One of the greatest advances in modern society is sanitation, but a new study suggests that hygiene itself may contribute to rising rates of depression*. The authors note that : 1) rates of inflammatory diseases like asthma, allergies, multiple sclerosis continue to rise in modern countries, 2) inflammatory diseases and depression tend to occur together, and 3) both inflammatory diseases and depression correlate with elevated levels of inflammatory markers in the blood.
Following this logic, researchers have recently started to look for connections between inflammatory processes and modern societies. Some guessed that reducing childhood illness may have reduced the “natural” activity of the immune system, leading to higher “abnormal” immune activity. However, this research has been disappointing and has failed to show an association between decreased childhood infection and increased autoimmune illness.
Another idea is the “Old Friends” hypothesis**, which suggests that the disruption of human association with ancient microorganisms once present inside and outside the human body has led to increased inflammatory disease. In other words, before modern hygiene and sanitation, humans were constantly surrounded by and colonized by benign microorganisms which “trained” the human immune system. Now that we wash dirt off our food and bodies and clean our water supply, we are no longer exposed to the “Old Friends” and our immune system overreacts due to lack of proper “training”. One recent study*** which supports this hypothesis showed that rats given probiotics reversed behaviors and changes in inflammatory markers in a model of stress and depression.
The implications of this work are fascinating and profound. If true, depression may be treatable by taking probiotic supplements or topical treatment. Will public health officials soon tell us to stop washing our food and get out in the dirt?
* Inflammation, Sanitation, and Consternation : Loss of Contact With Coevolved, Tolerogenic Microorganisms and the Pathophysiology and Treatment of Major Depression. Raison et al. (2010). Archives of General Psychiatry. 67:1211.
** Give us this day our daily germs. Rook and Brunet (2002) Biologist (London) 49(4):145-149.
***Effects of the probiotic Bifidobacterium infantis in the maternal separation model of depression. Desbonnet et al. (2010) Neuroscience.170(4):1179-88.