Does dirt make us happier?

“A rise in depression among young people may be due to the modern world being too clean,” reported The Daily Telegraph.

The authors have conducted a narrative review, using information from laboratory and human studies to explore the idea that there may be a relationship between major depressive disorder and exposure to some types of bacteria. They say that people with this condition can show more extreme immune responses (including inflammation) to stress and that artificially induced inflammation can trigger depression-like symptoms. They argue that the improvements to hygiene that have reduced the risk of infectious disease may also have disrupted evolutionary relationships with micro-organisms that may have a beneficial influence on health, including mental health.

Development and testing of new hypotheses is vital to scientific progress. In complex illnesses such as depression, new insights into the causes or risk factors may be gained from a variety of scientific fields. While this study does not provide any definitive evidence of a causal link between exposure to micro-organisms and the development of depression, it may provide researchers with a new line of investigation.

Where did the story come from?

This study was carried out by researchers from Emory University School of Medicine, Atlanta, the University of Colorado and University College London. It was published in the peer-reviewed medical journal Archives of General Psychiatry . The researchers were funded by the US National Institutes of Health, the US Centers for Disease Control, the National Science Foundation, the Bill and Melinda Gates Foundation and several other organisations.

The Daily Telegraph covered this story. It did not explain that it was a review of existing research and the headline implies a causal relationship between cleanliness and depression, which was not definitively supported by the findings. The emphasis on the prevalence of depression among young people does not reflect the study, which focused more on immunological processes and the role of specific types of micro-organism in moderating inflammation.

What kind of research was this?

The aim of this research was to investigate whether the reduction in levels of certain micro-organisms in our food, soil and the gut, is contributing to any increasing prevalence of depression. This was a narrative review of the scientific literature on studies relating to depression and inflammation. The authors present evidence on a number of related themes. They do not explicitly specify how they identified and included the studies in this review.

Previous research has identified that psychological stresses can trigger inflammatory responses in the immune system. Whether inflammation has a role in the development of psychological symptoms and illnesses is less clear, and it is this question that the authors set out to consider. They specifically look at the role of “old friends”, which are micro-organisms that have co-evolved in such a way that they may provide some benefit to human health.

As this was a non-systematic review, it is not possible to assess whether there were any biases in how the researchers selected the studies they included, or whether any studies that were left out may have given a different result had they been included. In addition, without carrying out a meta-analysis of the previous findings it is difficult to quantify any effect of inflammation on depression, and to compare this with the effect of other established risk factors for depression.

What did the research involve?

The authors identified a range of previous studies on the topic of inflammation, stress and depression. These ranged from laboratory studies in cells and animals, to long-term studies that had examined human health for a number of years. The researchers summarised the studies’ findings on several themes:

the role of stress as a trigger for inflammatory processes and how inflammation may in turn trigger depressive behaviours
how potential environmental triggers of inflammation (for example, sedentary lifestyles, diet and smoking) have changed in prevalence in recent decades
how the prevalence of depression has risen, particularly in the young

They discuss the “old friends” hypothesis that the increase in inflammatory disease may be partially explained by the disruption to evolutionary relationships between humans and micro-organisms found in the body and the environment, and suggest ways in which this may influence the risk of developing depression. They conclude by suggesting avenues for future research.

What were the basic results?

The researchers do not present any figures that summarise the collective data. The emphasis of this study was a narrative discussion of the current evidence and the generation of theories relating to the potential role of micro-organisms in the development of depression.

How did the researchers interpret the results?

The authors state that “multiple lines of circumstantial evidence point to a potential role for the old friends in the pathogenesis and development of [major depressive disorder]”. They suggest that the “same cultural practices that have decreased infectious morbidity have also deprived us of contact with a range of microorganisms, mostly derived from mud, animals, and feces, which had been entrusted through coevolutionary mechanisms with the task of modulating essential human immune regulatory systems”. In other words, by reducing the incidence of infectious disease through good hygiene, some of the beneficial effects of micro-organisms may have been lost.

The authors go on to suggest that certain micro-organisms might be useful in reducing depressive symptoms in affected people in industrialised countries. They point out that “studies addressing potential antidepressant properties of the old friends in a rigorous manner in humans are few and are suggestive rather than conclusive”.

Conclusion

This was an extensive overview of the potential role of inflammation and the immune system in the development of depression. The paper puts forward several arguments, which are mainly speculative and it is not possible to draw firm conclusions without further evidence to support the many biological mechanisms proposed here. Although there may be corresponding trends in the incidence of major depressive disorders and general standards of cleanliness, a causal relationship cannot be established without looking at individual people and their exposure to environmental risk factors and development of depression.

Depression and related mental health illnesses have complex causes, which will vary between individuals. Risk factors can include genetics, medical health, and environmental, social and life circumstances.