthesized by the body from beta-carotene, protects dark green, yellow and orange vegetables and fruits from solar radiation damage, and is thought to play a similar role in the human body. Carrots, squash, broccoli, sweet potatoes, tomatoes (which gain their color from the compound lycopene), kale, seabuckthorn, collards, cantaloupe, peaches and apricots are particularly rich sources of beta-carotene.


 * Vitamin C (Ascorbic acid) is a water-soluble compound that fulfills several roles in living systems. Important sources include citrus fruits (such as oranges, sweet lime, etc.), green peppers, broccoli, green leafy vegetables, strawberries, blueberries, seabuckthorn, raw cabbage and tomatoes. Linus Pauling was a major advocate for its use.


 * Vitamin E, including Tocotrienol and Tocopherol, is fat soluble and protects lipids. Sources include wheat germ, seabuckthorn, nuts, seeds, whole grains, green leafy vegetables, vegetable oil, and fish-liver oil. Recent studies showed that some tocotrienol isomers have significant anti-oxidant properties.

Vitamin cofactors and minerals

 * Coenzyme Q10 (CoQ10) is an antioxidant which is both water and lipid soluble. It is not classified as a vitamin in humans as it can be manufactured by the body, but quantities decrease with age to levels that may be less than optimal, and levels in the diet are generally low. Supplementation with CoQ10 has been clinically proven to improve the health of gums. There is evidence that CoQ10 helps protect the brain against Parkinson's disease.


 * Manganese, particularly when in its +2 valence state as part of the enzyme called superoxide dismutase (SOD).

Hormones

 * Melatonin is a natural hormone, occurring in every organism, which has many biological roles. Melatonin acts as an antioxidant and promoter of antioxidants in several different ways . Recent research supports a specific role as an antioxidant in mitochondria, which have an high level of reactive oxygen species produced during aerobic metabolism, but lack some of the protective mechanisms of cell nuclei. ,,,.

Carotenoid terpenoids

 * See main article at Carotenoid


 * Lycopene - found in high concentration in ripe red tomatoes.
 * Lutein - found in high concentration in spinach and red peppers.
 * Alpha-carotene
 * Beta-carotene - found in high concentrations in butternut squash, carrots, orange bell peppers, pumpkins, and sweet potatoes.
 * Zeaxanthin - the main pigment found in yellow corn.
 * 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.
 * Canthaxantin

Non-carotenoid terpenoids
Eugenol - has by far the highest Oxygen Radical Absorbance Capacity (ORAC) of all foodborn substances (in clove oil). Its concentration in clove oil ranges 5-20 times greater than where it is found in other sources such as in basil and cinnamon.

Saponins and limonoids Editor's note: Not certain if these are antioxidants; work in progress...

Flavonoid polyphenolics (also known as bioflavonoids)
Bioflavonoids, a subset of polyphenol antioxidants, are present in many dark berries such as pomegranate,seabuckthorn, noni, blueberries, and blackberries, as well as in certain types of coffee and tea, especially green tea.

Flavonols:
 * Resveratrol - found in the skins of dark-colored grapes, and concentrated in red wine.
 * Pterostilbene - methoxylated analogue of resveratrol, abundant in Vaccinium berries
 * Kaempferol
 * Myricetin - walnuts are a rich source
 * Isorhamnetin
 * Proanthocyanidins, or condensed tannins

Flavones: Flavanones:
 * Quercetin and related, such as rutin
 * Luteolin
 * Apigenin
 * Tangeritin
 * Hesperetin (metabolizes to hesperidin)
 * Naringenin (metabolized from naringin)
 * Eriodictyol

Flavan-3-ols (anthocyanidins):
 * Catechin
 * Gallocatechin
 * Epicatechin and its gallate forms
 * Epigallocatechin and its gallate forms
 * Theaflavin and its gallate forms
 * Thearubigins

Isoflavone phytoestrogens - found primarily in soy, peanuts, and other members of the Fabaceae family. Besides having antioxidant characteristics, isoflavones also protect and maintain the skeletal system.
 * Genistein
 * Daidzein
 * Glycitein

Anthocyanins protect plants from UV damage:
 * Cyanidin
 * Delphinidin
 * Malvidin
 * Pelargonidin
 * Peonidin
 * Petunidin

Phenolic acids and their esters

 * See main article: Polyphenol antioxidant


 * Ellagic acid - found in high concentration in raspberry and strawberry, and in ester form in red wine tannins.
 * 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.
 * Rosmarinic acid - found in high concentration in rosemary, oregano, lemon balm, sage, and marjoram.
 * 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.
 * Chlorogenic acid - found in high concentration in coffee (more concentrated in robusta than arabica beans), blueberries and tomatoes. Produced from esterification of caffeic acid.
 * Chicoric acid - another caffeic acid derivative, is found only in the popular medicinal herb Echinacea purpurea.
 * Gallotannins - hydrolyzable tannin polymer formed when gallic acid, a polyphenol monomer, esterifies and binds with the hydroxyl group of a polyol carbohydrate such as glucose.
 * Ellagitannins - hydrolyzable tannin polymer formed when ellagic acid, a polyphenol monomer, esterifies and binds with the hydroxyl group of a polyol carbohydrate such as glucose.

Other nonflavonoid phenolics

 * Curcumin

Other (someone please classify):
 * Other plant pigments such as anthoxanthins and betacyanins. (Are these antioxidants?  Are they flavonoids?)
 * Silymarin - mixture of flavonolignans extracted from milk thistle.

Other organic antioxidants

 * Citric acid


 * Lignan - antioxidant and phytoestrogen found in oats, flax seeds, pumpkin seeds, sesame seeds, rye, soybeans, broccoli, beans, and some berries.


 * Antinutrients - strong antioxidants that readily bind to needed dietary minerals, rendering them unabsorbable in the gastrointestinal tract. Examples: oxalic acid and phytic acid.


 * Bilirubin, a breakdown product of blood, has been identified as a possibly significant antioxidant.


 * Uric acid
 * R-&alpha;-lipoic acid - fat and water soluble
 * Silymarin - fat soluble; also available in water soluble form
 * N-acetylcysteine - water soluble

Beverages and foods highest in antioxidants


#1 Undutched cocoa powder

#2 Dark, semisweet chocolate; particularly that which is 85% cocoa solids

#3 White tea

#4 Green rooibos

#5 Green tea

#6 Red rooibos

#7 Oolong tea

#8 Black tea

Certain fruits and berries, especially:

#9 Blueberry (especially wild blueberry; AKA bilberry) contain more antioxidants than any other fruit or vegetable, when compared on the basis of equal calories. They are high in anthocyanins, chlorogenic acid, ellagic acid, catechins, and resveratrol.
 * Blackberry and raspberry
 * Cranberry
 * Cherry, esp. sour cherry
 * Dried plum ("prune")
 * Grape, esp. dark grape - high in the polyphenol resveratrol, and high in tannins. This includes raisins, purple grape juice, and red wine.
 * Crowberry
 * Kiwi
 * Pomegranate - high in tannins
 * Papaya - source of vitamin E & beta-carotene, three very powerful antioxidants.
 * Citrus fruit such as orange and grapefruit - the highest concentration of antioxidants are in the pulp, where its pectin is found

Leafy, dark green cruciferous vegetables:
 * Broccoli and all cruciferous vegetables are high in lutein, sulforaphane (a type of glucosinate), indoles, carotenoids, beta-carotene, zeaxanthin.
 * Brussels sprouts – high in glucosinates
 * Cabbage (both red and green)
 * Kale

Certain other vegetables, especially:
 * Artichokes
 * Asparagus
 * Avocado
 * Beans
 * Beets
 * Carrots
 * Red peppers
 * Russet potatoes
 * Spinach – high in carotenoids, especially zeaxanthin (related to lutein); but also high in the antioxidant antinutrient oxalic acid
 * Tomatoes, especially ripe red tomatoes – high in the extremely potent antioxidant known as lycopene. Eating tomatoes with olive oil helps in assimilation of the lycopene.  Tomatoes are also high in beta carotene and lutein.  Even ketchup has some lycopene (but is also high in corn syrup, so don't go crazy).
 * Olives in the form of extra virgin olive oil. Besides being high in polyphenols, extra virgin olive oil is also high in oleic acid, an omega-9 monounsaturated fatty acid.  Some studies suggest that olive oil can reduce blood pressure, reduce LDL, and ward off cancer.

Generally, the deeper and richer the color of fruits and vegetables, the higher the quantity of antioxidants. Many fruits and vegetables are also high in fiber, minerals, and vitamins. Note, however, that the most commonly eaten fruits and vegetables (apples, bananas, iceberg lettuce, and potatoes) are not on the list. Fruit juice can contain some antioxidants, but not nearly as much as the fruit from which they are made (antioxidants are concentrated in the skins and pulps), and fruit juice tends to consist primarily of corn syrup and water. To consume the greatest quantity of antioxidants, try to eat a variety of foods, and buy fruits and vegetables locally when they are in season.

Note that the color rule of thumb does not apply to varieties of tea. The darker the variety of tea, the lower is its antioxidant concentration.

Nuts, especially:
 * Walnut - high concentration of ellagic acid; high concentrations of tocopherols (especially gamma-tocopherol) in the kernel; high concentrations of phenolic antioxidants (found in the pellicle) such as ellagic acid, gallic acid, methyl gallate, and ellagitannins; so much antioxidizing power preserves its highly reactive short-chain fatty acids (especially alpha-linolenic omega-3) from rancidity
 * Pecan
 * Hazelnut

Besides being high in polyphenols, nuts are also high in beneficial, unsaturated fatty acids. There is a correlation between nut consumption and a reduced incidence of ischemic heart disease. This is most likely due partly to the favorable lipid content and partly to the high polyphenol content. Walnuts have the highest phenolic content, which is why they taste bitterer than pecans and hazelnuts. To help preserve the antioxidants in nuts, keep them in a freezer. They have almost no water, so the freezer won’t harm them.

Certain herbs and spices. Even though people typically use spices in small amounts, some spices have extremely high antioxidant content per unit mass, especially:
 * Allspice
 * Cinnamon
 * Cloves
 * Ginger
 * Lemon balm
 * Oregano
 * Peppermint
 * Rosemary
 * Sage
 * Thyme

Tea, jiaogulan tea and white tea - high in polyphenols and tannins.

Seeds and grains, especially:
 * Sunflower seeds
 * Oats – high in lignans (one type of phytoestrogen, the other type being isoflavones), caffeic acid (may be carcinogenic, but its phenethyl ester may be anticarcinogenic), and ferulic acid. Also contains omega-3 fatty acids.

Other plants:
 * Cacao and chocolate – high in flavonoid polyphenols. The darker and more bitter the chocolate, the higher the concentration of polyphenols.
 * Dog rose

List of the 20 foods with the highest concentration of antioxidants (“total antioxidant capacity”), according to the USDA:


 * 01.	Small red beans
 * 02.	Wild blueberries
 * 03.	Red Kidney beans
 * 04.	Pinto beans
 * 05.	Cultivated Blueberries
 * 06.	Cranberries
 * 07.	Artichokes
 * 08.	Blackberries
 * 09.	Prunes
 * 10.	Raspberries
 * 11.	Strawberries
 * 12.	Red Delicious & Granny Smith apples
 * 13.	Pecans
 * 14.	Sweet cherries
 * 15.	Black plums
 * 16.	Russet potatoes
 * 17.	Black beans
 * 18.	Plums
 * 19.	Gala apples
 * 20.	Walnuts

Foods that score well in Oxygen Radical Absorbance Capacity:
 * Beets
 * Brussels sprouts
 * Kale
 * Spinach
 * Many of the same berries that have high Total Antioxidant Capacity.

Antioxidants in fuels
Some antioxidants are added to liquid industrial chemicals, most often fuels and lubricants to prevent oxidation, and in gasolines to prevent polymerization leading to gumming. Some examples are:


 * AO-22 (N,N'-di-2-butyl-1,4-phenylenediamine), for turbine oils, transformer oils, hydraulic fluids, waxes, and greases
 * AO-24 (mostly N,N'-di-2-butyl-1,4-phenylenediamine), blended for low-temperature handling)
 * AO-29 (2,6-di-tert-butyl-4-methylphenol), for turbine oils, transformer oils, hydraulic fluids, waxes, greases, and gasolines
 * AO-30 (alkylated phenols, mostly 2,4-dimethyl-6-tert-butylphenol (>97%)), for jet fuels and gasolines, including aviation gasolines
 * AO-31 (alkylated phenols, mostly 2,4-dimethyl-6-tert-butylphenol (>72%)), for jet fuels and gasolines, including aviation gasolines
 * AO-32 (alkylated phenols, mostly 2,4-dimethyl-6-tert-butylphenol (>55%), and 2,6-di-tert-butyl-4-methylphenol (>15%)), for jet fuels and gasolines, including aviation gasolines
 * AO-36 (alkylated phenols), for gasolines
 * AO-37 (alkylated phenols, mostly 2,6-di-tert-butylphenol), for jet fuels and gasolines, widely approved for aviation fuels

Antioxidants are frequently used together with metal deactivators and corrosion inhibitors.