Our gut hosts an enormous population of bacteria, each species with its own niche (they feed on certain foods), which outnumbers our own cells 100-fold. Most of these bacteria are good bacteria, though. In fact, you couldn’t survive without most of them! They’re among the best decomposers, breaking down dead and organic matter otherwise impossible by the gut alone. But while bacteria help us digest food and ward off threatening microbes, it may be the case that bacteria aren’t serving us, but we are serving the bacteria. A recent study which analyzed recent scientific literature found that microbes influence human eating behavior and dietary choices to favor consumption of the particular nutrients they grow best on, rather than passively consuming what they have around.
Our bacterial overlords
It’s yet unclear how the gut microbiome influences our dietary patterns – if the authors are right – but the researchers hypothesize the bacteria must be 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. The research was carried out by scientists at UC San Francisco, Arizona State University and University of New Mexico.
“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.”
Fortunately, the researchers write that the relationship goes both ways. Because the bacterial population can evolve and change substance 180 degrees in as little time as 24 hours, we can influence the microbiome by deliberately altering our diet.
“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.”
Research shows that 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. By altering the neural signals in the vagus nerve, bacteria can influence mood and manipulate behavior by changing taste receptors, producing toxins to make us feel bad, and releasing chemical rewards to make us feel good. For instance, tests on mice showed these became more anxious when exposed to certain strains of bacteria. In humans, one clinical trial found that drinking a probiotic containing Lactobacillus casei improved mood in those who were feeling the lowest.
While still to be conclusive, the findings suggest that our gut bacteria greatly influence the decisions we make regarding what goes inside our tummies. Other researchers believe this should be further test out. For instance, the Japanese often include seaweeds in their diets, so they have a specialized bacteria that digest seaweed. Would transplanting Japanese gut bacteria, including seaweed bacteria, cause the person in question to crave for seaweed? It’s a tough question to answer, but with enough scrutiny it might shed further light.
An opportunity to exploit bacteria and fight bad health
In any event, scientists are especially interested in the microbiome because of its enormous potential to influence health. By changing food and supplement choices, ingesting specific bacterial species in the form of probiotics, or by killing targeted species with antibiotics we can purposely balance the bacteria species living inside our guts. This might allow us to lead less obese and healthier lives, according to the authors.
During an UCSF experiment, researchers recruited a pair of twins in which one was obese and the other lean. Gut bacteria from the twins was transplanted into mice, coupled with a low-fat diet. Bacteria from the lean twin took over the gut of a mouse that already had bacteria from a fat twin, prompting the mouse to lose weight. Regardless of diet, bacteria from a fat mouse did not take over in a mouse that is thin. This suggests it may be possible to fight obesity simply by changing the microbiome. Microbes are always busy digesting food, but some kinds of bacteria harvest more or less energy than others. In the case of obese individuals, you want those kinds of bacteria that leave fewer calories for you.
“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.