Scientists from the University of California–San Francisco, Arizona State University, and the University of New Mexico concluded from a literature review that gut microbes influence human eating behavior and dietary choices to favor consumption of the particular nutrients they grow best on instead of simply passively living off whatever nutrients we choose to send their way.
While it is unclear exactly how this occurs, the researchers believe the gut microbiome may influence our decisions by releasing signaling molecules into our gut. Because the gut is linked to the immune system, the endocrine system, and the nervous system, those signals could influence our physiologic and behavioral responses.
"Bacteria within the gut are manipulative," said Carlo Maley, Ph.D., director of the UCSF Center for Evolution and Cancer and corresponding author on the team’s paper (“Is eating behavior manipulated by the gastrointestinal microbiota? Evolutionary pressures and potential mechanisms”), which appears in BioEssays. "There is a diversity of interests represented in the microbiome, some aligned with our own dietary goals, and others not."
However, it turns out that we can influence the compatibility of these microscopic, single-celled microbes by deliberating altering what we ingest, added Dr. Maley, with measurable changes in the microbiome within 24 hours of diet change.
"Our diets have a huge impact on microbial populations in the gut," he continued. "It's a whole ecosystem, and it's evolving on the time scale of minutes.:
There are even specialized bacteria that digest seaweed, found in humans in Japan, where seaweed is popular in the diet.
Research suggests that gut bacteria may be affecting our eating decisions in part by acting through the vagus nerve, which connects 100 million nerve cells from the digestive tract to the base of the brain.
"Microbes have the capacity to manipulate behavior and mood through altering the neural signals in the vagus nerve, changing taste receptors, producing toxins to make us feel bad, and releasing chemical rewards to make us feel good," noted Athena Aktipis, Ph.D., co-founder of the Center for Evolution and Cancer with the Helen Diller Family Comprehensive Cancer Center at UCSF and who is currently in Arizona State University’s department of psychology.
In mice, certain strains of bacteria increase anxious behavior. In humans, one clinical trial found that drinking a probiotic containing Lactobacillus casei improved mood in those who were feeling the lowest. The researchers proposed further research to test the sway microbes hold over us. For example, would transplantation into the gut of the bacteria requiring a nutrient from seaweed lead the human host to eat more seaweed?
The speed with which the microbiome can change may be encouraging to those who seek to improve health by altering microbial populations. This may be accomplished through food and supplement choices, by ingesting specific bacterial species in the form of probiotics, or by killing targeted species with antibiotics. Optimizing the balance of power among bacterial species in our gut might allow us to lead less obese and healthier lives, according to the authors.
"[In our paper] we also review the evidence for alternative explanations for cravings and unhealthy eating behavior," the investigators wrote. "Because microbiota are easily manipulatable by prebiotics, probiotics, antibiotics, fecal transplants, and dietary changes, altering our microbiota offers a tractable approach to otherwise intractable problems of obesity and unhealthy eating."