How a year of regular exercise alters the biology of stress

A new year-long study reveals that engaging in regular aerobic exercise lowers long-term levels of a major stress hormone, which might help protect against heart disease and mood disorders. Published in the Journal of Sport and Health Science, the clinical trial demonstrates how meeting standard physical activity guidelines benefits our underlying stress biology. These results suggest that breaking a sweat provides lasting physiological relief from chronic stress.

Peter Gianaros, a psychology researcher at the University of Pittsburgh, led the investigation alongside a team of health and neuroscience specialists. The research team wanted to understand if training the body to handle physical exertion also trains the brain and body to handle mental pressure. They designed a trial to observe these biological adaptations over an entire year.

When a person works out, their heart rate rises and their body experiences a temporary form of physical stress. Over time, regular exercise makes the cardiovascular system more efficient at handling this physical load. Researchers have proposed a concept called the cross-stressor adaptation hypothesis to explain how this works.

This hypothesis suggests that the biological upgrades gained from regular physical activity might cross over to help the body manage emotional and psychological stress. A fitter cardiovascular system might react less intensely to an argument at work or a frustrating traffic jam. The research team set out to test this idea by looking at several biological markers, including cortisol.

The cross-stressor adaptation hypothesis also suggests that working out changes the physical structure of the brain. Regular physical activity might induce neuroplasticity, which is the brain’s ability to rewire itself. This rewiring could stabilize brain circuits that control the autonomic nervous system.

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The autonomic nervous system acts as the control center for our unconscious bodily functions, including heart rate and digestion. By improving the efficiency of this system, aerobic fitness could theoretically calm our biological reactions to negative emotions.

Cortisol is the primary stress hormone in humans. It plays a role in regulating metabolism, immune responses, sleep cycles, and overall mood. High levels of long-term cortisol exposure are linked to a higher risk of heart disease and metabolic disorders.

Most physiological tests measure cortisol in saliva or blood plasma. These methods only capture a brief snapshot of stress at a specific moment in time. However, cortisol also accumulates in human hair as the hair grows out of the scalp.

Testing hair clippings allows researchers to measure systemic cortisol exposure over several months. This retrospective measure gives scientists a much broader picture of a person’s chronic stress load. Gianaros and his team used this technique to see if physical training altered this long-term hormone accumulation.

The clinical trial involved 130 healthy adults between the ages of 26 and 58. Before the trial began, none of the participants exercised regularly. The researchers divided these participants into two distinct groups using a randomized process.

One group was assigned to complete 150 minutes of moderate to vigorous aerobic exercise every week for an entire year. This routine included brisk walking, jogging, or using stationary bikes. Participants attended supervised workout sessions and wore heart rate monitors to track their physical exertion.

To monitor their progress, the participants in the exercise group utilized wearable heart rate sensors. During the first six weeks, they aimed to keep their heart rates at a moderate intensity. After this initial adjustment period, they were encouraged to increase their workout intensity to a more vigorous level. Certified trainers checked in regularly to ensure everyone was exercising safely.

The second group served as a health information control group. These individuals received general educational materials about blood pressure and body composition. They were asked not to change their normal physical activity habits over the course of the year.

Throughout the year, the investigators tracked multiple biological and psychological markers in a laboratory setting. They drew fasting blood samples to check for systemic inflammation, blood sugar levels, and cholesterol. They also measured arterial stiffness by tracking how fast a pulse wave traveled from the heart to the calf.

The researchers also monitored heart rate variability, which tracks the tiny time differences between consecutive heartbeats. A higher heart rate variability usually indicates a relaxed, adaptable nervous system. A lower variability often points to elevated physical or emotional stress.

In addition to these cardiovascular checks, the participants underwent testing inside a magnetic resonance imaging machine. While inside the brain scanner, they completed frustrating psychological tasks to provoke a temporary stress response. These tasks involved processing conflicting information and receiving negative feedback while under a strict time limit.

During another scanner task, the participants viewed upsetting images chosen from a standardized psychological database. They were asked to actively try to change how they felt about the images by reframing the negative scenes into something more positive. This allowed the research team to monitor real-time brain activity, blood pressure changes, and emotion regulation skills during moments of mental strain.

After a year of observation, the researchers found a clear difference in the collected hair samples. Participants who completed the weekly aerobic exercise routines showed a drop in their accumulated hair cortisol compared to the control group. A year of brisk walking and jogging successfully reduced their overall systemic exposure to this stress hormone.

Gianaros explained the potential importance of this biological change. He stated, “The effect of exercise on long-term cortisol levels could be one of the mechanisms or benefits of exercise that protect against several diseases and some mental health conditions, but more research is needed to fully explore this possibility.”

The rest of the biological and psychological markers did not show obvious improvements. The exercise group did not experience notable changes in their cholesterol, blood sugar, or markers of systemic inflammation. Any minor shifts in these specific variables were not statistically significant.

During the brain scan tasks, the differences in mental stress reactions between the exercise group and the control group were also not statistically significant. A year of aerobic training did not seem to alter how the participants’ brains and blood pressure reacted to sudden, acute stressors. Their self-reported emotional reactions to the negative images remained largely the same as before the trial.

The research team noted a few reasons why the physical activity routine might not have improved all the measured health markers. One primary factor was the study’s strict enrollment criteria. To ensure safety, the investigators only selected adults who were remarkably healthy and free of underlying medical conditions.

Because the participants were already in good health, their biological markers might not have had much room to improve. This ceiling effect often makes it difficult to detect slight changes in cardiovascular disease risk factors among a healthy population. The clinical trial also took place during the early stages of the global pandemic.

Another limitation of the clinical trial involved the lack of questionnaires regarding daily life stressors. The investigators did not ask the participants to track their daily mood swings or environmental pressures at home. Because of this missing context, it is hard to know if the reduction in hair cortisol perfectly mirrored a reduction in everyday psychological stress.

Pandemic-related disruptions led to a higher rate of participant dropout than the team originally anticipated. A smaller final group size can limit the mathematical ability to detect subtle biological shifts. It remains unclear whether differing amounts of physical activity could produce broader biological changes.

Future studies might involve larger groups of people, including those who already have existing health conditions like high blood pressure or metabolic disorders. Testing different intensities of workouts could reveal exactly how much exertion is needed to optimize stress biology. Exploring these variables will help scientists build a more detailed map of how physical fitness protects the brain and body.

The study, “Effects of a year-long aerobic exercise intervention on neuroendocrine, autonomic, and neural correlates of stress, emotion, and cardiovascular disease risk in midlife adults,” was authored by Peter J. Gianaros, Lu Wan, Mia K. DeCataldo, Cristina Molina Hidalgo, Mark R. Scudder, George Grove, Abigail Shell, Chae Ryon Kang, E. Lydia Wu-Chung, Anna L. Marsland, Thomas W. Kamarck, Javier Rasero, and Kirk I. Erickson.