Is Your Gut Ruling Your Brain on Food Choices?

Pam Harrison

September 03, 2014

People may no longer have to blame their food cravings — and giving in to them — on poor self-control.

Now scientists are arguing that microbes living in the gut are responsible for manipulating eating behavior by causing cravings for food they favor for fitness or that suppress their competition. Alternatively, microbiota in the gut may send out signals via the vagus nerve to the brain to induce dysphoria and goad people into eating what the microbe needs whether it's good for the host or not, a diverse group of researchers are suggesting.

"Bacteria within the gut are manipulative," Carlo Maley, PhD, director, Center for Evolution and Cancer, University of California at San Francisco, states in a press release. "There is a diversity of interests represented in the microbiome, some aligned with our own dietary goals and others not."

In their overview of eating behavior and the microbiome published online August 7, 2014 in BioEssays, Joe Alcock, MD, University of New Mexico, Albuquerque, and colleagues argue that certain microbes are highly dependent on the nutrient composition of the diet.

For example, Prevotella grows best on carbohydrates while dietary fiber provides a competitive advantage to Bifidobacteria. "Even microbes with a generalist strategy tend to do better on some combinations of nutrients than others," the authors write, "and competition will determine which microbes survive."

Microbes can manipulate host behavior in a variety of ways, but one way may be by "hijacking" the host's nervous system. As the authors point out, evidence shows that microbes can have dramatic effects on behavior through the microbiome-gut-brain axis.

 
The vagus nerve is a central actor in this communication axis, connecting the 100 million neurons in the enteric nervous system in the gut to the base of the brain at the medulla.
 

"The vagus nerve is a central actor in this communication axis, connecting the 100 million neurons in the enteric nervous system in the gut to the base of the brain at the medulla," they explain. And, they add, enteric nerves have receptors that react to the presence of particular bacteria as well as to bacterial metabolites. Research has also shown that blockade or transection of the vagus nerve causes drastic weight loss while stimulation of its activity through norepinephrine appears to drive excessive eating behavior in satiated rats.

Other pathways through which microbes may influence host eating behavior is through secretion of hormones involved in mood and behavior, including dopamine and serotonin. Microbes may also manipulate eating behavior by altering receptor expression. Changes in taste receptor expression and activity have been reported following gastric bypass surgery, a procedure that changes gut microbiota and alters satiety and food preference, as the authors point out.

"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," coauthor Athena Aktipis, PhD, Arizona State University, Phoenix, states in a press release.

"Together, these results suggest that microbes have opportunities to manipulate vagus nerve traffic in order to control host eating. Exerting self-control over eating choices may be partly a matter of suppressing microbial signals that originate in the gut."

Happily, researcher add, the use of prebiotics, probiotics, antibiotics, fecal transplants, and dietary changes can rapidly alter the microbiome within 24 hours of administration.

The study was funded by the National Institutes of Health, the American Cancer Society, the Bonnie D. Addario Lung Cancer Foundation, and the Institute for Advanced study in Berlin, Germany. The authors have disclosed no relevant financial relationships.

BioEssays. Published online August 7, 2014. Abstract

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