A relatively short run, if taken as fast as possible, is more stressful for the body than you might think, according to research from LiU. The results, published in the journal PLOS ONE, show, among other things, that the level of the stress hormone cortisol remains high for several hours.

How are blood sugar regulation and metabolism affected by a relatively short run, if you run as fast as possible? In order to answer this question, researchers at Linköping University have compared starting the day with a five-kilometre run at maximum intensity with not performing any particular exercise during the day. Seventeen healthy volunteers aged around 20 years, all of whom were experienced runners, participated in the investigation. The participants started one day with a running session before breakfast, and started another day by an equally long rest before breakfast. The order in which the days were taken was decided randomly for each participant. The researchers took blood samples on both days, and used these to analyse factors related to stress, cardiac health and diabetes risk. They continued to take blood samples throughout the day at regular intervals. The participants measured the level of the stress hormone cortisol in their blood and in their saliva. This could be done at home.

“We were surprised to find that running caused the level of cortisol in the blood to rise rapidly and that the level of cortisol in saliva remained high on running days, right up until bedtime. It wasn’t until the following morning that the cortisol level in saliva was back to normal,” said Professor Fredrik Nyström of the Department of Medical and Health Sciences, Division for Cardiovascular Medicine at Linköping University, Sweden, who led the study.

The rise in cortisol level was accompanied by a reduced sensitivity to the hormone insulin, which is important in regulating blood sugar.

“Sensitivity to the body’s insulin was lower, and after a run the participants needed to double the level of insulin in the blood to try to achieve control of their blood sugar. Despite the significantly increased levels of insulin, blood sugar levels were clearly elevated at the first measuring point after the run, for these healthy individuals,” said Fredrik Nyström.

Another result that the researchers point to concerns troponin T, which is the marker currently used to diagnose myocardial infarction. It was possible to detect troponin T in the blood of most participants after the run. In three people, the level of troponin T reached the limit for a clinical diagnosis of myocardial damage. However, by the next day the level of troponin T (in all participants but one) was again so low that it could not be detected.

These results are compatible with those of previous studies showing that intense, long-lasting physical exertion, such as running a marathon, often leads to increased blood levels of markers for cardiac damage. What has not previously been shown, however, is that the same signs of physical stress can be seen in healthy 20-year-olds who run as short a distance as five kilometres.

“It was remarkable to discover that the stress hormone effect can be so strong that blood sugar rises when running on an empty stomach, even though the insulin level is twice as high. It’s probably not a good idea to exercise at such high intensity, if the goal is to improve cardiac health and counteract diabetes. It’s interesting to speculate what the results would have been if we had tested older or overweight people,” said Fredrik Nyström.