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.