If a person with high genetic vulnerability is exposed to adverse environmental factors, such as an infection or mental trauma, that person can develop a mental illness — in the same way that genes can predispose a person to physical illnesses, such as obesity and heart disease.
This made Nils Eiel Steen from the University of Oslo and his colleagues wonder: could there be an overlap in genetics when it comes to the relationship between serious mental disorders and weight?
In other words: Can certain gene variants cause both mental illness and obesity?
In all, 63 gene variants were associated with both BMI and schizophrenia, 17 variants were associated with both BMI and bipolar disorder, while 32 variants were associated with both BMI and severe depression.
Most people with schizophrenia may have many genes that protect them from being obese. However, some may have inherited schizophrenia genes that are linked to an increased risk of being overweight. At the same time, a person with the protective version of the genes can still become overweight due to medication and lifestyle.
In other words: Two people with fairly similar mental and physical symptoms may have completely different underlying causes for their problems.