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Depression may be the result of brain inflammation, a new study has suggested.

Psychiatrists have found that the brains of patients suffering from depressive episodes may be significantly inflamed.

They found that a protein known to be a marker of inflammation was up to a third higher in the brains of depressed patients compared to healthy ones.

Excessive inflammation in the brain may interfere with the way neurons (above) communicate with each other

Those with the most severe forms of depression also had the most inflammation.

The finding may help to explain why depression can cause physical symptoms such as loss of appetite and poor sleep.

It could also lead to new ways of treating the mental health condition, which can affect up to a fifth of adults at some point in their lives.

Dr Jeffrey Meyer, one of the researchers behind the study from the Centre for Addiction and Mental Health in Toronto, Canada, said: 'This finding provides the most compelling evidence to date of brain inflammation during a major depressive episode.

'This discovery has important implications for developing new treatments for a significant group of people who suffer from depression.

'It provides a potential new target to either reverse the brain inflammation or shift to a more positive repair role, with the idea that it would alleviate symptoms.'

Depression is one of the most common forms of mental illness and is estimated to affect around one in five adults in the UK at some point in their lives.

An estimated 14 million American adults suffer from major depressive disorder.

Dr Meyer and his colleagues, whose work is published in JAMA Psychiatry, conducted brain scans on 20 patients with depression and 20 healthy control participants.

Using a technique called positron emission tomography, they were able to measure the levels of a protein that is associated with inflammation, called translocator protein.

These PET scans above show how depression reduces the activity of neurons in patients (right) compared to healthy people who are not suffering from the condition (left). It is thought that inflammation may play a role

Dr Meyer, pictured above, scanned the brains of 20 depressed patients and 20 healthy individuals in the study

They found that in patients suffering from major depression episodes, the levels of this protein were 26 per cent higher in the prefrontal cortex of the brain, 32 per cent higher in the anterior cingulate cortex and 33 per cent higher in the insula area of the brain.

The researchers say that the inflammation in the anterior cingulate cortex appeared to be most linked with the severity of the depression.

It is thought that this inflammation may impact on the activity of the neurons in the brain and the way they communicate with each other. 

Although the researchers were not able to determine from their study whether the inflammation was the direct cause of the depression or a result of it, other studies have shown that inflammation is linked to many of the symptoms of depression - such as low mood, loss of appetite and inability to sleep.

Depression can lead to physical symptoms including loss of sleep, loss of appetite and low mood

Inflammation itself is one of the body's key ways of protecting itself, but excessive inflammation is also known to be harmful for many tissues.

Current treatments for depression do not target inflammation.

Dr Meyer said: "Depression is a complex illness and we know that it takes more than one biological change to tip someone into an episode.

'But we now believe that inflammation in the brain is one of these changes and that's an important step forward.

'Previous studies have looked at markers of inflammation in blood, but this is the first definitive evidence found in the brain.'