Academics have identified 12 subtypes of depression which could help experts better treat the thousands of New Zealanders who seek help each year.

Auckland University PhD candidate Severi Luoto is one of the authors of a new review article in the journal Brain, Behaviour and Immunity which calls for a major overhaul of the way depression is diagnosed and treated, saying the current way was ineffective.

Mental Health Foundation figures showed 14.3 per cent of New Zealand adults were diagnosed with depression at some time in their lives.

Luoto argued depression was not a single disease.

"The evidence that major depressive disorder is a group of separate syndromes comes from the observations that patients not only have many hundreds of unique symptom profiles, but many of the symptoms often have opposite features such as insomnia or hypersomnia, or an increase or decrease in appetite," he said.

The authors observed patterns in literature and used those to classify depressive episodes into 12 subtypes based on evolutionary psychiatry.

They argued the 12 subtypes were induced by:
• Infection, in which sickness behaviour to combat pathogens and parasites may lead to symptoms such as loss of appetite, sleep disturbances, anhedonia, impaired concentration.
• Long-term stress which is known to activate the immune system, causing an increase in pro-inflammatory cytokine levels that influence mood.
• Loneliness.
• Traumatic experience.
• Hierarchy conflict where events such as unemployment, exclusion from a social group, bullying at school or professional hierarchy conflicts may trigger a depressive episode.
• Grief.
• Romantic rejection.
• Postpartum events which lead to depression in 10-15 per cent of women.
• The season, where Seasonal Affective Disorder affects the individual at the same time each year.
• Chemicals such as alcohol and cocaine.
• Somatic diseases such as Alzheimer's, Parkinson's, migraine, epilepsy, stroke and traumatic brain injury.
• Starvation which is known to reduce mood and, when prolonged, can lead to apathy and social withdrawal.

One of the other authors, adjunct professor Markus J. Rantala from the University of Turku in Finland, said the subtypes would make it easier to find more effective treatments for depression.

"This is because the focus will be on treating the underlying reasons of depression instead of merely focusing on the symptoms, which is how traditional psychiatry treats depression.

"We argue that the occurrence of symptoms, or patterns of symptoms, depends on the subtype of the depressive episode. The particular manifestation of depressive symptoms may have more to do with what triggered the depression than the personality of the patient."

The authors said certain depressive states were designed to benefit a person. For example, starvation-induced depression could be an adaptation to save energy to survive through a famine.

But sometimes those responses could turn into clinical depression if they did not serve the purpose they were designed for.

"Chronic clinical depression is what we could call an evolutionary novelty that arises from a mismatch between our current environment and our ancestral environment," Luoto said.

"Some depressive responses to adverse life circumstances can be beneficial to the patient. So understanding the psychological and physiological underpinnings of depression is important and might remove some of the stigma around it.

"Future depression treatments should employ an analysis of symptom patterns together with an in-depth interview and a blood test to reveal inflammation and stress hormone levels."

Rantala said if a depressive episode appeared to be a response to an adverse life event, clinicians should evaluate whether the symptoms were adaptive or whether the depression episode had exacerbated into pathological depression.

The authors believed their system would lead to a focus on patients' long-term mental and physical well-being instead of fixating on the short-term alleviation of symptoms.