This article is an excerpt from the Shortform book guide to "I Contain Multitudes" by Ed Yong. Shortform has the world's best summaries and analyses of books you should be reading.
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What’s the relationship between gut health and mental health? Can poor gut health cause or worsen depression or anxiety?
Ed Yong wrote I Contain Multitudes to raise awareness about the trillions of microbes that exist in our bodies and how they influence everything, including our mental health. Your gut health and mental health are closely connected—a lack of certain microbes can even lead to abnormal brain development, claims Yong.
Read on to learn more about the relationship between gut health and mental health, according to Yong.
Ed Yong on the Gut-Brain Connection
In I Contain Multitudes, Ed Yong explains that beyond immunity, research illuminates links between the gut microbiome, the immune system, and brain development. Because of the vagus nerve, which directly connects the brain to the gut (where trillions of microbes live), certain brain activities depend on signals from gut microbes. Therefore, Yong posits that the absence of certain microbes is linked to abnormal brain development. This two-way communication between the brain and the gut is called the gut-brain axis. In this article, we’ll explain how gut health and mental health are connected, according to Yong’s explanation of the gut-brain connection.
(Shortform note: As far back as the early 1800s, one British doctor proposed that mental disorders can all be traced back to “gastric derangement.” In the absence of modern technology, the doctor’s theory was based on his observations that emotional conditions like sadness and anxiety impact our appetite, physical damage to the stomach can impact mental health, and digestive issues also seem to cause mental decline. He concluded that the gut and mind were linked through the nervous system and recommended that people eat more natural, unprocessed foods—advice that aligns well with modern research.)
Examples of the Impact on Mental Health
In one of Yong’s examples of this phenomenon, people who experience major infections during their pregnancy are more likely to have autistic and schizophrenic children, and those children are also likely to have unusual gut microbiomes and gastrointestinal problems. The implication is that the abnormal microbiome (caused by the infection) gets transferred to the child, and the missing microbes then contribute to conditions in the gut as well as the brain.
(Shortform note: A more recent study concluded that infections during pregnancy are unlikely to directly cause autism, and the association between the two is more likely due to genetic and environmental factors.)
Yong also describes evidence showing how gut health is correlated with mental health. For example, research indicates that inserting a certain type of Bif (the bacteria initially fed by breast milk) into people with Irritable Bowel Syndrome (IBS) reduces symptoms of depression in those patients.
(Shortform note: Studies show that about 84% of people with IBS experience depression, and people with IBS are more likely to suffer from both anxiety and depression compared to healthy individuals. Although the association between IBS, depression, and anxiety is clear, the causes are complex and multifaceted. This means that while some scientists are hopeful that microbes can be used therapeutically, there’s no conclusive evidence that the presence or absence of a specific microbe actually causes IBS and depression.)
According to Yong, this is the first compelling evidence that gut health can change the pathology of mental health conditions in humans. In addition, Yong points to studies showing that when researchers manipulate the microbiomes of mice, it drastically changes their brain chemicals and social behavior. Although there are limited studies showing that microbes or gut health actually cause differences in brain development and activity, Yong argues that abnormal microbiomes are at least a symptom of many mental health conditions and illnesses.
(Shortform note: One of the challenges of studying mental conditions like depression and autism spectrum disorder is that they manifest in people in different ways, and diagnosis is not an exact science. This leaves room for misinterpretation of data when these conditions are translated into social behaviors in mice for research. Nonetheless, mouse models can still provide insight into potential mechanisms behind illnesses and social behavior.)
Western Lifestyles Alter Microbiomes
According to Yong, Western lifestyles are drastically changing the gut microbiomes of people living in wealthier, developed countries, which can be a cause of the negative mental health outcomes mentioned above. Yong cites contributing factors like our changing diet, lower exposure to microbes in our environment, and use of antibiotics. As a result of these factors, many species of microbes that were once ubiquitous in humans are going extinct, leading to a decrease in microbiome diversity. Although scientists don’t agree on the implications of this pattern, Yong suggests it may be responsible for the rise in inflammatory conditions (like allergies, asthma, and inflammatory bowel disease) in Western countries.
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Here's what you'll find in our full I Contain Multitudes summary:
- A deep dive into the mysterious and fascinating world of microbes
- How commercial probiotics have oversold health benefits
- How modern sanitation practices are harming us
