The role of psychological and physical stress in relation to the microbiome

Chronic stress from psychological or physical factors has been shown to increase stress hormones, like cortisol, as well as inflammation and there is some evidence that mind-body interventions can be helpful in reversing these effects[80]. Chronic stress also can increase vulnerability to infection[81] and imbalances in the intestinal microbiome[82]. Greater psychological and physiological stress can result from many sources[12] and lead to reduction in secretory IgA[83], which is essential for protection from microbial invasion. A study in mice showed that lack of secretory IgA led to a COPD-like condition, as bacteria invaded the epithelial lining of the lung[84].
Stress reduction methods, such as diaphragmatic breathing, have shown benefits and a recent article[85] aimed at health care workers that must cope with intense psychological pressures in the current pandemic have suggested a number of “micropractices,” including breathing practices. These methods could potentially help reduce stress even when minimal time is available. A recent review of diaphragmatic breathing (taking a deep breath, allowing the lung to expand toward a relaxed abdomen, holding it briefly and letting the breath out slowly) showed that such breathing practices reduced cortisol and resulted in other benefits as well[86].
The elevated cortisol associated with chronic stress and aging appears to be associated with inflammation[87] and has been linked to susceptibility to infection and difficulty recovering from stressors such as surgery[88]. Elevated cortisol is also associated with insulin resistance, which is linked to obesity, cardiovascular disease, hypertension and diabetes[89]. Interestingly, research is still clarifying the effects of cortisol on immune function. There is evidence that the effects are complex, with moderate levels of cortisol having delayed immune effects that can potentiate a pro-inflammatory response[90,91].
The hypothesis that stress, in some circumstances, may be part of a protective mechanism to cause avoidance of toxins and microbes that elicit allergic host defense responses has been discussed in detail elsewhere[12]. In 2 studies stress-related neuropsychiatric disorders have been associated with allergies[92,93]. In support of the causal role of allergy in this association, anxiety-like behavior has been observed in mice in response to small amounts of allergens in their cages, and this was found to be dependent on classical allergic mechanisms involving mast cells and IgE[94]. Cortisol releasing factor (CRF) increases in the brain along with the anxiety-like behavior resulting from trace amounts of allergen in the cage was observed in similar rat model experiments[95]. Thus, the potential for physiological stress to result from allergy/hypersensitivity responses to both inhaled and ingested substances should be considered.