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Prof Jim Mann, of the University of Otago Dunedin School of Medicine, said the research was "unquestionably" a wake-up call for the New Zealand Government to do more in the way of lifestyle management support, given our obesity epidemic, and growing problems with adult-onset diabetes.
"An improved approach to the management of mental illness and enhancing and hopefully achieving equity in health outcomes" were clearly stated Government priorities in health care.
Current guidelines suggested cardiovascular risk assessment - which included screening for diabetes - should be carried out at a younger age in those with mental illness, and others considered at a high risk.
New Zealand had achieved considerable health benefits by reducing smoking.
However, the facilities available for lifestyle management - including dietary and physical activity advice and support, which were "the cornerstone of preventive treatment", along with smoking cessation - were "limited, if not non-existent" in most of the country.
"These neglected components of personal and public health would seem to be good starting points for a national response to the alarming message of the Lancet Psychiatry Commission's report, as well as demonstrating a commitment to reducing inequities in health outcomes more generally," he said.
"The finding that life expectancy in those with a mental illness may be reduced by 20 years when compared with the general population should be of enormous concern to Government, all agencies and individuals involved in the health care of those with mental illness and indeed all New Zealanders," Prof Mann said.
The "novel and immensely important" finding of the commission's report, published in The Lancet, was the magnitude of the inequity in health outcomes and life expectancy.
The report found people with mental health problems were up to twice as likely to suffer from obesity, diabetes and cardiovascular disease and were at risk of dying 20 years earlier than those without a mental illness.
The report was of special relevance to New Zealand, given our rates of mental illness were high.
More health action, not only to improve mental health support, but also to counter obesity and adult-onset diabetes was needed, Prof Mann, who is director of the Healthier Lives National Science Challenge, said.
The report makes lifestyle and health-care recommendations to counter the huge health outcome disparities facing the mentally ill, the commission chairman calling the current situation a "human rights scandal".