Can Gut Microbes Cause Aggression?

“The tendency to aggression is an innate, independent, instinctual disposition in man… it constitutes a powerful obstacle to culture.” –Sigmund Freud

Does gut health affect aggression?

Source: Midjourney

Omry Koren and colleagues at Bar Ilan University in Israel transplanted fecal microbes from human infants to mice who then became aggressive. What was it about those infants that caused the aggression? The one-month-old babies had received antibiotics which damaged their collection of gut microbes called the microbiome. When a wild-mouse microbiome was subsequently transferred to those mice, the aggression disappeared. However, the reduction in aggression was more pronounced in younger mice, making early intervention an important factor in recovery.

Degrees of Aggression

Aggression is deeply rooted in our psyche and is at least part of the story of our global dominance. We all exhibit some degree of aggression and in wartime we valorize it. But in peacetime, overly aggressive behavior is often met with jail time. Extremes of aggression are considered pathological and create a heavy burden on society.

Surprisingly few studies have been done on human aggression, partly because there are so many factors involved and they are difficult to disentangle. Therefore, much of what we know is based on animal studies that are easier to control.

As a general rule, administering glucocorticoids to animal brains increases aggression, and that behavior in turn increases glucocorticoid production, resulting in a vicious cycle. Studies with mice have shown that lower levels of dopamine are also involved in aggressive behavior. Both factors are influenced by the gut microbiome.

Mice born and raised in a sterile environment, so-called germ-free mice, were found to be more aggressive than mice with a normal microbiome. This behavior was often long-lasting.

Katsunaka Mikam and colleagues at Tokai University School of Medicine in Japan looked at aggression in other species as well. Their studies with dogs show a similar association between gut microbes and aggression. They found a significant decrease in Bacteriodaceae and an increase in Erysipelotrichaceae among aggressive dogs. The connection was good enough that an examination of gut microbes could diagnose aggressive behavior before it manifested. Interestingly, levels of hormones like testosterone didn’t explain the difference in behavior, but microbial metabolites did.

Another study, by Thomas Sharpton and colleagues at Oregan State University, found a similar association when looking at gut microbes of pit bulls rescued from dogfighting. The most aggressive dogs had a noticeably different microbiome than the more docile dogs. Although these are associational studies and can’t identify causation, when you combine them with the fecal transplant studies, a direction emerges: unbalanced gut microbiomes can trigger aggression.

This has implications for the treatment of these dogs, since their problems may be temporary. Instead of putting these animals down, it might be possible to rescue them with diet or probiotics.

Human Studies

Among the few studies with humans, Bangtao Chen and colleagues at Chongqing University found that the level of aggression in schizophrenic patients coincided with a difference in their microbiomes. Those patients with aggressive behavior had markers of leaky gut and systemic inflammation. The diversity of their gut microbiome was lower overall. The abundance of beneficial Bifidobacteria was lower, while Prevotella was higher.

Importantly, levels of butyrate – an important nutrient for the gut lining – were much lower in the aggressive group. Butyrate, which is created by beneficial bacteria, is also important because it can make its way to the brain where it stimulates the growth of new neurons, improving brain function.

All of these studies found that an unbalanced, diminished, or less diverse gut microbiome lies at the root of aggression. Several factors affect gut health, but a diet that is high in fats and sugars and low in fiber can severely damage the microbiome.

Adding to Aggression

Is our national diet promoting aggressive behavior? There are many variables involved, but it’s interesting to note that processing food by extracting fiber commenced around 1950. Give kids a few years to grow up with processed food, and it might not be surprising to see an uptick in crimes of aggression among young adults. In fact, rates of violent crime took off in the 1960s.

A 2009 study by Simon Moore and colleagues at Cardiff University found that 70% of violent offenders ate candy every day throughout their childhood, versus only 40% of nonviolent people. In light of subsequent gut-brain revelations, it is possible that this result is due to gut-microbiome damage caused by excess sugar in the diet.

Could diet be part of a program to help aggressive prisoners chill out? It may be worth a try; the cost to the prison is nothing. Studies have shown that a high-quality diet is no more expensive than a poor-quality one. Some prisons have heeded the call and set up agricultural programs that have improved mental health and reduced recidivism.

In Sum

We need more studies to flesh out the specific microbial components of aggression, but these studies are a promising start. Some seemingly innate personality traits may be due more to what’s in our gut than what’s in our head. Since we have some control over our microbiome, that gives all of us hope that we can be our best, just by attending to our diet.