In the UK, the number of prescriptions for antidepressants has more than doubled in 10 years, with 80% of GPs admitting they overprescribe drugs such as Prozac and Seroxat because of the lack of alternative forms of treatment. But though they might not be available on the NHS just yet, alternatives are starting to emerge - and with some promising results.

Later this month Dr David Servan-Schreiber, a clinical professor of psychiatry and founding member of Medecins Sans Frontières in the US, will visit Britain to launch his book, Healing Without Freud or Prozac, which has already received an enthusiastic response in Belgium, Switzerland and Canada, where it has sold half a million copies. After a career in conventional medicine, Servan-Schreiber's theory is that exercise can be as effective in treating depression and stress as antidepressants. "It is not that I am against antidepressants," he says. "But there are some natural methods of treatment that have been demonstrated to work for milder forms of depression. It doesn't make any sense to ignore them any longer."

In this country, meanwhile, there is a growing interest in an orthomolecular, or nutritional approach to mental health problems. Food as medicine is not a revelatory proposition - it was the hobbyhorse of Hippocrates, the father of western medicine.

But there is a lot more to the science of nutritional therapy than adding the odd grilled fish and dollop of spinach to your shopping list. Differences from those affected by manic depression, schizophrenia, autism or attention deficit and hyperactivity disorder (ADHD) are only achieved by giving patients large amounts of precise nutrients to alter their brain biochemistry.

All psychiatric drugs work to rebalance out-of-kilter brain chemistry. But the success rates achieved by providing the brain with the same molecules it uses to build brain cells and neurotransmitters are eye-opening - hence the well-publicised success of the essential fatty acid omega 3.

Twenty per cent of the brain is made up of essential fats. These fats are missing from the modern diet, and the body can't manufacture them itself. Each of our 100 billion brain cells links up to 20,000 others, and when essential fats are in short supply, the link-ups become difficult. The potential consequences are manifold: our mood, concentration, memory and intelligence can all suffer. So it is unsurprising that essential fats have been proven to help with numerous brain-based afflictions.

While 40% of patients who walk into doctor's surgeries are suffering from a mental health problem, an astonishing two-thirds of GPs have had no mental health training. As a result, most GPs do not even consider their patients' nutritional status. Professor André Tylee is chairman of the National Institute for Mental Health and is responsible for educating all British GPs in the treatment of mental health. He is passionate about the benefits of nutritional therapy and describes it as "the breakthrough we've been waiting for". He is hoping to ensure that nutritional approaches will become the first step that doctors use to defeat mental illness.

Tylee is working with Patrick Holford, founder of the Institute of Optimum Nutrition and author of the bestseller Optimum Nutrition for the Mind. Holford's latest enterprise is The Brain Bio Centre, which is dedicated to helping patients recover from all forms of brain-centred illness, from depression to Alzheimer's, using nutritional therapy. Tylee is anxious to introduce the clinic's approaches to the NHS and to conduct a clinical study that confirms their anecdotal success rate of 80%. The clinic defines success as freedom from symptoms, the ability to socialise with friends and family, and the paying of income tax.

According to Holford, a nutritionist and psychologist, nine out 10 people eat less than the recommended daily amounts of our 39 essential nutrients. "They're not called essential for nothing," he says. When this is combined with other factors such as high homocysteine levels, which leave one twice as likely to succumb to depression, blood-sugar and neurotransmitter imbalances, it is hardly startling that people's brain chemistry goes awry.

James Maclean, 21, is a typical Brain Bio Centre success story. James developed manic depression in his second year at university. He was given antipsychotics and antidepressants and received guidance from an NHS psychiatric nurse who "normally just spoke about his own family".

"I had to leave university," says James. "I hated taking so much medication - it made me put on five stone." When he started to believe that the radio was speaking to him, he knew he needed further help. "Within a space of 10 days I was sectioned, then released, then detained again three times." He was eventually hospitalised for two months. He calls the mental hospital the most soulless place on earth.

On his release, his sister-in-law suggested he see a nutritionist. After an internet search which yielded only a quack who wanted $10,000 for the first consultation, James eventually tracked down the Brain Bio Centre. After tests he was found to be very low in minerals and to be yeast- and gluten-intolerant; he was also suffering from blood-sugar problems. He changed his diet and took a comprehensive range of supplements, including high doses of niacin and essential fats. "The difference was startling," he says. "I feel sharper than before and I'm now supermotivated. I have lost three stone in the last two months and I feel whole again." James has reapplied to read sports science at university and hopes to play rugby professionally.

Holford may be regarded as being outside the mainstream, but increasingly his approach is being fostered in conventional medicine. Many respected scientists and physicians are reporting unprecedented success with the orthomolecular approach, so named by the American chemist Linus Pauling, who died in 1994. In the UK, Malcolm Peet, an NHS psychiatric consultant, recently led separate studies with schizophrenics and depressives who were failing to respond to drugs. Both studies concluded that the essential fat, EPA, is effective. Earlier this year, Dr Basant Puri, a consultant psychiatrist at Imperial College School of Medicine, published an entire book dedicated to explaining why EPA is so good at treating depression.

In the US, Dr Mary Megson, a fellow of the American Academy of Paediatrics, has treated over 2,000 children for autism and uses under a teaspoon of cod liver oil every day. The majority of subjects come out of the autistic spectrum within six months - some within weeks, she says. She has seen children making eye contact for the first time in their lives after just three days of treatment.

Similarly, Dr Alex Richardson, a senior research fellow at Oxford University, recently published a study showing remarkable turnarounds in concentration, attention and disruptive behaviour after combining the fish oil EPA with omega 6 to treat dyslexics and ADHD sufferers.

"We're not talking about easily labelled disease entities," says Richardson. "We are simply saying there is a frightening epidemic of children falling into categories where extra help and special education is needed. One in four children are now affected by underlying problems that stop them achieving their potential." Richardson, director of a new charity, Food and Behaviour Research, is anxious to emphasise that a single nutrient cannot work in isolation. "You can't carry a single nutritional supplement around as a talisman for superhealth, because usually specific vitamins, minerals or enzymes are necessary to ensure the key nutrient is absorbed."