The world is getting fatter and it’s making us sicker. But could rising stress levels be playing an important role in our growing waistlines?
Obesity is now one of the leading causes of death worldwide and is associated with increased risk of developing a host of chronic health conditions. There is great public interest in the reasons some people struggle with their weight while others find it easy to stay slim, with blame often attributed to genes or health conditions, such as thyroid problems.
Stress is another potential risk factor that has attracted research attention. People tend to report overeating and “comfort eating” foods that are high in sugar, fat and calories when stressed. And because the stress hormone cortisol plays a role in metabolism and fat storage, there are plausible biological mechanisms behind a possible link between stress and putting on weight.
In research published in Obesity this week we found that chronic stress was consistently linked with people being more heavily, and more persistently, overweight.
Our data were collected over a four-year period as part of the English Longitudinal Study of Ageing, a study that follows a large group of people aged 50 and older. We found that people who had higher levels of cortisol in their hair tended to have a larger waist circumference, were heavier, and had a higher body-mass index (BMI). People classified as obese on the basis of their BMI (≥30) or waist circumference (≥102cm in men, ≥88cm in women) had particularly high levels of cortisol in their hair.
When we looked back at people’s weight over a period of four years, we saw that those who had more persistent obesity had higher hair cortisol measurements than those whose weight had fluctuated or who had consistently been a healthy weight.
Measuring long-term stress
Why did we use hair to measure cortisol levels? Previous studies looking at the link between cortisol and obesity have relied mainly on measurements of the hormone in blood, saliva or urine which may vary according to the time of day and other “situational factors”, such as diet or illness. Because these methods give a very short-term picture of a person’s stress levels, these studies were not able to evaluate the relationship between obesity and longer-term stress. The distinction between acute (short-term) and chronic (longer-term) stress is important because the former is thought to serve as a protective fight or flight response whereas the latter can have a damaging effect on the body.
Over the last decade, a new method for measuring cortisol levels in hair has been developed, and has been shown to be a reliable way of assessing chronic stress exposure.
For our research, a lock of hair 2cm long was taken from each participant, cut as closely as possible to the person’s scalp. Hair grows at an average rate of 1cm per month, so our samples represented approximately two months’ hair growth with associated accumulated levels of cortisol.
We measured people’s weight, height and waist circumference, and we used these measures to assess the relationship between levels of hair cortisol and adiposity (fatness).
A new target for treating obesity?
We cannot be sure from our research that stress is causing people to become obese, but if causation can be proved through further investigation, the link between chronic stress and obesity offers a potential target for interventions aimed at preventing and treating obesity. Tried and tested stress-reduction techniques such as mindfulness meditation and yoga are cheap, widely accessible options that could help people reduce their risk of developing obesity. It may also be possible to use drugs that reduce cortisol levels to treat obesity in more severe cases.