While depression is a well-known phenomenon that affects the mind, new research is uncovering the role the gut may play.
Research published on December 6 in the journal Nature revealed that certain strains of bacteria might protect from—while others contribute to—a depressive state.
The study identified over a dozen groups of bacteria correlated with an increased chance of experiencing depressive symptoms as an adult.
Certain gut bacteria were depleted in some of the patients experiencing depression, while some others with the condition had high levels of certain other strains of bacteria.
The findings, said the authors, are not conclusive. The results do not prove that gut bacteria are the ultimate cause of or protector against depression.
While not involved in the study, Dr. Mayer, director of the Oppenheimer Center for Neurobiology of Stress at the University of California, Los Angeles David Geffen School of Medicine, said the results point to a “general disease effect,” not necessarily bacteria as the source of depression.
Scientists have increasingly investigated the role of the gut microbiome, the trillions of bacteria and microorganisms that reside in the digestive system. More than just assisting with digestion, these microbes are active in the body’s immune defense, inflammation systems, and other critical functions.
While past research found that gut bacteria may affect the chance of developing depression, the latest findings further verify the linkage. They also identified new microbes, according to the study’s coauthor, Najaf Amin of Oxford University in England.
However, like Mayer, Amin stressed that the findings do not prove a causal relationship.
For example, depression can lead people to consume unhealthy food, which affects a person’s gut microbiome makeup. In other words, the bacteria may not be the source of depression. Instead, depression may drive poor diet, which results in a particular microbiome profile.
Still, Amin’s team said gut bacteria may play a role. Some bacteria analyzed during the study were found to assist in synthesizing certain brain chemicals like serotonin, responsible for regulating mood, among other functions.
The study authors said that regardless of the role gut bacteria plays in causing or preventing depression, understanding the correlation is valuable since it may help with diagnoses.
If the bacteria associated with depression can be identified, said Amin, it could help identify “a biomarker for depression.” This insight would allow doctors to objectively assess the disorder, something “which is lacking at the moment,” said Amin.
The findings were generated by analyzing two studies, one with nearly 2,600 Dutch adults and another with over 3,000 Dutch adults. The studies revealed that, for the most part, people with depression exhibited low levels of gut microbes. However, some bacteria, like Sellimonas, were abundant in individuals with depression.
Mayer is more skeptical. He said the latest findings implicate numerous groups of gut bacteria each with a range of strains that have, in some cases, opposite functions. As a result, he doubts the results are “clinically relevant.”
One theory is that depression may be driven by an imbalance in gut bacteria, with too little or too much causing the condition. Further research is needed to test the theory, explained Amin.