Poor nutrition is contributing to the increasing numbers of people suffering mental illness, a large psychiatry conference has been told.

Professor of Clinical Psychology at the University of Canterbury Julia Rucklidge says a well-nourished body and brain is better able to withstand ongoing stress and recover from illness.

She said it's time Australian and New Zealand psychiatrists and psychologists "get serious" about the critical role nutrition plays in mental health.

"Not a single study has shown that a western diet that is heavily processed, high in refined grains, sugary drinks and takeaways and low in in fresh produce is good for us," Rucklidge said.

"The western diet is associated with poor mental health and eating a diet more akin to the Mediterranean diet improves mental health," she said.

For more than a decade Rucklidge has been leading research investigating the role of nutrition in mental health.

A previous paper - led by Rucklidge - published in the British Journal of Psychiatry showed taking macronutrients improved ADHD symptoms, including attention, hyperactivity and impulsivity, compared to participants on a placebo.

Professor Rucklidge will tell the the New Zealand Conference of the Royal Australian and New Zealand College of Psychiatrists in Tauranga on Tuesday that nutrition matters and that optimising nutrition is a safe and viable way to avoid, treat or lessen mental illness.

"Every time we put something in our mouths we can choose to offer ourselves something nutritionally deprived or something nourishing," said Rucklidge.

The Royal Australian and New Zealand College of Psychiatrists (RANZCP) agrees psychiatrists need to think about the "whole person" and the relationship between mind and body, in particular nutrition.

Research has shown people with a severe mental illness die up to twenty five years earlier than those without a serious mental illness, often due to preventable physical health conditions.

They experience much higher rates of cardiovascular disease, diabetes and respiratory conditions.

"Psychiatrists have a key role to play in ensuring that people with mental illness are not further burdened by avoidable chronic physical health conditions," said Dr Kym Jenkins, President of the RANZCP.