The terms ‘antioxidant’ and ‘free radical’ are used prolifically with reference to supplements, health foods and fresh produce, but how many people actually know what they mean? By definition, the prefix ‘anti’ indicates that antioxidants work in opposition to the effects of oxidation. And by-products of this oxidation process are commonly known as free radicals. But those short definitions give little insight into their actual role and functions and, as the terms are bandied about with ever-increasing frequency, it is important to understand what they are.
Oxygen – the good and the bad
Most life on earth requires oxygen to survive. The human body needs oxygen to function efficiently and fuel the countless physiological processes that continuously take place. Oxygen is consumed in the body during metabolism by the oxidation process – one of the body’s natural chemical functions. While we cannot survive without oxygen, this life-giving substance also has some negative side-effects because oxygen is a highly reactive molecule that can also damage living organisms.
The presence of oxygen facilitates a chemical reaction that alters the structure of substances during the oxidation process. In the same way that oxygen causes iron to rust, peeled apples to turn brown and oils to turn rancid, oxidation inside the body causes a breakdown of cells. The by-products of this process are known as free radicals and these molecules can initiate chain reactions that have the potential to cause cell damage. This chain of events can weaken immunological functions and is believed to be linked to numerous diseases.
A Radical Response
Free radicals are chemically active molecules that have one or more unpaired electrons, which makes them unstable and highly reactive. These free radicals affect everything they come into contact with as they scavenge the body to hijack electrons. They disrupt the equilibrium of biological systems as they damage the cells of lipids, proteins, carbohydrates and DNA genetic material by starting chemical chain reactions such as lipid peroxidation, or by oxidizing DNA or proteins.
Free radicals are short-lived and derived mainly from oxygen and nitrogen. Free radicals containing oxygen, known as reactive oxygen species (ROS), are the most biologically significant and include superoxide and hydroxyl radical, plus derivatives of oxygen that do not contain unpaired electrons, such as hydrogen peroxide, singlet oxygen, and hypochlorous acid.
These destructive molecules are generated by sources both inside and outside the body. Those that develop internally form as a result of respiration, metabolism, inflammation and stress. Externally generated free radicals arise from things such as pollution, sun exposure, strenuous exercise, unhealthy food, environmental contaminants, tobacco smoke, X-rays and alcohol. Because of the numerous unnatural elements and chemicals in the modern world, there are more free radicals today than ever before.
In small quantities free radicals can fight bacteria and viruses; in larger quantities they cause damage known as ‘oxidative stress’ which is thought to accelerate the ageing process and contribute to age-related diseases, as well as play a role in the development of a variety of diseases including Alzheimer’s disease, cancer, eye disease, heart disease, Parkinson’s disease, neuro-degeneration, diabetes and rheumatoid arthritis.
Oxidative Stress
Oxidative stress occurs when the production of harmful free radicals is beyond the protective capability of the body’s antioxidant defences. As we age, cell parts damaged by oxidation accumulate. Oxidative stress is thought to contribute to the development of a wide range of diseases. In some cases it is not clear if oxidants trigger the disease or if they are produced as a secondary consequence of the disease and from general tissue damage. However, one case in which this link is particularly well-understood is the role of oxidative stress in cardiovascular disease. Here, low density lipoprotein (LDL) oxidation appears to trigger the process of atherogenesis, which results in atherosclerosis and finally cardiovascular disease. Free radicals can also damage DNA and cause mutations which may contribute to cancer, and damage to proteins thereby causing enzyme inhibition, denaturation and protein degradation. The brain is particularly vulnerable to oxidative injury due to its high metabolic rate and elevated levels of polyunsaturated lipids, the target of lipid peroxidation.
Free Radical Scavengers
Antioxidants – often referred to as free radical scavengers – are nutrients such as vitamins and minerals and enzymes that assist in the chemical reactions which retard or prevent deterioration, damage or destruction caused by oxidation. They also protect the cells against the effects of free radicals and can furthermore help repair damage already sustained by cells.
Antioxidants block the process of oxidation by neutralising free radicals. They reduce the effects of these dangerous oxidant molecules by binding together with them, thereby decreasing their destructive power. In doing so the antioxidants themselves become oxidised, which is why there is a constant need to replenish antioxidant resources. The function of antioxidant systems is not to remove oxidants entirely but to keep them at an optimum level.
The first research into the role of antioxidants in biology concentrated on their use in preventing the oxidation of unsaturated fats, which resulted in rancidity. Antioxidant activity was measured by placing the fat in a closed container with oxygen and measuring the rate of oxygen consumption. However, it was the identification of vitamins A, C and E as antioxidants that revolutionised the field and led to the realisation of the importance of antioxidants in the biochemistry of living organisms.
Living organisms – plants, animals and humans – maintain a complex network system of multiple types of antioxidant metabolites and enzymes that work together to prevent oxidative damage to cellular components such as DNA, proteins and lipids. These include antioxidants such as glutathione, vitamin C and vitamin E, as well as enzymes such as catalase, superoxide dismutase and various peroxidases.
Numerous antioxidants are now widely available in the form of dietary supplements. The use of antioxidants in pharmacology is being intensively studied for the prevention of diseases such as cancer, coronary heart disease, stroke and neurodegenerative diseases. Several antioxidant enzymes such as superoxide dismutase, catalase, glutathione peroxidase, glutathione reductase and glutathione S-transferase are believed to protect DNA from oxidative stress.
Interactions between different antioxidants are very complex and the various metabolites and enzyme systems seem to exert a synergistic and interdependent effect on one another. The amount of protection provided by any one antioxidant will depend on its concentration, its reactivity towards the particular reactive oxygen species being considered and the status of the antioxidants with which it interacts. The effectiveness of any given antioxidant in the body depends on which free radical is involved, how and where it is generated and where the target of damage is situated. Thus, while in one particular system an antioxidant may protect against free radicals, in other systems it could have no effect at all. Or, in certain circumstances, an antioxidant may even act as a ‘pro-oxidant’ that generates toxic oxygen species.
Antioxidant Sources
Certain antioxidants are produced within the body, including superoxide dismutase, catalase, ubiquionol (coenzyme Q10) and glutathione. Other antioxidants are found in foods, for example the ACE vitamins – vitamin A, vitamin C and vitamin E – which are considered to be the most beneficial.
Fruit and vegetables are good sources of these, for example vitamin A and carotenoids can be found in carrots, squash, broccoli, sweet potatoes, tomatoes, kale, collards, cantaloupe, peaches and apricots; vitamin C is in citrus fruits like oranges and limes, green peppers, broccoli, green leafy vegetables, strawberries and tomatoes and vitamin E in nuts and seeds, whole grains, green leafy vegetables, vegetable oil and liver oil.
Antioxidants are found in varying amounts in different foods – vegetables, fruits, grain cereals, eggs, meat, legumes and nuts. Some antioxidants such as lycopene and ascorbic acid can be destroyed by long-term storage or prolonged cooking. Other antioxidant compounds are more stable, such as the polyphenolic antioxidants in whole-wheat cereals and tea. Cooking can increase the bioavailability of some antioxidants, such as some carotenoids in vegetables. However, processed foods usually contain fewer antioxidants than fresh and uncooked foods.
Numerous dietary supplements are available for people who don’t eat enough antioxidant-producing foods. These may include specific antioxidant chemicals such as the grape-seed derived polyphenol resveratrol, combinations of antioxidants like the ‘ACES’ products that contain beta carotene (provitamin A), vitamin C, vitamin E and selenium, as well as antioxidant-rich herbs including rooibos and green tea.
Levels of antioxidants can be altered by certain dietary compounds that act as pro-oxidants which cause oxidative stress, thereby stimulating the body to respond by inducing higher levels of antioxidant defences like antioxidant enzymes. Some of these compounds, such as isothiocyanates and curcumin, may be chemo-preventive agents that either block the transformation of abnormal cells into cancerous cells or even kill existing cancer cells.
Phytochemicals are a rich source of antioxidants and can be found in specific foods, for example: Allyl sulfides - Onions, garlic, leeks, chives; Carotenoids (lycopene, lutein, zeaxanthin) – Tomatoes, carrots, watermelon, kale, spinach; Curcumin - Turmeric; Flavonoids (anthocyanins, resveratrol, quercitin, catechins) – Grapes, blueberries, strawberries, cherries, apples, grapefruit, cranberries, raspberries, blackberries; Glutathione - Green leafy vegetables; Indoles – Broccoli, cauliflower, cabbage, Brussels sprouts, bok choy; Isoflavones (genistein, daidzeins) – Legumes such as peas, soybeans; Isothiocyanates (sulforaphane) – Broccoli, cauliflower, cabbage, Brussels sprouts, bok choy; Lignans - Seeds such as flax seeds, sunflower seeds); Monoterpenes - Citrus fruit peels, cherries, nuts; Phytic acid - Whole grains, legumes; Phenols, polyphenols, phenolic compounds (ellagic acid, ferulic acid, tannins) – Grapes, blueberries, strawberries, cherries, grapefruit, cranberries, raspberries, blackberries, tea; Saponins – Beans, legumes.
The body produces its own antioxidant enzymes to keep free radicals under control. However, to produce sufficient quantities of these enzymes the body needs fuel in the form of antioxidant nutrients. Unfortunately, not even a balanced diet guarantees the ideal intake of sufficient antioxidants to keep us healthy.
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