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A recent study from the University of Denver, United States, suggests that dogs might have a deeper and more biologically complex effect on humans than scientists previously believed.

"The human response to stress is a finely tuned and coordinated set of various physiological pathways. Previous studies of the effects of dogs on human stress focused on just one pathway at a time. For our study, we zoomed out a bit and measured multiple biological indicators of the body’s state, or biomarkers, from both of the body’s major stress pathways," co-authors Kevin Morris and Jaci Gandenberger write.

This allowed the team to get a more complete picture of how a dog’s presence affects stress in the human body.

The stress pathways the scientists measured are the hypothalamic-pituitary-adrenal, or HPA, axis and the sympathoadrenal medullary, or SAM, axis.

"When a person experiences a stressful event, the SAM axis acts quickly, triggering a 'fight or flight' response that includes a surge of adrenaline, leading to a burst of energy that helps us meet threats. This response can be measured through an enzyme called alpha-amylase," Morris and Gandenberger explain.

At the same time, but a bit slower, the HPA axis activates the adrenal glands to produce the hormone cortisol. "This can help a person meet threats that might last for hours or even days. If everything goes well, when the danger ends, both axes settle down, and the body goes back to its calm state."

Today, we get uncomfortable even hearing the word stress, but it has been important to human survival. Our hunter-gatherer ancestors had to respond effectively to animal attacks and other threats.

In such instances, over-responding could be as ineffective as under-responding, and staying in an optimal stress response zone maximized humans' chances of survival.

"After cortisol is released by the adrenal glands, it eventually makes its way into your saliva, making it an easily accessible biomarker to track responses. Because of this, most research on dogs and stress has focused on salivary cortisol alone," Morris and Gandenberger say.

For example, past studies have found that people exposed to a stressful situation have a lower cortisol response if they are with a dog than if they are alone—even lower than if they are with a friend. And while these papers have shown that having a dog nearby can make us calmer during a stressful event, the team from the University of Denver suspected that was just part of the story and recruited about 40 dog owners to participate in a 15-minute gold-standard laboratory stress test, which involves public speaking and oral math in front of a panel of expressionless people posing as behavioral specialists.

"The participants were randomly assigned to bring their dogs to the lab with them or to leave their dogs at home. We measured cortisol in blood samples taken before, immediately after, and about 45 minutes following the test as a biomarker of HPA axis activity. And unlike previous studies, we also measured the enzyme alpha-amylase in the same blood samples as a biomarker of the SAM axis," Morris and Gandenberger explain.

As was expected based on previous studies, the people who had their dog with them showed lower cortisol spikes. But the results also showed that people with their dog present experienced a clear spike of alpha-amylase, while those without their pet showed almost no response.

"No response may sound like a good thing, but in fact, a flat alpha-amylase response can be a sign of a dysregulated response to stress, often seen in people experiencing high stress responses, chronic stress, or even PTSD," the co-authors add.

(A lack of response is exacerbated by chronic or overwhelming stress that can change the way our nervous system responds to stressors.)

So, the participants with their dogs had a more balanced response: their cortisol didn't spike too high, but their alpha-amylase still activated.

"This shows that they were alert and engaged throughout the test, [and] then able to return to normal within 45 minutes. That's the sweet spot for handling stress effectively. Our research suggests that our canine companions keep us in a healthy zone of stress response," Morris and Gandenberger conclude.

Dogs aren't just good company, they might be one of the most accessible and effective tools for staying healthy in a stressful world. No wonder we're so attached to them!