The results show that cholesterol tests could be used to back up a diagnosis of depression, say the researchers.

They believe depression may lower blood cholesterol levels, leading to a chemical reaction which allows aggressive instincts free rein.

A recent US study of women showed similar links between low cholesterol levels and depression.

It suggested boosting cholesterol levels could improve mental health.

The eight-year Finnish study of 29,133 men aged 50 to 69, published in the British Journal of Psychiatry, found that those reporting depression had significantly lower average blood cholesterol levels than those who did not.

This was despite the fact that they ate a similar diet.

Blood tests

The researchers analysed the men's total blood cholesterol levels, including a type of cholesterol associated with heart disease and another thought to protect the heart.

They asked the men to report any history of depression and checked medical records for treatment for a depressive illness.

Some 15% reported having been depressed in the four months before the study began and 22% said they felt anxious.

The researchers followed the men up five to eight years later.

They found that 280 had been hospitalised for major depression and 111 had committed suicide.

Oily fish contains 'good' cholesterol which could improve mental health

There was "a significant association" in both cases with low blood cholesterol levels.

The scientists, led by Professor T. Partonen from the National Public Health Institute in Helsinki, also noted a link beween non-suicidal violent deaths and low blood cholesterol levels.

They say previous research suggests a relationship between low levels of blood cholesterol and a fall in levels of the neurotransmitter serotonin in the brain.

Serotonin is thought to control moods and low levels have been linked to depression and aggression.

Another finding was that high levels of HDL cholesterol, the so-called healthy cholesterol found in food such as oily fish, were linked to risk of death from suicide.

They believe this is because high HDL cholesterol is associated with increased alcohol intake, a suicide risk, and that this, rather than HDL levels, accounts for the finding.