Having genes that put you at risk of depression could also make you susceptible to a variety of other health conditions, often seemingly unconnected. According to new research, this could include things like coronary heart disease and even bacterial infections.

The new study assessed the genetic risk factors of major depressive disorder in relation with more than 900 other diseases; the research team found that having poor mental health isn't always a consequence of serious illness – it can be directly responsible for it.

"Data shows that people living with serious mental diseases, like depression, have much higher rates of physical illness than those in the general population," says genetic epidemiologist Anwar Mulugeta from the Australian Centre for Precision Health at the University of South Australia.

Suffering from severe depression while dealing with another major illness is maybe not all that surprising, but researchers have long wondered whether the root causes of depression might be somehow more directly responsible.

Previous research has indicated that depression isn't just a disease of the mind, but can have a debilitating influence over the entire body.

The team used what's known as a Mendelian randomisation approach to a pool of genomic data taken from UK Biobank records on nearly 340,000 individuals to work out which comes first.

While many similar studies have already established links between depression and individual diseases, this kind of approach applies various controls suited to establishing a more causal relationship.

It turns out that having a high genetic risk score for major depressive disorder also makes it more likely a patient will have been admitted to hospital with – or even died from – at least one of 20 different diseases.

In the context of the analysis, these genes predisposed the individuals to a serious illness affecting just about every system in the body. These include such diverse conditions as asthma, high cholesterol, gastroenteritis, oesophagitis, urinary system disorders, and even infections from E.coli.

The study also highlighted potential sensitivities and adverse reactions to some drugs, suggesting a strong need to monitor prescriptions for individuals with a diagnosis of major depressive disorder.

"This research puts the 'chicken and egg' conundrum to rest, showing that depression causes disease, rather than only the other way around," says Mulugeta.

"Importantly, this research signals that an individual diagnosed with depression should now also be screened for a defined set of possible comorbidities, enabling much better clinical management and significantly improved outcomes."

Exactly how genes for depression might lay the groundwork for developing a range of other diseases isn't clear. Given the number of gastrointestinal illnesses on that list, the researchers speculate that medications used to treat mental illness could be having an adverse effect on our guts.

This isn't out of line with previous studies pointing at similar conclusions, but further research should help nail down the mechanisms behind the relationship.

"Understanding the connections between depression and other diseases is critical to ensure people with depression receive the support they require. The more we can look at the individual patient, the better their outcomes are likely to be," says study lead Elina Hyppönen, also from the Australian Centre for Precision Health.

"Our results suggest that it is important to look beyond the obvious, and that we need to screen and effectively manage depression-related comorbidities if we want to minimise the longer-term negative implications on health."

With more than 300 million people all over the globe experiencing depression, it ranks among one of the most common illnesses of our time.

It's clear that finding better ways to treat and prevent the effects of depression without putting us at risk of other conditions is becoming more important than ever.

This research was published in Molecular Psychiatry.