Research: Maintaining moderate exercise levels could reduce the impact that stress has on your health, according to a recent scientific review published last month in Exercise and Sport Science Reviews.
"Physical activity may prevent stress-induced suppression of the immune system," explains Dr Monika Fleshner from the University of Boulder, Colorado, the author of the review. "This could reduce the increased susceptibility and severity of infectious disease caused by stress."
An early study suggested that sedentary teenage girls suffered more infectious diseases when under high levels of psychological stress, according to Dr Fleshner. In contrast, girls who were more physically active were protected against the stress-induced increases in disease incidence.
Dr Fleshner's own research uses animals to explore the biological mechanisms by which exercise could act as a defence against the detrimental effects of stress. She vaccinates mice with a harmless protein called KLH, which elicits an immune response in the same way as a virus or bacteria.
When the animals are stressed, they have poorer immune responses to the vaccine. If this had been a real infection, instead of a vaccination, they would have been more likely to get ill than the animals that were not stressed.
Importantly, this negative effect of stress was only observed in "sedentary, but not in physically active, rats," according to Dr Fleshner. This suggests that exercise has a "stress-buffering effect," she says.
When an animal or person is stressed, a branch of the nervous system called the sympathetic nervous system is activated. This is part of the "fight or flight" response and prepares the body for action. When stress causes the sympathetic nervous system to be activated repeatedly, or for a long time, the immune system can be weakened.
Dr Fleshner suggests that regular physical activity may reduce the nervous system response to stress. However, the "specific adaptations that occur in the brain which result in a more constrained nervous system response to stressor exposure are currently unknown," she explains.
She points out that the implications for humans are "speculative at this time", but that "a physically active lifestyle incurs many health benefits" in addition to its possible role in stress reduction.
Dr Vikki Burns is a scientist from the University of Birmingham on placement at The Irish Times as a British Association for the Advancement of Science Media Fellow.