THE link between a healthy mind and body is not a new one. More than 400 years BC, Hippocrates, the Greek philosopher and founding father of medicine, recommended exercise as a treatment for mental illness. He described walking as "man's best medicine". Several centuries later, William James, the physiologist, philosopher and founder of American psychology, wrote: "Muscular vigour will always be needed to furnish the background of sanity, serenity and cheerfulness to life, to give moral elasticity to our disposition to round off the wiry edge of our fretfulness, and make us good-humoured and easy of approach."

You do not have to be a medical expert or a philosopher to recognise and appreciate the physical and emotional benefits derived from being physically active. Reflect on your childhood or your children for a moment and recall the pleasure and happiness you felt when you were active - crawling, walking, hopping, jumping, running and dancing. Physical activity or exercise never fails to have a positive impact when done safely.

So exercise is good for body and mind, but just how good remains a question currently being investigated by academics throughout the world. The importance of this subject is made even more pertinent given the alarming rise in the numbers of people using anti-depressants. Such is the growth in usage, that the World Health Organisation predicts that in 14 years, depression will create the greatest burden of illness. Closer to home, in Scotland, every third or fourth person seen by their GP has a mental health problem, according to latest Scottish Executive figures. Moreover, last year, £55m was spent on anti-depressant drugs north of the border, which means Scots are now using 40% more anti-depressants per head of population than the rest of the UK.

How we have arrived at this situation is another matter entirely, but how society tackles the fallout is a question that Professor Marie Donaghy, head of the school of health sciences at Queen Margaret University, Edinburgh, and others, are currently considering.

"My interest in the mind-body link' began in 1979, when I worked as a physiotherapist with adolescent and adult patients who had learning disabilities and had been living in institutional care since birth," she says. "My physiotherapy colleagues and I learned how to develop their confidence and self-esteem through structured exercise activities. These included developing skills in balance and strength in the gym, cycling, swimming and horse-riding. Exercise brought them pleasure, laughter and a sense of achievement.

"In 1986, I moved into psychiatry and began using exercise as an adjunct to treatment with people suffering from anxiety and depression," adds Donaghy, Scotland's leading expert in the exercise-psychology field.

How exercise actually imp- roves mental health can be explained through a number of physiological and psychological explanations.

At a physical level, the increased blood flow to the brain stimulates the release of naturally occurring mood-enhancing chemicals known as endorphins. These natural opiates are similar to morphine and have been linked to the "runner's high".

Also, studies have found that chemicals known to be depleted during depression - norepinephrine, dopamine and serotonin - are released during exercise. These neurotrans- mitters have been associated with elevating mood, which may partly explain why exercise offers a degree of protection from depression and is effective as a treatment intervention (anti-depressant medication such as Prozac works by boosting these chemicals).

Exercise is also known to increase levels of brain-derived neurotrophic factor (BDNF), a substance associated with enhancing mood and helping the brain cells survive longer. This may also be linked to improved cognitive function. It has been suggested that increased levels of phenylethylamine, a known stimulant in the brain which occurs during exercise, is linked to the release of dopamine and endorphins which act as a natural anti-depressant.

Exercise has an important impact on an individual's psychology. According to a National Association for Mental Health (Mind) survey in 2001, 50% of participants said that exercise boosted their self-esteem. Mind's findings are supported by the wider body of psychological research which links exercise and self perceptions such as body image, physical self-worth and self-esteem.

"The planning and undertaking of exercise builds people's self-confidence because they've planned and achieved something," says Donaghy. "It may also provide social support if they exercise with others. The anxiety-reduction effects of exercise have been linked to improved cardiovascular fitness, reducing reactivity to, and recovery from, psychosocial stressors.

"I believe the brain can be changed through experience. The human genome the totality of our chromosomes cannot account for the entire structure of the brain, but it helps set the circuits in the older part of the it. It is this part of the brain which is pre-set for survival - it ensures we continue breathing, have a regular heartbeat and a balanced metabolism. Although pre-set, these circuits can adapt with experience so we can adjust to different environments across our lifespan.

"Studies have shown that we have reward circuits' in the brain linked to the limbic system older part of brain. This means memories associated with rewarding experiences release dopamine telling the brain to do it again. Memories associated with rewarding experiences are then laid down in the cerebral cortex and are triggered by certain cues, which stimulate the recurrence of behaviour," explains Donaghy.

IT WAS not until the early 1990s that such explanations from exercise psychologists emerged. The first significant text to be published in the UK came in 1991. Written by Stuart Biddle and Nanette Mutrie, the textbook was the first to pull together the scientific evidence of the benefits of exercise in relation to mental wellbeing. It incl- uded evidence for exercise in the treatment of depression and anxiety, and highlighted the early evidence of exercise in the treatment of addiction.

More recently, a number of studies carried out throughout the UK, Europe and the US have provided evidence for the premise that exercise can improve mental health. These studies have looked into how exercise can be used in the intervention and prevention of mental illnesses such as depression and anxiety, as well as drug and alcohol addiction.

A prominent American study measured exercise and depression and showed that people who are physically active and exercise regularly are less likely to be diagnosed with depression. Six years ago, researchers at Duke University also found that a modest exercise programme of 16 weeks (that is three 30-minutes sessions of physical activity per week) was as effective as medication in the treatment of people with major depression.

The most current, well- funded investigation is being led by Professor Glyn Lewis at the University of Bristol, and will report in 2011 on the effects of exercise on people with depression.

"Our main aim in research now is to convince the medical profession, in particular GPs, to get on board with us in this," says Donaghy.

"I think there is enough evidence for GPs to start prescribing exercise either as a treatment on its own or as an adjunct to anti-depressants and cognitive behavioural therapy (CBT). Some doctors are taking on the message, but others are still speculative. However, the literature, particularly in regard to depression, is well on its way to being fully developed. When you consider all the other health benefits of exercise, other than it being good for the mind, any risks are outweighed.

"We need to increase the exercise referral scheme and provide training in cognitive behavioural therapy techniques for physiotherapists, occupational therapists and fitness trainers so that they are able to help patients to start exercising, stay motivated and keep on going."