As our awareness of healthy eating has grown so, too, has our consumption of dietary supplements. In fact, 42 per cent of us take some form of vitamin or herbal preparation. In the US, a survey developed with the US Food and Drug Administration (FDA) estimated that 158 million consumers spend approximately $8.5 billion (£4.5 billion) per year on dietary supplements.

The over-the-counter market in the UK is currently worth about £277 million a year, excluding sales from health-food shops and mail order. While there is little evidence that taking supplements actually works — and most nutritionists agree that it is healthier to get your vitamins and minerals from food — the market continues to grow.

Women are the biggest consumers of vitamins, minerals and herbal supplements and, as such, are the prime targets for advertising. However, most women don’t seek advice from a nutritionist or doctor before taking supplements. They self-diagnose, and are often unaware that boosting consumption of one nutrient can interfere with others. For example, iron supplements can impede the absorption of zinc and vitamin E; taking high levels of zinc may mean you also need to take copper; and calcium supplements should be taken with magnesium, as they work together.

The Department of Health says that supplements should be taken only after medical advice has been sought because long-term use of some substances — particularly vitamin B6, beta-carotene, niacin, zinc, manganese and phosphorus — is thought to pose a serious health risk.

SOME WOMEN DO NEED SUPPLEMENTS

Doctors recommend that women who are trying to get pregnant, or who are pregnant, should take folate, and iron is prescribed for anaemia. Monopausal women may be advised to take calcium and women over 65 can sometimes need vitamin D too.

Doctors will also consider recommending supplements to women who have a disease of the liver, gall bladder, intestine or pancreas; who have had surgery on their digestive tract; who have a poor diet or are on a very low calorie diet; who suffer from severe food allergies; are vegetarian or vegan; are on large amounts of antibiotics, laxatives or diuretics that interfere with nutrient absorption; who smoke or drink alcohol excessively; or who don't get 15 minutes of sun each day on their skin.

Although there is a multitude of products on the market, labelling restrictions mean that vitamins and minerals can't make health claims, so it is often quite difficult to work out what they are supposed to treat. Many women simply buy on impulse, and vitamin positioning in pharmacies is indicative of this. Vitamins are usually placed near the till, so a woman who feels under the weather will often pick up a bottle on impulse. In fact, any woman who eats a balanced diet is unlikely to need supplements so, since it is impossible to overdose on vitamins and minerals absorbed from food, it makes sense to source the nutrients we need from shat we eat.

SUPPLEMENTS THAT WORK

Some food supplements, such as evening primrose oil, are well tested and have proven benefits. Others remain a mystery to nutritionists and consumers alike. Because supplements are sold as food, manufacturers are not obliged to pay for large clinical trails, so scientific evidence of their effectiveness is scarce. This means that they can’t claim to cure anything specific, but it also means that they are not subject to the rigorous medical testing that drugs have to go through before they are allowed on to the market, Whether they work or not is one issue; what goes into them is another. Without proper regulation, manufacturers can sell and promote products containing substances that may be toxic. The British Dietetic Association believes that supplements should come under tighter regulation and be classed as medicines, not foods. This would oblige a manufacturer legally to prove that its products were effective. Preparations would also have to contain exactly the same dose or ingredient. At present the upper limits vary.

Although supplements have the potential to do good, and many of the more popular herbs have been used in traditional medicine for centuries, they also have the potential to harm, so you should consult a doctor before taking any of them. For example, St John’s wort can interfere with the effectiveness of antidepressants and the contraceptive pill; evening primrose oil can provoke epileptic attacks in susceptible patients; and echinacea may contribute to a range of allergic reactions. Other supplements containing iron, manganese, silver and gold (which can be found in certain Chinese preparations) can lead to renal failure, and ginkgo biloba has been linked to excess bleeding. Listed below are the ones that do work:

• Coenzyme Q10 Our bodies make this enzyme naturally and it is sourced from fish and meat. It is thought to strengthen the heart, increase exercise tolerance, keep gums healthy and act as an antioxidant (but hasn’t been tested).

• Valerian. Proven effective as a natural cure for insomnia and stress.

  SUPPLEMENTS THAT DON'T WORK

• In Body&Soul on Saturday: Suzi Godson on looking after your skin

  Are dietary supplements a waste of money? E-mail debate@thetimes.co.uk

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