Poor mental health is associated with lower life expectancy, but the connection has not been well explored, hampering efforts to address the problem. Now an enormous study has identified the factors that matter the most, at least in Denmark, and it turns out the biggest dangers are not those that get the most attention.

The exceptional databases Scandinavian countries keep on their citizens' health might arouse privacy concerns elsewhere, but represent an enormous resource for medical researchers. Professor John McGrath of the University of Queensland has been drawing on Danish research for a long time, for example throwing light on the causes of schizophrenia by showing it correlates with vitamin D deficiency at birth.

For his latest research, McGrath used data from all 7.4 million people who lived in Denmark for part or all of the period 1995-2015.

“This is the first time we have measured life expectancy for distinct types of mental disorders in a sex and age-specific way – it’s well known that people with mental disorders die earlier than the general population, but we have used more accurate approaches than those used in the past,” McGrath said in a statement. 

On average Danes diagnosed with mental illness – a third of the population – live 10 years less than everyone else.

In The Lancet, McGrath reveals the detail behind that figure. “All types of mental disorders had higher mortality rates – some are attributable to suicide but surprisingly, most were due to general medical conditions such as heart disease, infection, and cancer,” he said. 

The largest effect, particularly for men, is from cardiovascular and lung diseases.

The minimum life years lost (LYLs) compared to healthy peers was 5.4 years for women with mood disorders, while the heaviest toll was from substance use disorders, which on average took 14.8 years off men's lives.

McGrath told IFLScience “There could be many different reasons” for the associations he found. “Those with mental disorders may have an unhealthier lifestyle (less exercise, poor diet, smoking, alcohol etc). As well, they may be less likely to seek care, and/or when they get care, may not get optimal care.”

The exceptional nature of the Danish health system means the situation could easily be worse elsewhere, but McGrath told IFLScience no international comparison is available.

Solving a problem like this is inevitably a herculean task, but much good can be done without making everything perfect. In particular, “Regular health check-ups for those with mental health disorders and improving coordination between specialists and general health practitioners,” are things McGrath believes could make a substantial difference.