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In a study, students completed a 20-week exercise program just ahead of exams, then their heart rates were measured for variations in beats.istockphoto

The engineering students at Karlsruhe Institute of Technology in Germany are like students everywhere: Exams stress them out.

So, when researchers at the university were looking for a way of testing how exercise affects the body's response to real-world stressors, they had a perfect population to study. Their results support a controversial theory that, as your body learns to adapt to one form of stress (such as exercise), the benefits extend to other forms of stress (such as exams). And they add to growing evidence that stress management may be one of the key links between exercise and lower rates of heart disease.

The idea that exercise combats stress may seem astoundingly obvious, particularly to those who already swear by it. But the links are surprisingly hard to disentangle.

"The stress response is a huge cascade, starting in the brain and going all the way down to the immune system," says Dr. Eli Puterman, a Canada Research Chair in Physical Activity and Health who heads the University of British Columbia's Fitness, Aging & Stress Lab.

Exercise, too, has wide-ranging effects throughout the body – including well-known mood-boosting benefits. So does exercise simply make you feel better about stress, or does it directly fight some of the negative physiological effects of stress, like inflammation and heightened risk of heart disease?

The new German study, led by researcher Birte von Haaren and published last month in the European Journal of Applied Physiology, zeroes in on the heart's response to stress. The researchers recruited 61 students, half of whom completed a 20-week exercise program involving walking or running for 30 to 60 minutes twice a week. The exercise program was timed to finish right before the students' exam period.

To measure stress response, the researchers relied on a physiological measure called "heart rate variability," or HRV, which refers to slight variations in the time between successive heart beats.

"The heart is happiest when it dances," explains Dr. Richard Sloan, a professor of behavioural medicine at Columbia University in New York. "It's good to have a lot of heart rate variability."

By analyzing this variability, researchers are able to probe the balance between two components of the autonomic nervous system: the parasympathetic (sometimes referred to as "rest and digest") and sympathetic ("fight or flight") systems. A decrease in high-frequency HRV indicates a shift away from the parasympathetic system – it's a signal that your body and mind are gearing up to deal with a stressful challenge.

Sure enough, when the Karlsruhe researchers measured heart rhythms for 36 hours during the exam period, the students who had completed the exercise program had heart-rate variability patterns indicating less stress response.

The results are significant because of continuing controversy surrounding the "cross-stressor adaptation hypothesis," the idea that adapting to one form of stress confers protection from other forms of stress. Previous studies of the idea have produced conflicting results; a 2011 study by Sloan, for example, found no improvement in the HRV response to a stressful laboratory task after 12 weeks of exercise.

The new study has several strengths. The 20-week training program is longer than many previous studies, and the exams were likely far more stressful than the usual mild stressors used in lab-based studies. "It's really exciting to see an intervention that changes the physiological response to stress," Puterman says.

Puterman is also using real-world settings to study stress. He's currently leading a six-month study of caregivers of people with Alzheimer's disease, to see whether regular exercise can halt or reverse some of the health problems often experienced under these stressful circumstances. He'll be analyzing a wide range of outcomes related to inflammation, immune function, and even telomere length – a property of DNA associated with aging.

That's a good reminder that fighting stress isn't just about feeling more relaxed. One of the unexplained mysteries of exercise science is why exercise is so effective at reducing heart disease, for example. Factoring in all the usual benefits like reduced cholesterol, blood pressure, and weight only explains about 60 per cent of the observed benefits.

Where does the rest come from? It could be from improved stress response, von Haaren and her colleagues argue. There's evidence that chronic stress of the type seen in their exam-period HRV data raises the risk of later heart problems. If exercise keeps HRV values (and, by extension, the autonomic nervous system) closer to normal, that could be a hidden long-term health benefit.

That debate is far from settled, and the full picture of how exercise and stress interact remains dauntingly complex – but the practical message is clear. "Exercise helps pretty much everything," Sloan says, "even if we don't understand all of it."