The link between heart disease and depression could be inflammation, research suggests.
The two conditions have been heavily linked to each other for years but scientists have struggled to explain why this is.
Now experts have found inflammation - the body's first line of defence - could be to blame.
Both heart disease and depression were found to trigger the release of inflammatory substances into the blood.
Inflammation has been linked to higher levels of 'bad' cholesterol and triglycerides - the fat found in our blood.
Researchers at the University of Cambridge analysed data from more than 360,000 people to make the discovery.
Heart disease increases the risk of depression - and vice versa, research suggests (stock)
Dr Golam Khandaker, study author, said: 'It is possible heart disease and depression share common underlying biological mechanisms.'
He added these could 'manifest as two different conditions in two different organs - the cardiovascular system and the brain'.
Dr Khandaker said: 'Our work suggests inflammation could be a shared mechanism for these conditions.'
Past research suggests around 40 per cent of patients experience depression after a heart attack.
While 15 per cent of cardiovascular-disease sufferers experience severe forms of the mental-health condition.
This association was thought to be too strong to be down to the emotional turmoil of battling heart disease alone.
To determine whether environmental or genetic factors play a role, the researchers analysed 367,703 middle-aged people of European ancestry from the UK Biobank.
Participants were asked if they had ever felt depressed or unable to enjoy things for at least a week, as well as if they have ever sought professional help for a low mood.
They also gave details on whether their mother or father ever had heart disease.
DNA of other Biobank participants, not involved in the first part of the study, were also analysed to determine if there was a genetic link between the two conditions.
Results - published in the journal Molecular Psychiatry - revealed those who lost one of their parents to heart disease are more at risk of depression themselves.
Triglycerides are the most common form of fat in the body and come from the extra calories in food.
When the body does not need calories straight away, they get converted into triglycerides and stored in fat cells.
And when the body then needs energy, it releases triglycerides.
Having a high level of triglycerides is associated with heart disease.
This can be caused by obesity, smoking, excessive drinking, certain medication or genetic disorders, poorly controlled type 2 diabetes, and liver or kidney diseases.
Normal levels are less than 150 milligrams per deciliter (mg/dL).
Borderline high is 150-to-199mg/dL; high 200-to-499mg/dL; and very high
