The bacteria found in our digestive tracts may be affecting both our cravings and moods, and may even push us toward obesity, according to a new analysis published in the journal BioEssays.
Based on a review of recent scientific literature, researchers from University of California, San Francisco (UCSF), Arizona State University, and University of New Mexico found that the microbes living in our digestive tracts cause us to crave the particular nutrients they need to grow on, rather than passively living off whatever nutrients we happen to consume.
Each bacterial species thrives on specific nutrients. Some prefer fat, and others sugar, for example. They compete with each other for food and try to retain a niche within their ecosystem (our digestive tracts).
While the exact mechanism is still unknown, the researchers believe this diverse community of microbes — known as the gut microbiome — may influence our food choices by releasing signaling molecules into our gut. Since 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 paper. “There is a diversity of interests represented in the microbiome, some aligned with our own dietary goals, and others not.”
“The good news is that it goes both ways, and the bacteria is easily altered. We can influence the compatibility of these microbes by deliberating changing what we eat,” Maley said, “with notable changes within 24 hours.”
“Our diets have a huge impact on microbial populations in the gut,” Maley said. “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.
“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,” said senior author Athena Aktipis, Ph.D., co-founder of the Center for Evolution and Cancer with the Helen Diller Family Comprehensive Cancer Center at UCSF.
In mice, certain strains of bacteria increase anxious behavior. In humans, one study found that drinking a probiotic containing Lactobacillus casei enhanced the mood of depressed people.
The researchers have proposed more research to determine the influence microbes hold over us. For example, would transplantation of a gut bacteria that requires a nutrient from seaweed lead the person to eat more seaweed?
“Targeting the microbiome could open up possibilities for preventing a variety of disease from obesity and diabetes to cancers of the gastro-intestinal tract. We are only beginning to scratch the surface of the importance of the microbiome for human health,” said Aktipis.
It is encouraging that the microbiome is quick to change when we alter our food and supplement choices, consume probiotics, or kill a 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, said the researchers.
“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,” the authors wrote.