The study, published in the journal Scientific Reports, indicated that people with higher levels of cortisol are at an increased risk of heart attack. 

"The levels of the stress hormone cortisol differed between people who have had a heart attack and those not affected. This suggests that cortisol in hair may be a new risk marker for heart attacks," said Tomas Faresjo from the Linkoping University in Sweden. 

Cortisol is a steroid hormone that regulates a wide range of processes throughout the body, including metabolism and the immune response. It also has a very important role in helping the body respond to stress. 

To study the long-term stress due to a lack of reliable methods, the team improved the use of a new biomarker, in which they measure the levels of the stress hormone cortisol in hair. 

This enables measurements of cortisol levels backwards in time, similar to the growth rings in a tree. This analysis method is at the moment only available in research settings. 

In the present study, the researchers used hair samples of length between 1 and 3 centimetres, corresponding to 1-3 months of hair growth. They measured cortisol levels in hair samples from 174 men and women in professional life who had been admitted for myocardial infarction to cardiology clinics. 

As control group, the researchers used hair samples from more than 3,000 similarly aged participants in the Swedish SCAPIS study (Swedish CardioPulmonary bioImage Study). 

The researchers showed that patients who suffered a heart attack had statistically significant higher levels of cortisol during the month preceding the event. 

"It's surprising that this biomarker for long-term stress seems to be strong even compared with traditional cardiovascular risk factors," said Tomas Faresjo.