Of the 18,019 subjects, 14.11% were exposed to probiotics. These subjects tended to be white, older, female, and healthier, and tended to have higher family income. Moreover they were less likely to be smokers, had more fiber in their diet, and consumed fewer calories than subjects not exposed to probiotics.
Among those subjects who had been exposed to probiotics, 91.63% were exposed only through food, 6.53% only through supplements, and 1.85% through both.
For mild or more severe depression, the overall prevalence was 22.80% (PHQ-9 score ≥5) and 7.59% for moderate or more severe depression (PHQ-9 score ≥10). Subjects exposed to probiotics were less likely to have depression (unadjusted OR for mild or more severe depression 0.69, 95% CI 0.58-0.83); unadjusted OR for moderate or more severe depression 0.58, 95% CI 0.45-0.76).
However, the association between probiotic exposure and depression disappeared after adjustment for confounding factors, and was no longer significant. For mild or more severe depression, the OR was 0.85 (95% CI 0.70-1.03) and for moderate or more severe depression, the OR was 0.82 (95% CI-0.61-1.1). The results remained unchanged, even after excluding subjects who were exposed to probiotics through supplementation rather than through food. Moreover after adjustment for potential confounders, it was found that consuming higher amounts of probiotics did not contribute to reduction in depressive symptoms.
After adjustment, women, current smokers, and subjects with very lower income were found to have higher odds of experiencing depression. Higher fiber intake was associated with lower odds of depression, while poor health status was the strongest predictor of depression.
Analysis and Discussion
Major depressive disorder is “being viewed as a psychoneuroimmunological disorder that includes not only the traditional monoamine deficiency, but also a persistent low grade inflammation,” the authors comment. Major depression is associated with increased levels of inflammatory markers, and the gut microbiota is “now being viewed as a virtual endocrine organ that has bidirectional communication with the central nervous system through the microbiome-gut-brain axis.”
The authors theorize that probiotics could, directly or via influence of other gut microbes, inhibit gut and systemic inflammation and activate the microbiome-gut-brain axis. They note that probiotics “directly protect the intestinal barrier; limit bacterial growth in the small intestine; decrease systemic cytokines, oxidative stress, and inflammatory markers; and increase microbial production of GABA and other neurotransmitters.”