Antioxidants: their role in health

Oxygen is essential to life, but as our bodies use oxygen, we generate by-products called free radicals. These can damage healthy cells and are especially likely to attack the fats that provide structure to the membranes surrounding body cells. Free radicals are also produced from exposure to cigarette smoke, excess exposure to the sun, drinking alcohol, and from exposure to large amounts of heavy metals. Antioxidants neutralise the effects of free radicals.

Great publicity given to the anti-ageing effects of antioxidants has increased sales of antioxidant supplements and cosmetics, but their true role is much more than skin deep. Antioxidants function in many body systems and it’s important to sort out the genuine research from the marketing hype.

Why are antioxidants in food important?

The body produces a range of its own protective antioxidants. Those in foods may boost the body's own supply and these may become especially important as we age and produce more free radicals.

Plants produce hundreds of antioxidants for their own protection. Many that are also useful to us are present in vegetables, fruits, herbs and spices, nuts and wholegrains. Tea, coffee, extra virgin olive oil, red wine and dark bitter chocolate are also rich in antioxidants.

Antioxidant action is also part of the role of vitamins C, E, folate and beta carotene and also the minerals selenium, manganese, copper and zinc. Much of the marketing of antioxidants concentrates on these vitamins and selenium, but the complex array of antioxidants present naturally in plants may be more important.

Antioxidants produce the bright colours in fruits and vegetables and the flavour of extra virgin olive oil, tea or coffee. Bitter compounds in foods such as rocket are also potent antioxidants.

Claims that a particular food is the 'richest source of antioxidants' can be confusing as the claim may depend on the type of measurement used or refer to a particular antioxidant, such as lycopene in tomatoes, and does not necessarily consider how well particular antioxidants can be absorbed. The variety and combination of antioxidants in their natural food sources may also be important as isolated antioxidants may not function in the same beneficial way.

Can antioxidants prevent disease?

Research into antioxidants is in its infancy. Epidemiological studies (studies of large populations that try to link disease in that population with a cause) show that a diet rich in foods with high levels of antioxidants is associated with longevity and good health. The antioxidant properties of vitamins are well established and evidence from laboratory studies indicates that particular antioxidants may have specific roles in disease prevention. Clinical trials are underway, although those using antioxidant vitamins have not shown expected results.

Cardiovascular disease.

There is widespread agreement that a diet high in fruits and vegetables reduces the risk of cardiovascular disease and many types of cancer. Growing evidence also shows that wholegrains offer protective effects against both types of disease, although whether this is specifically due to their antioxidant capacity is less clear. Nuts are also protective against cardiovascular disease, but whether this is due to their fat, minerals or antioxidants – or a combination of these – is unclear. Research on green and black tea looks promising.

With red wine, there is good evidence for protection against cardiovascular disease, but benefits are wiped out if intake is high. It is still unclear how much of the benefit comes from alcohol’s effect of raising HDL (so-called ‘good’) cholesterol and how much the antioxidant compound, resveratrol, is responsible. The evidence is insufficient to suggest that non-drinkers should start drinking.

Much research is attempting to elucidate the potential benefits of dark chocolate, although researchers agree that fruit and vegetables are a wiser dietary choice, since their antioxidants come without chocolate's high load of fat and kilojoules.

Cancer.

With a huge volume of research currently examining possible protective roles against cancer of various foods, including spices, herbs and tea, more specific advice lies in store. Currently, cancer and cardiovascular experts do not think there is evidence to support antioxidant supplements, but all recommend a high intake of fruits and vegetables. There is concern about possible interactions between high doses of some antioxidant supplements and chemotherapy drugs that work by using free radicals to kill cancerous cells.

Macular degeneration.

Some positive messages have come from studies of particular antioxidants in macular degeneration, the major cause of blindness in elderly people. Some (but not all) studies show that specific antioxidant supplements help protect against further degeneration, while others back greater benefits from vegetables and fruits rich in the antioxidants lutein and zeaxanthin. These dietary components may decrease the risk of developing macular degeneration.

Immune system.

Frequent claims suggest that antioxidants benefit the immune system. In theory, that sounds valid, but specific evidence is currently lacking.

Anti-ageing.

For many people, the greatest interest is in antioxidant's anti-ageing potential. Since the body's production of its own antioxidants decreases in old age, few doubt the potential value of dietary sources. However, there is no evidence that extra antioxidants stop hair greying, prevent wrinkles or provide a ‘fountain of youth’.

Should people take high doses of antioxidants?

Antioxidants can act in different ways, depending on the dose and the environment in which they are operating. Laboratory studies show that some antioxidants (including minerals the body uses to produce its own antioxidants) can become pro-oxidants at high doses which could potentially damage DNA.

The best evidence that antioxidant supplements are of value involves their role in decreasing the risk of macular degeneration.

In most areas of health, experts agree there is insufficient evidence to support antioxidant supplements, but more than enough studies to back recommendations to increase our antioxidant intake by eating more fruit, vegetables and wholegrains.

Dr Rosemary Stanton, Nutritionist, Visiting Fellow, Faculty of Medicine, University of New South Wales.

myDr, 2006
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Last Reviewed: 15 December 2006