People suffering from a mental illness such as depression and schizophrenia also stand a higher risk of developing chronic physical illnesses such as heart disease and type 2 diabetes, suggesting certain biological mechanisms may hold the key to treating both types of illness. A recent study led by scientists from the University of Cambridge has revealed a person’s immune system may play a major role in their chances of developing a mental illness such as depression or psychosis.

"Our immune system acts like a thermostat, turned down low most of the time, but cranked up when we have an infection,” Dr. Golam Khandaker, lead researcher  from the Department of Psychiatry at the University of Cambridge, said in a statement. “In some people, the thermostat is always set slightly higher, behaving as if they have a persistent low level infection — these people appear to be at a higher risk of developing depression and psychosis. It's too early to say whether this association is causal, and we are carrying out additional studies to examine this association further."

Khandaker and his colleagues gathered data on 4,500 individuals participating in the Avon Longitudinal Study of Parents and Children, also known as Children of the 90s. Each participant who had blood samples taken at the age of 9 were divided into three groups based on low, medium, or high levels of the protein interleukin-6 (IL-6). In response to an infection such as influenza or a stomach bug, our immune cells expose the blood stream to these so-called “inflammatory markers,” but traces of IL-6 can be found in the immune system when we are healthy. Participants were asked to return for a follow up at 18 to see if they had experienced episodes of depression or psychosis.

Children included in the high IL-6 group were twice as likely to show signs of depression or psychosis by the age of 18 compared to those in the low IL-6 group. Results of the study indicate a common mechanism tied to both physical and mental illnesses, considering high levels of IL-6 are also a tell-tale sign of heart disease and diabetes. High levels of IL-6 may also explain why a healthy diet and increased physical activity are suggested preventive measures for reducing our risk of heart disease and diabetes as well as depression and anxiety. The research team also suggests that anti-inflammatory drugs may be a fairly unconventional method for treating signs of a mental illness.

"Inflammation may be a common mechanism that influences both our physical and mental health,” said Professor Peter Jones, the study’s senior author and Head of the Department of Psychiatry. “It is possible that early life adversity and stress lead to persistent increase in levels of IL-6 and other inflammatory markers in our body, which, in turn, increase the risk of a number of chronic physical and mental illness."

Source: Khandaker G et al. Serum Interleukin-6 and C-Reactive Protein in Childhood as Predictors of Depression and Psychosis in Young Adult Life: A Population-Based Longitudinal Study. JAMA Psychiatry. 2014.