Jearelle Wolhuter investigates the connection between diet and mental health

can you imagine Monday without your morning cup of coffee? Or facing an uncomfortable party without a glass of wine? Could you get through the end of a relationship without chocolate?

Eating is not just about surviving. We eat for enjoyment or as part of our social rituals. Most of us also accept that we are what we eat in a physical sense - we agree that eating a balanced diet will improve our health and longevity and certain foods can relieve or aggravate illnesses ranging from diabetes to heart disease. But we also use food and drink to influence our moods. And while most of us would easily agree that chocolate, alcohol or caffeine can be a short term pick-me-up, the evidence seems to suggest that what we eat can affect our mood and behaviour in a much more fundamental way.

Depression is a common mental illness that is estimated to affect up to one in five Britons during their lifetime. The total cost to the UK, in terms of medication, benefits and lost working days is around £8 billion a year. Globally more working days are lost through depression than any other illness. But before you reach for your anti-depressants, you might want to take a look at your diet. According to Joseph Hibbeln, a psychiatrist and biochemist from the National Institutes of Health near Washington DC, we can eat ourselves happy by consuming the right kind of fats. Hibbeln knew that fats in the brain could easily be altered by diet and started to wonder if the composition of brain fat might influence mental health. "One day I was standing in a lab, holding a human brain in my hand, and it suddenly hit me - the brain is all fat. There is no difference between a stick of butter and your brain."

While certain fats can predispose us to heart disease and cancer, Hibbeln believes we are not eating enough a type of polyunsaturated fatty acid called omega-3, found in foods such as oily fish, walnuts and olive oil. The exact mechanism is unclear, but researchers in many different studies have shown that giving patients high doses of fish oil can relieve symptoms of depression, even when they are unresponsive to drugs. Hibbeln has also found that countries with a diet rich in fish have lower levels of depression, and vice versa.

Relieving depression is one thing, but can we change criminal behaviour just by eating right? It sounds simplistic but research funded by Natural Justice, a charity that researches criminal behaviour, seems to show just that. At a young offender's institution in Aylesbury, Buckinghamshire, the behaviour of 231 violent young offenders was monitored after half were given daily supplements of vitamins, minerals and fatty acids and half were given placebos. The tests were double blind so neither guards nor inmates knew who received real pills and who received dummies. The results were remarkable - prisoners on the nutrient pills committed 37 per cent fewer serious or violent offences than the placebo group. When the trial finished, levels of violence returned to normal.

There are some problems with both the findings on criminal behaviour and depression. Plenty of badly behaved children and criminals eat a perfectly balanced diet. It is also difficult to measure something as subjective as mood or bad behaviour, versus something as concrete as insulin levels or blood pressure, since you have to rely on the participants to tell you how they feel, or how badly someone else is behaving. People have different ideas of what constitutes depression or violent behaviour.

But as Stephen Schoenthaler, a criminologist at California State University in Long Beach points out, most crimes are committed by a small, underprivileged section of society, and it is these people who are most likely to be malnourished. So even if we can improve the behaviour of only a fifth of all offenders (he found a change in diet affected only about 20 per cent of offenders in his work), then you could significantly reduce crime.

There are cost and ethical implications. Supplementing a prisoner's diet costs about £1 a day, but Britain's prisons budget currently allows for just £1.49 a day on food for a young offender. Forcing prisoners to take supplements might seem benign, but is there a real moral difference between handing out vitamins and handing out drugs such as Prozac?

Many researchers believe nothing has been proven yet, but these findings have to be encouraging. Even if it is never conclusively proven that supplements can alter mood or improve behaviour, it has also not done any damage. Even if eating a balanced diet rich in fruit, vegetables, and oily fish as the Food and Mood Project (supported by mental health charity Mind) suggests doesn't improve your behaviour, it will certainly improve your health. It is relatively easy, relatively cheap and has no side-effects. Eating a balanced diet may not make you happier, but at least you will lead a long and healthy life of misery.