By Michelle Roberts
Health reporter, BBC News
Anxiety can manifest as physical problems
The true burden of mental ill health is unrecognised since many "physical" problems, like cancer and obesity, are really "mind" problems, say experts.
Most lung cancers are caused by addiction to smoking, and some obesity by a brain-driven compulsion to eat, says UK psychiatrist Dr Peter Jones.
And to tackle such problems experts need to go back to delving the mind.
He and other leading mental health experts are calling for a trebling of funding to £200m a year for research.
The Research Mental Health initiative, along with public figures including Alistair Campbell, Jo Brand and Stephen Fry, are taking their declaration to Downing Street.
Mental illness in its "classic" sense, including depression and schizophrenia, affects one in four people in the UK each year but receives just 5% of total health research spending.
Currently, around £74 million a year is spent on researching mental illness.
Yet the economic, social and human cost of mental illness totals £100 billion a year in the UK alone.
And many "physical" health problems involve a strong mental component, they say.
Professor Peter Jones, head of psychiatry at the University of Cambridge, said: "Mental health and illness are seen as separate from physical health and disorders but it's becoming increasingly clear that is wrong.
"Take smoking and lung cancer. People think of it as a physical illness but lung cancer is a behaviour disease due to smoking habit."
Similarly, he said research showed that some cases of obesity could be explained by a hormonal deficiency that acts on the brain circuitry that tells the body when it is full or hungry.
"We need to zip together physical and mental health. It is absurd to think that biological processes would stop at the neck."
People with severe mental illnesses are nearly three times more likely to develop diabetes and other cardiovascular disease risk factors and, on average, die 25-30 years younger.
Research Mental Health says more research investment is desperately needed to match the impact mental health has on people in terms of premature death and disability.
"The long term aim must be to put mental health research on the same footing as that for physical illness," it says.
Mental illness and cancer both account for about 15% of the total disease burden in the UK, yet cancer gets more than 25% of research investment, while mental health gets 5%.
Andrew McCulloch, chief executive of the Mental Health Foundation, said: "Our understanding of mental illness is moving at a snail's pace.
"Whilst treatments have improved, we have not yet seen the breakthroughs needed to significantly reduce the massive economic and social damage caused by mental illness."
Meanwhile, experts and advocates in mental and physical health are working together for the first time in a European policy initiative - the Mental and Physical Health Platform - to improve the understanding of the interaction between body and mind in disease.
The chairman of the initiative, John Bowis, said: "It is time to bridge the gap between mental and physical health by taking actions across policy areas and countries."