Poorer physical health across multiple organ systems has been significantly associated with depression because of the role the brain plays linking physical and mental health, new research has shown.
The study published in Nature Mental Health on Friday has identified for the first time the biological pathways through which poor organ health may lead to poor brain health, which may in turn lead to poor mental health.
The research led by the University of Melbourne, in collaboration with the University College London and the University of Cambridge, studied a subset of 18,083 people in the UK Biobank cohort.
Among these, 7,749 people had no clinically diagnosed major medical and mental conditions, while 10,334 had a lifetime diagnosis of one of the four common mental disorders including schizophrenia (67), bipolar disorder (592), depression (9,817) and generalised anxiety disorder (2,041).
They were aged between 40–70 years (with a mean age of 53.7 ) at the time of recruitment in 2006–2010 where researchers also separately assessed the health of their seven organ systems: the lungs, muscles and bones, kidneys, liver, heart, and the metabolic and immune systems. As well as physical assessments, the participants also undertook questionnaires for environmental and lifestyle factors.
For each of the seven organ systems, the researchers found that poorer organ health was significantly associated with higher depressive symptoms.
Similarly, poor organ health scores, except kidney and lung scores, were significantly associated with higher anxiety symptoms.
Poorer health scores of all organs, except the kidneys, were significantly associated with higher neuroticism (participants with schizophrenia or bipolar disorder).
“The musculoskeletal system consistently showed the strongest associations with the three mental health measures, followed by the immune, metabolic and hepatic [liver] systems,” according to the research.
The researchers found a consistent relationship for each reduction in musculoskeletal health, a consistent worsening of mental health symptoms.
Physical–mental health associations were modest for heart and lung systems and the weakest for the kidney system, they found.
The researchers then investigated whether the association between organ and mental health is mediated by the brain by gathering brain imaging data through participants undergoing an MRI between 4–14 years later.
For each person, a score was calculated for the volume of each brain gray matter (GM) region and the brain’s white matter tract – the fiber pathways that connect different components of the neural system. The participants then undertook an online assessment for mental health outcomes.
The researchers found “multiple significant pathways through which poor organ health may lead to poor brain health.” Across the three mental health measures, they found the volume of brain gray matter had the largest impact on depressive symptom severity, while the influence was weakest on anxiety.
They found the extent to which brain structure mediates physical–mental health varies across organ systems. “In general, the brain showed a strong mediating effect on organs that had strong direct effects on mental health outcomes; namely, the musculoskeletal and immune systems.”
The lead author, Dr Ye Ella Tian, a research fellow in the University of Melbourne’s department of psychiatry, said “by integrating clinical data, brain imaging and a wide array of organ-specific biomarkers in a large population-based cohort, we were able to establish for the first time multiple pathways involving the brain as a mediating factor and through which poor physical health of body organ systems may lead to poor mental health.”
The authors also assessed 14 lifestyle factors commonly associated with physical and mental health, including smoking, alcohol use, physical inactivity, poor nutrition and sleep, finding most were significantly associated with all three mental health measures.
The authors noted several limitations of the study. They acknowledged brain imaging and mental health assessments not available at the first study wave of the UK Biobank, when physical health was assessed.
Because of the sequence of participant assessment, they were unable to assess pathways where poor mental health may lead to poor physical health via influencing brain structure, or other combinations. They noted the relationship between physical and mental health is likely bi-directional.
They also acknowledged the UK Biobank cohort was predominantly white British participants and further work is needed to assess the generalisability of findings across the adult lifespan and in a diversity of ethnicities and socioeconomic backgrounds.
Dr. Matthew Ahmadi, a research fellow at the University of Sydney’s faculty of medicine and health who was not involved in the research, said “the authors’ thorough integration of several lifestyle and health factors, brain imaging, organ-specific biomarkers and analytical modelling allowed them to identify potentially key pathways that the brain, physical health, and organ function may directly and indirectly impact mental health through biological pathways”.
