Antioxidants - Superfoods for Good Health
Antioxidants are the key to what dieticians and nutritionists refer to as superfoods, i.e. foods that contain a greater than average amount of the chemicals which help to make us healthier and live longer. I little bit of chemistry is required to help understand what antixoidants are and why they are the source of our superfoods. Antioxidants are chemicals that reduce the rate of oxidation reactions. Oxidation reactions are chemical reactions that involve the transfer of electrons from one substance to an oxidizing agent. Antioxidants can slow these reactions either by reacting with intermediates and halting the oxidation reaction directly, or by reacting with the oxidising agent and preventing the oxidation reaction from occurring.
Antioxidants are particularly important in organic chemistry and biology. All organisms maintain a reducing environment inside their cells and contain complex systems of antioxidants to prevent damage by oxidation. These antioxidants include glutathione and ascorbic acid and these chemicals are substrates for enzymes such as peroxidases and oxidoreductases. Low levels of antioxidants or inhibition of antioxidant enzymes causes oxidative stress and may damage or kill cells. Therefore, large supplies of antioxidants help to prevent cell breakdown, and can help to boost the body's natural defences and strnegthen the immune system.
Antioxidants are widely used as ingredients in dietary supplements used for health purposes such as attempting to prevent cancer and heart disease. Research has indicated that antioxidant supplements can benefits health, but several large clinical trials did not demonstrate a definite benefit for the formulations tested, and excess supplementation may even be harmful.
Early nutrition researchers focused on the use of antioxidants for preventing the oxidation of unsaturated fats, the cause of rancidity. 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. Research into how vitamin E prevents the process of lipid peroxidation led to the current understanding of antioxidants as reducing agents that prevent oxidative reactions, often by scavenging reactive oxygen species before they can cause damage to the cells.
The Biology of Superfoods and Antioxidants
All living organisms contain antioxidant enzymes and chemicals. These work together to prevent oxidative damage to cellular components such as DNA, proteins and lipids. In cells, the oxidising agents that cause this damage are called reactive oxygen species. In general, antioxidant systems either prevent these reactive species from being formed, or remove them before they can damage vital components of the cell.
Oxidative damage
Without antioxidants in our diet, we suffer greater rates of oxidative damage. This is when the reactive oxygen produced in cells include hydrogen peroxide (H2O2), hypochlorous acid (HOCl), and free radicals such as the hydroxyl radical (·OH) and the superoxide anion (O2−). These molecules are unstable and highly reactive, and can damage cells by starting chemical chain reactions such as lipid peroxidation, or by oxidising DNA or proteins. Damage to DNA can lead to mutations and cancer if not reversed by DNA repair mechanisms. Damage to proteins can cause enzyme inhibition and denaturation.
Applications in nutrition and medicine
Health preservation
Antioxidants are chemicals that reduce oxidative damage to cells and biomolecules. Researchers have found a 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, atherosclerosis, 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. This is the basis for the free-radical theory of aging.
Some of the reactions in the body that produce free radicals involve metal ions. Some antioxidants, such as the tannins in walnuts and tea, 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.
Some studies suggest that by destroying free radicals and reducing cellular damage, antioxidants in the diet can have positive health effects, such as preventing macular degeneration (studied in the Age-Related Eye Disease Study); maintaining the immune system; potentially preventing neurodegeneration due to oxidative stress; preventing DNA damage; and lowering the risk of cardiovascular disease. Any specific antioxidant may perform only a small fraction of these functions. The mixed results from controlled studies using antioxidant vitamins suggest that other antioxidant substances in fruit and vegetables at least partially explain the better health of those who consume more fruit and vegetables. 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.
Problems with too much Antioxidants
Relatively strong reducing acids can have anti-nutritional effects by binding to dietary minerals in the gastrointestinal tract and preventing them from being absorbed. Notable examples are oxalic acid and phytic acid, which are high in plant-based diets. Some 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. These anti-nutrients can result in deceptively high oxygen radical absorbance capacity (ORAC) ratings given to various "healthy" beverages and foods, particularly:
| Foods | Reducing acid |
|---|---|
| Cocoa and 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.
While antioxidants supplementation is widely hypothesized to prevent the development of cancer, antioxidants may, paradoxically, interfere with cancer treatments. One explanation for this effect is that the growth-promoting environment of cancer cells leads to high levels of redox stress under baseline conditions, and this makes cancer cells more susceptible than normal cells to the further stress of chemotherapy or radiation therapy. So by reducing the redox stress in cancer cells, antioxidant supplements could decrease the effectiveness of the therapy designed to kill them.
Calorie restriction
Many studies of mammals have shown that a restricted calorie diet extends median and maximum lifespan. This effect appears to be partly due to reduced oxidative stress. 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.
Physical exercise
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) 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. 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 1000 mg 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 300 mg 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
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. 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.
While several trials have investigated supplements with high doses of antioxidants, the "Supplémentation en Vitamines et Mineraux Antioxydants" (SU.VI.MAX) study tested the effect of supplementation with doses comparable to those in a healthy diet. Over 12,500 French men and women took either low-dose antioxidants (120mg of ascorbic acid, 30 mg of vitamin E, 6 mg of beta carotene, 100g of selenium, and 20mg 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
Oxygen radical absorbance capacity (ORAC) has become the current industry standard for assessing antioxidant strength of whole foods, juices and food additives. Other measurement tests include reducing power, free radical scavenging, superoxide anion radical scavenging, hydrogen peroxide scavenging, and metal chelating.
Food preservatives
Antioxidants used as food additives to help guard against food deterioration include:
- Ascorbic acid (vitamin C)
- Tocopherol-derived compounds
- BHA, BHT, EDTA
- Tert-Butylhydroquinone
- 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
Superfoods
Antioxidants are found naturally in varying amounts, in vegetables, fruits, grain cereals, legumes, nuts etc. Antioxidant sources / superfoods include:
- Fruits: berries and peppers, cider, wine
- Vegetables: spinach, tea
- Fungi: mushrooms
- Whole grain cereals: hops, barley, millet and maize (including beer)
- Nuts: pecans, pistachios, and almonds
- Beans: cacao (including chocolate), coffee
- Olive leaf tea
Dietary supplements
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. However, to date, the FDA finds insufficient evidence to endorse health claims that have been made regarding antioxidant supplementation.
Further reading
- Antioxidants and Exercise by Bryan Helwig, PhD
- Survey of sport nutrition experts PPOnline article
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