Antioxidants

The use of oxygen in the body's normal processes creates chemicals known as free radicals. These have unpaired electrons and so they try to steal them from other molecules. These attacks damage the body's cells - a process called oxidation. In much the same way that air turns a cut apple brown or rusts a nail or makes butter go rancid, so oxidation damages the cell membranes, genetic material in cells (DNA), fatty acids and other body structures. 

Free radicals can:

1. Affect the rate at which we age
2. Start cancers by damaging the DNA in cells
3. Increase heart disease by making LDL cholesterol more likely to stick to artery walls
4. Produce cataracts and encourage degeneration of the lens of the eye that ultimately leads to blindness. 
5. Contribute to inflammation of the joints, as in arthritis
6. Damage brain cells, promoting neurological conditions such as Parkinson's or Alzheimer's disease

They appear to have a greater protective effect against atherosclerosis than against cancer. In the Nurses’ Health Study there was a 34% reduction of Coronary Heart Disease (CHD) in women whose vitamin E intake was in the top quintile. Potential mechanisms for protecting against CHD include: 

1. Prevention of lipoprotein oxidation 
2. Possible restoration of normal endothelial function in people with early atherosclerosis 
3. Possible protection of the myocardium from ischaemic damage.

Antioxidants (AO) come to the rescue and neutralise free radicals. Although the body produces its own antioxidants to deal with free radicals produced each day as part of normal oxidation in the cells, an overload - caused by smoking, alcohol excess, exercise excess, pollution, radiation from the sun or x-rays - may leave the body's system unable to cope. That is when you need plenty of AO in your diet. 

Early research centred on the antioxidant vitamins A, C and E also known as the ‘ACE’ vitamins and minerals such as selenium, copper and zinc. But in the last few years, researchers have discovered many, many more naturally occurring anti-oxidants which are not strictly nutrients - known as phytochemicals (from plants) and zoochemicals (from animals). These are more potent antioxidants than vitamins and minerals. 

Where to find antioxidant vitamins, minerals, phytochemicals and zoochemicals:

The antioxidant vitamins and phytochemicals act as free radical scavengers, mopping up these highly reactive species and neutralising them. 

The antioxidant minerals are components of the primary antioxidant enzymes (e.g. copper as part of superoxide dismutases) which reduce the production of free radicals in the cell.

The best sources are vegetables, fruit, tea and wine. Variety is essential – yellow, orange, green, red, brown and blue-purple plant foods provide a variety of antioxidants, and the more brightly coloured, the richer in anti-oxidants. 

Tufts University has developed a test (oxygen radical absorbance capacity) which measures the ability of the blood, or any other substance such as food to subdue free radicals. They have ranked some foods in order of their levels of AO, from highest to lowest: prunes, raisins, blueberries, blackberries, kale, strawberries, spinach.

Carotenoids - there are over 600, of which about 50 can be present in our diet; they include:

b-carotene inhibits the early stages of tumour development and improves immune function (orange/yellow fruits & vegies like carrots, pumpkin, yellow squash, rockmelons, apricots, mangoes, pawpaw; dark green leafy vegies like spinach, parsley, endive);

lycopene reduces the risk of prostate cancer (tomatoes, watermelon, pink grapefruit); 

lutein protects the macula of the eye from degeneration (green leafy vegetables like spinach and yellow vegetables like squash); 

cryptoxanthins (mangos, pawpaw, red capsicum, pumpkin).

Flavonoids (quercetin, kaempferol) - minimise oxidation of LDL-cholesterol and decrease tendency for blood clots (tea, especially green tea, berries, lemons, limes, oranges, apples, fruit juice and skin, apples, vegetables, onions, red wine, red grape juice); resveratrol found in grape skin (which is used to make red wine but not white wine) can assist in inhibiting tumour growth at three different stages of cancer, preventing the spread of malignant cells.

Isoflavonoids – apart from being phytoestrogens which help to relieve hot flushes and other menopausal symptoms and possibly prevent osteoporosis, they are also antioxidants and may help to reduce risk of breast and prostate cancer (soy beans, other beans, tofu, soy milk, lentils, peas)

Lignans – are similar to isoflavonoids (flaxseeds, sesame seeds, bran, whole grains, beans, vegetables)

Anthocyanins - mild anti-bacterial effect (berries, grapes, eggplant)

Polyphenols e.g. rosmarinic acid – oregano, thyme

Catechins – may help protect against heart disease, skin cancer and stomach cancer (tea, red wine).

Indoles – trigger the release of anti-cancer enzymes; reduce tumour development (broccoli, cabbage, cauliflower, mustard)

Allium sulphur compounds – help the liver to detoxify/neutralise carcinogens, may reduce the risk of colon cancer and are anti-bacterial (garlic, onions, leeks)

Zoochemicals – glutathione (meat), ubiquinone (coenzyme Q10, meat, meat organs, fish)

Vitamin C – Inhibits nitrosamine formation; reduces cancers of the digestive tract and regenerates vitamin E (high in oranges, guava, mango, blackcurrants, strawberries, grapefruit, kiwi, potatoes, peas, capsicum, parsley, broccoli, spinach, pineapple and cabbage).

Tocopherols (vitamin E) & tocotrienols – protect polyunsaturated fats and b-carotene from being broken down/oxidised; help maintain stability of fats in cell membranes (found in vegetable oils, mayonnaise, wheatgerm, nuts, seeds, wholegrains and is added to fish oil capsules and polyunsaturated margarines to keep them stable. Red palm oil is high in tocotrienols.

Selenium - part of the enzyme glutathione peroxidase which reduces production of free radicals; enhances the immune response and affects DNA repair which can prevent the development and growth of cancer; works in combination with vitamins C and E (found in brazil nuts, seafood, liver, kidney, lean meat and whole grains, although depends on the soils where they are grown). 

Copper (cocoa, wheat bran, yeast), manganese and zinc (seafood e.g. oysters, lean meat, chicken, milk, wholegrains, legumes, nuts) are also components of antioxidant enzymes e.g. (e.g. copper as part of superoxide dismutases)

Despite these impressive defences, significant oxidative damage can occur if the system is overtaxed. This may result from acute exposure to an oxidising influence or may occur at a chronic level if the dietary intake of antioxidants is sub-optimal. Unless controlled, free radicals may damage components of our cells and initiate the processes which lead to chronic disease. 

In addition, cell damage per se generally leads to a release of free iron which catalyses the production of free radicals, thus amplifying the damage. 

Evidence

Free radical damage has been implicated in a growing number of acute (exercise excess, sunburn, radiation injury, oxidising pollutants) and chronic clinical conditions (ageing, arthritis, atherosclerosis, cancer, inflammation, cataract formation, inflammation, iron overload, macular degeneration). 

Evidence of free radical damage in the aetiology of atherosclerosis, cancer and ocular cataract has emerged persuasively. 

There is circumstantial evidence that dietary antioxidants, especially vitamin E and flavonoids protect against CHD mortality. A large study from Finland has recently reported less than half the rate of CHD death among people who ate the most flavonoids (Knekt et al. BMJ 1996; 312: 478-81). Other polyphenols e.g those in extra virgin olive oil, tea and red wine have also been proposed as protecting against CHD because of their proven antioxidant potential (Renaud et al. Lancet 1992; 339; 1523-26) but proof through either prospective clinical trials or clear-cut observational studies is still lacking. The preventive potential of AO with respect to these conditions have been based on retrospective studies; very few have been prospective intervention trials, except for vitamin E and b-carotene trials. 

In summary:

Foods rich in vitamin E and flavonoids are recommended components of a healthy diet. These foods include vegetables, fruits, unsaturated and monounsaturated fats (especially extra virgin olive oils), nuts, soybeans, tea and grape products. Possible benefits of supplemental antioxidants are limited to vitamin E on current evidence, although even in this case the evidence is equivocal. 

Existing evidence suggests that intakes of antioxidant vitamins may need to be rather higher than current recommended dietary intakes (RDIs) – the protective effects of a diet high in fruit and vegetables occurs at dietary levels of nutrient antioxidants in the 2-3 times RDI range. 

However, because of the complexity of the antioxidant system, it may be that optimal benefit from antioxidant supplementation will only be obtained from a mixture of these substances (Dreosti. APJCN 1993; 2: 21-25). Ideally, the best way to get them is from food – this will ensure you are also consuming other unidentified but beneficial compounds.

See also an interesting article on antioxidant supplements in Choice Magazine (on-line) published in 2000.

Last Updated: March 2002.