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Antioxidants, like oxidative injury causing pro-oxidants, have a profound role in health and diseases in humans. Some of the major beneficial roles include those in disease prevention and treatment.
There are several antioxidant systems within the body that help cope with the oxidative stress that results from regular metabolic processes. Antioxidants in diet can also cancel out the cell-damaging effects of free radicals. These antioxidant supplements act in addition to the endogenous systems and their lack can cause several ill-consequences of oxidative stress.
There is evidence that some types of vegetables and fruits protect against a number of cancers and other diseases. Large studies have shown that people who took regular antioxidants in fruits and vegetables seemed to have lesser incidence of these diseases. In addition, those who took fewer amounts of antioxidants, or had excessive exposure to pro-oxidants like cigarette smoking etc., had a higher risk of these disorders.
For example, oxidation of low density lipoprotein (LDL) in the blood contributes to heart disease. Those taking Vitamin E supplements had a lower risk of developing heart disease.
The exact amounts of antioxidant supplement and their exact preventive role, however, could not be determined. This meant that some people did get cancers and other oxidative stress related disorders despite adequate fruits and vegetables and antioxidant consumption.
In prevention of heart disease, for example, seven large clinical trials were conducted to test the effects of antioxidant supplement with Vitamin E, in doses ranging from 50 to per day. None of these trials found a statistically significant effect of Vitamin E on overall number of deaths or on deaths due to heart disease.
This said, there are several essential vitamins, minerals and antioxidants that include resveratrol (from grape seeds or knotweed roots), beta carotene (provitamin A), vitamin C, vitamin E and Selenium, or herbs that contain antioxidants - such as green tea and jiaogulan.
Several vital organs like the heart, lungs and the brain are vulnerable to oxidative injury. Brain in particular is vulnerable because of its high content of oxygen, high metabolic rate and elevated levels of polyunsaturated lipids - the target of lipid peroxidation.
Several antioxidant supplements are available to treat neural injury with oxidative stress. Brain injury may result in damage to parts of the brain after a stroke, Alzheimer’s disease, Parkinson’s disease and other neurodegenerative disorders. After a stroke for example the brain undergoes reperfusion injury that is mediated by oxidative stress.
Superoxide dismutase mimetics, sodium thiopental and propofol are used to treat reperfusion injury and traumatic brain injury, while the experimental drug NXY-059 and ebselen are being applied in the treatment of stroke. These compounds appear to prevent oxidative stress in neurons. They help in preventing neural cell death.
Antioxidants are also being investigated as possible treatments for neurodegenerative diseases such as Alzheimer's disease, Parkinson's disease, and amyotrophic lateral sclerosis.
Some of the antioxidants when taken in excess in diet may cause more harm than good. For example, when a person takes in excessive amounts of strong reducing agents as antioxidants, he or she may develop deficiency of several minerals like iron and zinc. The absorption of these minerals is prevented from the gastrointestinal tract.
Notable examples are oxalic acid, tannins and phytic acid, which are high in plant-based diets. In addition, there may be Calcium and iron deficiencies in persons who take too much phytic acid from beans, legumes, maize and unleavened whole grain bread. Similarly oxalic acid is present in cocoa, chocolate, spinach, turnip and rhubarb and tannins are present in cabbage, tea and beans. Excess of these in diet may prevent mineral absorption.
Eugenol, an antioxidant present in oil of cloves, also possesses toxic effects in high levels.
Toxicity associated with high doses of water-soluble antioxidants such as ascorbic acid are less of a concern since these can be excreted rapidly in urine. Very high doses of some lipid soluble antioxidants may have harmful long-term effects.