An Israeli-Canadian study used hair to study stress hormone, and found, not surprisingly, that chronic stress plays an important role in heart attacks.
Cortisol, a hormone secreted in higher levels during times of stress, is usually measured in blood, urine or saliva. But those measurements reflect stress only at a certain period of time, not over long stretches.
Hair, however, captures cortisol levels over a longer period, said study author Dr. Gideon Koren.
Since hair grows a half-inch per month on average, a three-inch-long hair sample can show cortisol levels over six months, he said.
"It gives us, for the first time, a biological marker for chronic stress," Koren said.
The hair shaft records cortisol levels through time, similar to the way tree rings reflect age.
In the study, researchers collected 1.5-inch-long hair samples from 56 men admitted to the Meir Medical Centre in Kfar-Saba, Israel, suffering heart attacks or acute myocardial infarction (AMI).
A control group of 56 who were treated in hospital for reasons other than a heart attack also provided hair samples.
"In conclusion, hair cortisol concentrations were found to be elevated in the three months prior to the event in patients admitted with AMI, than in controls," the study's authors wrote.
"Hair cortisol measurements can be used to identify patients at high risk for AMI who may benefit from strategies targeted to manage chronic stress, and as an impetus for more aggressive treatment of other modifiable risk factors."
After taking known risk factors into account, hair cortisol levels was the strongest predicator of heart attack, the researchers said.
But the study's authors noted the hair sample needs to be long enough and the findings could be subject to contamination by creams containing cortisol.
The researchers also plan to test cortisol levels in the hair of women.