Researchers found that individuals who had persistently high levels of the "stress hormone" cortisol over long periods of time weighed more, had a higher body mass index (BMI), and a larger waist, compared with those who had low levels of the hormone.
The study - led by Dr. Sarah Jackson from the Department of Epidemiology and Public Health at University College London (UCL) in the United Kingdom - was recently published in the journal Obesity.
Obesity has become an epidemic in the United States, affecting more than a third of adults.
Past research has shown that stress can fuel obesity; stress has been linked to "comfort eating," whereby individuals turn to foods high in fat and sugar in an attempt to make them feel better.
Studies have also suggested cortisol - a hormone released in response to stress - plays a role in obesity and metabolic syndrome.
Hair cortisol levels used to assess stress-obesity link
For their study, Dr. Jackson and colleagues sought to determine how chronic stress influenced the risk of obesity. They did so by analyzing cortisol levels in hair samples of 2,527 adults aged 54 years and older who were part of the English Longitudinal Study of Ageing.
"Hair cortisol is a relatively new measure which offers a suitable and easily obtainable method for assessing chronically high levels of cortisol concentrations in weight research and may therefore aid in further advancing understanding in this area," notes Dr. Jackson.
Also, hair sampling allows researchers to get a more accurate, longer-term picture of cortisol levels; many studies measure cortisol levels in blood, saliva, or urine, and these often vary by time of day and other factors.
"The analysis of cortisol in scalp hair reflects systemic cortisol exposure over a prolonged period - in this study, 2 months - and is therefore not affected by the timing of sample collection or acute stress," say the authors.
The researchers took a lock of hair from each subject that measured at least 2 centimeters in length and 10 milligrams in weight. The hair was cut as near to the scalp as possible, representing around 2 months of hair growth.
Additionally, the team measured the weight, BMI, and waist circumference of each participant at several points over 4 years.
'Consistent evidence that chronic stress is linked to obesity'
Compared with adults who had lower levels of hair cortisol, those who had higher levels were found to have a larger waist circumference, a higher BMI, and a heavier weight.
Adults considered obese based on their BMI or waist circumference - defined as greater than 102 centimeters in men and greater than 88 centimeters in women - had the highest hair cortisol levels, the team reports.
Based on their results, the researchers suggest long-term stress - as determined by cortisol levels in hair - may raise the risk of obesity.
"These results provide consistent evidence that chronic stress is associated with higher levels of obesity.
People who had higher hair cortisol levels also tended to have larger waist measurements, which is important because carrying excess fat around the abdomen is a risk factor for heart disease, diabetes, and premature death."
Dr. Sarah Jackson