In the March 2013 issue of Evolution, Medicine, and Public Health, Graham Rook et al. review a multitude of epidemiological data to highlight links between sanitation, psychiatric disorders and stress resilience.
At the core of this concept is the Hygiene Hypothesis and “Old Friends” mechanism, which suggest that increased sanitation accounts for increases in inflammatory disorders, specifically because decreasing exposure to the microbes, chronic infections, or helminths with which we co-evolved leads to unchecked inflammation. In the past, and currently in developing countries, humans were more frequently exposed to animals, mud, untreated water, and feces. When ingested, the helminths and pseudocommensal bacteria contained therein alter the microbiota, expand regulatory T cell populations, and modulate dendritic cells, leading to net immunoregulation.
Various studies have shown elevated proinflammatory cytokines in depression, as well as exaggerated cytokine response to social stressors. Both of these are more prominent in higher income countries and in urban as compared to rural areas. In addition, when comparing immigrants from a lower to higher income country with their birth population, the immigrants have higher rates of both various chronic inflammatory diseases and psychiatric disorders.
The findings summarized by Rook et al. suggest that defective immunoregulation secondary to diminished “Old Friends” exposure impairs stress resilience, thus leading to a vicious cycle of depression, psychosocial stress, and unmitigated inflammation. Rook et al. furthermore suggest specific research questions that have yet to be investigated to further clarify these complex interrelationships.
Source: Evolution, Medicine, and Public Health. 2013; 1:46-64.