Food Cravings: Can Bacteria Control Your Brain?

Food Cravings

Some new science is suggesting that your food cravings could be influenced by forces other than YOU.

No way you say?

Consider this.

You're actually more microbe than human. Yep, 10 to 1 your cells are outnumbered by microbial cells living in and on you. And the federally funded Human Microbiome Project, is uncovering some bigger news scientists believe will affect the future of medicine.

Namely, our gut microbiome benefits us more than we ever knew- breaking down our food, fighting off infections and nurturing our immune system.

It’s a living, microscopic civilization we should be nurturing for our own well-being.

This year, in the journal Bioessays, a team of scientists has raised a creepier possibility... that perhaps our native germs are also influencing our behavior, such as, giving us cravings for certain foods they need to thrive.

The idea that a simple organism could control a complex animal may sound like science fiction. But in fact, there are many well-documented examples of parasites controlling their hosts.

"Operation Takeover"

Some species of fungi, for example, infiltrate the brains of ants and coax them to climb plants and clamp onto the underside of leaves. The fungi then sprout out of the ants and send spores showering onto uninfected ants below.

Just how parasites control their hosts remains mysterious. But it looks as if they release molecules that directly or indirectly influence their brains.

Our microbiome has the biochemical potential to do the same thing. In our guts, bacteria make some of the same chemicals that our neurons use to communicate with one another, such as dopamine and serotonin. And the microbes can deliver these neurological molecules to the nerve endings that line the gastrointestinal tract.

A number of recent studies have shown that gut bacteria can use these signals to alter the biochemistry of the brain.

Mind Control of Mice

One experiment compared ordinary mice with those raised free of gut bacteria and found the germ free mice behave differently in a number of ways. They are more anxious, for example, and have impaired memory.

Adding certain species of bacteria to a normal mouse’s microbiome reveals other ways germs can influence behavior. Some bacteria lower stress levels in the mouse. When scientists sever the nerve relaying signals from the gut to the brain, this stress-reducing effect disappears.

Another experiment suggests that bacteria also can influence the way their hosts eat. Germ-free mice develop more receptors for sweet flavors in their intestines, for example. They also prefer to drink sweeter drinks than normal mice do.

Scientists have also found that bacteria can alter levels of hormones that govern appetite in mice.

Dr. Maley and colleagues (Center for Evolution and Health) argue that our eating habits create a strong motive for microbes to manipulate us. “From the microbe’s perspective, what we eat is a matter of life and death,” Dr. Maley said.

A highly magnified view of Enterococcus faecalis, a bacterium that lives in the human gut. Microbes may affect our cravings, new research suggests. Image: Centers for Disease Control and Prevention

Different species of microbes thrive on different kinds of food. If they can prompt us to eat more of the food they depend on, they can multiply. Right?

Are Food Cravings Good or Bad?

Microbial manipulations might fill in some of the puzzling holes in our understandings about food cravings, Dr. Maley, said. Scientists have tried to explain food cravings as the body’s way to build up a supply of nutrients after deprivation, or as addictions, much like those for drugs like tobacco and cocaine.

But both explanations fall short. Take chocolate: Many people crave it fiercely, but it isn’t an essential nutrient. And chocolate doesn’t drive people to increase their dose to get the same high. “You don’t need more chocolate at every sitting to enjoy it,” Dr. Maley said.

John F. Cryan, a neuroscientist at University College Cork in Ireland suggested that microbes might also manipulate us in ways that benefit both them and us. “It’s probably not a simple parasitic scenario,” he said.

Research by Dr. Cryan and others suggests that a healthy microbiome helps mammals develop socially. Germ-free mice, for example, tend to avoid contact with other mice.

Social bonding is good for the mammals. But it may also be good for the bacteria.

“When mammals are in social groups, they’re more likely to pass on microbes from one to the other,” Dr. Cryan said.

“I think it’s a very interesting and compelling idea,” said Rob Knight, a microbiologist at the University of Colorado. If microbes do in fact manipulate us, Dr. Knight said, we might be able to manipulate them for our own benefit — for example, by eating yogurt laced with bacteria that would make us crave healthy foods.

“It would obviously be of tremendous practical importance,” Dr. Knight said. But he warned that research on the microbiome’s effects on behavior was “still in its early stages.”

Research to come..

The most important thing to do now, Dr. Knight and other scientists said, is to run experiments to see if microbes really are manipulating us.

Mark Lyte, a microbiologist at the Texas Tech University Health Sciences Center who pioneered this line of research in the 1990s, is now conducting some of those experiments. He’s investigating whether particular species of bacteria can change the preferences mice have for certain foods.

“This is not a for-sure thing,” Dr. Lyte said. “It needs scientific, hard-core demonstration.”

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