Stress disorders of various kinds could go on to do more damage to the body according to a new study covering data on over one million people. The study has found an association between a range of such disorders and the risk of developing autoimmune diseases like arthritis or Crohn's disease.

While links between mental stress and physical deterioration have been highlighted before, few previous studies have closely examined the relationship between psychiatric stress and the immune system.

Now it looks as though the risk of autoimmune diseases – where the body's defence mechanism turns on itself – could be pushed up by disorders including Post-Traumatic Stress Disorder (PTSD).

That's a worrying link, but it might also give us clues for developing better methods of treatment.

"We know from previous research that too much stress can disrupt our immune system, but this is the first study that shows the link between PTSD and other stress disorders and increased risk of autoimmune diseases in a large sample of individuals," says one of the researchers, Unnur Anna Valdimarsdóttir from the University of Iceland.

Looking at data collected by the Swedish national health system, the researchers identified 106,464 people diagnosed with stress disorders, including PTSD. Over 30 years of records, they were checked against individuals without stress disorders – 126,652 related siblings and over a million people from the general population.

Those with stress disorders were 30-40 percent more likely to go on to develop one of 41 autoimmune diseases as well, on average – 9.1 people per thousand for the stress disorder group, 6.5 people per thousand for their siblings, and 6 people per thousand for a matched group without PTSD or other stress disorders.

The exact relationships between disorders and diseases varied – for instance, individuals with PTSD taking antidepressants known as selective serotonin reuptake inhibitors (SSRIs), were at a particularly high risk of developing autoimmune diseases.

This might be a marker for the severity of the stress disorder, the researchers say, with the people who are worse affected more likely to be prescribed antidepressants. It's also worth noting that the longer SSRI use continued for, the more the risk fell.

"The main message to patients suffering from severe emotional reactions after trauma or other life stressors is to seek treatment," one of the researchers, Huan Song from the University of Iceland, told Lisa Rapaport at Reuters.

"There are now several treatments, both medications and cognitive behavioural approaches, with documented effectiveness."

As usual with these types of studies, we can't say for sure that stress disorders cause autoimmune diseases. Only that there appears to be an association between the two.

The researchers suggest people living with PTSD might end up drinking more or sleeping less, for example, which could push up the risk of autoimmune problems.

Another possibility is that some unknown third factor increases the risk of both autoimmune diseases and stress disorders together.

Ultimately, we need more research and more data to refine our understanding of what's really happening.

Even if the conclusions that researchers can take right now are limited, there is a statistically significant link here, and the sheer size of the cohort and the length of the time involved means this is definitely worthy of further investigation.

As the research progresses, it's possible that improving treatments for individuals with stress disorders could also cut down the risk of autoimmune disease later in life.

The next challenge is to work out the biological mechanisms causing this association.

According to Mayer Bellehsen from the Unified Behavioral Health Center for Military Veterans and Their Families in New York, who wasn't involved in the study, it's more evidence that extreme stress has a direct impact on our bodies.

"Regardless of cause, this study adds to the evidence of the link between stress conditions and physical well-being, warranting further attention to the reduction of trauma and other causes of stress conditions, as well as improving treatment of these conditions," Bellehsen told Steven Reinberg at HealthDay.

The research has been published in the Journal of the American Medical Association.