Contrary to popular belief, alcohol does not cause depression, Australian scientists have found.

Researchers also debunk the view that mild to moderate alcohol consumption may reduce the risk of depression.

Until now it was believed that alcohol caused people to become depressed, particularly if consumed at excessive levels, according to Professor Osvaldo Almeida, from The University of Western Australia.

"Even one of the diagnoses we have for depressive disorders - Substance Induced Mood Disorder - is a diagnosis where alcohol plays a role," Almeida said.

"However, because of the observational nature of the association between alcohol and depression, and the risk of confounding and bias that comes with observational studies, it is difficult to be entirely certain that the relationship is causal.

"For example, people who drink too much may also smoke, have poor diets and other diseases that could explain the excess number of people with depression among heavy drinkers," he said.

Almeida and fellow researchers with the Health in Men Study (HIMS), including 12,201 men aged 65-83 when recruited in 1996, decided to search for a causal link via physiological pathways instead: specifically the genetic polymorphism, or mutation, most closely associated with alcohol metabolism.

"We now know that certain genetic variations affect the amount of alcohol people consume. There is one particular genetic variation that affects the enzyme responsible for the metabolism of alcohol," Almeida said.

"This variation produces an enzyme that is up to 80 times less competent at breaking down alcohol. Consequently, people who carry this variation are much less tolerant to alcohol. In fact, there is now evidence that alcohol-related disorders are very uncommon in this group.

"Now, if alcohol causes depression, then a genetic variation that reduces alcohol use and alcohol-related disorders, should reduce the risk of depression.

"The great advantage of looking at the gene is that this association is not confounded by any other factors - people are born like that," he said.

The researchers analysed the triangular association between the genetic mutation, alcohol and depression in 3,873 elderly male participants of the study, using data collected over three to eight years.

"We found (as expected) that this particular genetic variant was associated with reduced alcohol use, but it had no association with depression whatsoever," Almeida said.

"The conclusion is that alcohol use neither causes nor prevents depression in older men. Our results also debunk the view that mild to moderate alcohol consumption may reduce the risk of depression," he added.