Your Bacteria May Be Telling You What to Eat

You hold in your hands a delicious slice of pizza. Globs of grease sparkle on its cheesy surface. The toppings are piled on and arrayed splendidly. Is this food or art? You start to salivate. But you're not the only one.

Within your gut, bacteria of the phylum Bacteroidetes are salivating, too (figuratively, of course). Fat is one of their favorite foods, and they're about to receive a feast.

Researchers have long pondered our relationship with the 100 trillion bacteria residing within our guts. It seems these squatters dip their flagella in a multitude of pots. Our resident bacteria aid in digestion, influence the immune system, change how we store fat and metabolize sugar, and even prevent allergies.

Therefore, it might come as little surprise that they may also be controlling our minds.

In an article published in the September issue of BioEssays, scientists Joe Alcock, Carlo C. Maley, and C. Athena Aktipis reviewed the research on how microbiota affect the brain, and believe there's a strong case that bacteria influence overall eating behavior. It seems that the bacteria in our guts don't simply wait for whatever leftovers we have to offer. They actively seek out their preferred meals through tricksy deception.

“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,” Aktipis says.

Around 100 million neurons are stationed in the gut, collectively forming the enteric nervous system, also called the "Second Brain." The enteric nervous system is connected to the human brain via the vagus nerve. Thanks to this setup, bacteria are granted streamlined pass to the brain, and they're equipped to take advantage. For example, microbes have genes that allow them to produce hormones like serotonin and dopamine.

Studies in humans have shown that probiotics can improve mood. In mice, the affects are even more pronounced. When a team transplanted the gut bacteria from fearless mice into more anxious mice, the anxious mice began displaying markedly bolder behavior. The behavioral change worked in reverse, too.

"Like microscopic puppetmasters, microbes may control the eating behavior of hosts through a number of potential mechanisms including microbial manipulation of reward pathways, production of toxins that alter mood, changes to receptors including taste receptors, and hijacking of neurotransmission," the researchers write.

Basically, bacteria will send positive signals to the brain when you eat foods that they like, and negative signals when you eat foods they don't like.

Since fecal and oral bacteria can be transferred between individuals, particularly among people living together, the authors consider an intriguing possibility.

"The obesity epidemic could be contagious as a result of obesity-causing microbes transmitted from person to person. A social network study of 12,067 people found that a person’s chance of becoming obese increased by 57% if a friend had become obese. This raises up the possibility that cravings and associated obesity might not be socially contagious as the authors of the social network study suggest, but rather truly infectious, like a cold."

The authors offered up some routes for further research. For instance, they hypothesize that populating an animal's gut with a microbe that feeds on seaweed will cause the animal to develop a preference for seaweed. They also suggest that a diverse microbiome will inhibit manipulation on the host because the microbes will be too busy competing or cooperating with each other.

Far more controlled research on humans will be needed before we can truly gauge the extent to which bacteria control our food preferences.

Source: Alcock, J., Maley, C. C. and Aktipis, C. A. (2014), Is eating behavior manipulated by the gastrointestinal microbiota? Evolutionary pressures and potential mechanisms. Bioessays, 36: 940–949. doi: 10.1002/bies.201400071

(Image: San Diego State)