The notion that what goes on inside our heads can profoundly influence our body's typical functions has gained momentum over time, with decades of research revealing how gut health can influence mood and even neurological disease.

A new study by a team of researchers from Australia now suggests mental illness might manifest in the body even more obviously than it does in the brain. An analysis of more than 175,000 individuals suggests that poor physical health scores could be a better indicator of mental illness than changes detected on brain scans.

Lead author, psychiatrist and neuroscientist Ye Ella Tian, explained on Twitter that those diagnosed with a mental illness showed subtle brain changes as you might expect, but they also had "considerably poorer physical health across multiple body systems compared to their healthy peers."

In some respects, this isn't so surprising. Research shows that mental illness is associated with poor physical health and chronic illness, such as obesity and diabetes, which may be related to the side effects of medication or disparities in access to healthcare.

For example, people with schizophrenia are around three times more likely to develop diabetes than the general population; they also have a two-fold increased risk of coronary heart disease, Tian says.

However, the impact of severe mental health conditions on other aspects of physical health, such as lung health, liver dysfunction, and bone loss, is less well-studied. To make matters worse, physical health more generally has been "underestimated, inadequately treated, and often overlooked in psychiatry" for decades, the University of Melbourne's Tian and colleagues write.

Compiling a suite of measurements from MRI brain scans and health assessments from a number of different study collectives, the team produced scores of brain health and the function of seven body systems, including the immune system, liver (hepatic) function, heart, lung and kidney health. This allowed them to compare health scores among people diagnosed with mental illness to populations of people the same age.

Differences in brain imaging results could accurately differentiate between the four mental health conditions studied: schizophrenia, bipolar, depression, and generalized anxiety disorder.

And yet, despite the neural basis of mental illness, the study found that individuals with one or more of these disorders could be differentiated "with modest accuracy" from healthy controls of a similar age based on body health alone.

Body health scores for liver and kidney function, the immune system, and metabolism were consistently poor across mental health conditions compared to healthy controls – and turned out to be better predictors of a mental health diagnosis than brain imaging scores.

That physical health measures would surpass differences in brain imaging results "was quite a surprising finding" given how mental illness is rooted in the disordered brain, University of Melbourne neuroscientist Andrew Zalesky explained in a podcast.

Even so, there are many possible reasons that explain why poor physical health goes hand in hand with mental severe illness: from the side effects of antipsychotic medications and mood-stabilizing drugs, to the chronic stress, increased infections, and enhanced immune responses that people with mental ill-health experience. For instance, research shows that for people with schizophrenia, their risk of dying from COVID-19 was nearly three times higher than people without the disorder.

The researchers note that their findings – which are mostly from white, British populations – aren't meant to be used as a diagnostic tool. "We should note here that these are individuals with an established diagnosis," Zalesky added. "Whether this would hold before the onset of the disorder we don't know."

Instead, the findings should help psychiatrists to appreciate how different aspects of physical health are affected by mental illness, and encourage them to treat both the body and the mind when managing people with mental health conditions.

In this particular study, chronic physical health issues were often not diagnosed in people with mental health conditions but the findings suggest "that poor body health and function may be important illness manifestations that require ongoing treatment in patients," the researchers conclude.

The study has been published in JAMA Psychiatry.