Could the gut microbiome be responsible for food cravings?

• A new study by University of Pittsburgh researchers suggests that intestinal microbes may trigger foraging behaviors for foods containing certain nutrients.
• In the study, researchers saw that mice bred to have no microbiome but later colonized with the gut microbiomes of different animals showed significant variations in their dietary behaviors.
• The authors believe that gut bacteria could affect our food choices and cravings by influencing the availability of essential amino acids.

The decisions we make about what to eat might not just come from our brains. The mechanisms driving our dietary choices may reach back to evolutionary processes with gut microbes.

For decades, scientists have wondered if intestinal flora drives food urges. However, no one had ever directly tested the hypothesis on animals larger than a fruit fly.

Dr, Kevin Kohl and Dr. Brian Trevelline from the University of Pittsburgh, Pennsylvania explored this possibility with germ-free mice.

Dr. Kohl is an assistant professor in the Department of Biology at the Kenneth P. Dietrich School of Arts and Sciences at the University of Pittsburgh. Dr. Trevelline is now a Rose postdoctoral fellow at the Cornell Lab of Ornithology.

Their research showed that the mice that received diverse types of microbiota voluntarily changed their diet preferences.

“Our work shows that animals with different compositions of gut microbes choose different kinds of diets,” Dr. Kohl said.

This research appears in the journal Proceedings of the National Academy of Sciences.

Microbes and amino acids

In an interview with Medical News Today, Dr. Trevelline explained how microbes play a role in nutrient making.

“Animals need a suite of essential amino acids to survive. But the microbes that live inside of our guts need [to] grow and have some of these same nutrients or make nutrients that the human body or the animal body can recognize. For instance, they make these essential amino acids, and then they’re released into the gut where they can be absorbed by the host,” he said.

Dr. Kohl, meanwhile, suggested that the microbes might also be broken down and digested to release nutrients as well.

Dr. Trevelline said the microbes in the gut were beneficial contributors to a lot of processes, particularly by synthesizing the nutrients humans need and supplementing our diets.

“[N]ow we’re showing that some of those molecules can actually enter the host bloodstream and affect their behavior,” he told MNT.

Wild cocktails

Drs. Kohl and Trevelline collected microbes from three wild rodent species with different natural diets. They gave these microbic “cocktails” to 30 mice engineered without gut microbiota.

The mice in each group began selecting foods rich in significantly different macronutrients.

Mice with microbes from herbivorous wild rodents chose a diet with a higher protein-to-carbohydrate (P:C) ratio. Those with microbes from carnivorous and omnivorous rodents chose a lower P:C diet.

Tryptophan’s role

Tryptophan, an essential amino acid, is a precursor to the neurotransmitter serotonin. Scientists believe that this hormone may regulate diet selection.

Drs. Trevelline and Kohl discovered that mice with different microbiota also had varying levels of tryptophan in their blood before choosing different diets. The mice with more tryptophan also had more microbes that could produce the amino acid in their guts.

Tryptophan is only one of many chemical messengers that communicate with the gut and brain. This experiment uncovers how tryptophan might directly affect day-to-day food behavior.

The study’s limitations

As intriguing as the results are, it may be too soon to blame intestinal flora for all food cravings.

Drs. Kohl and Trevelline implanted microbiota from wild-type mice and voles into bioengineered members of the same species with no intestinal flora, quite an unnatural undertaking. However, the researchers say it was necessary to be able to observe how gut microbes modulate behavior in isolation.

This pioneering research hints at just one aspect of how microbes may interact with animal hosts. It did not compare the influence of microbes with any other factors.

“It could be that what you’ve eaten the day before is more important than just the microbes you have. Humans have way more going on that we ignore in our experiment, pointed out Dr. Kohl.

However, it’s not feasible to test larger mammals or humans in such context. Drs. Kohl and Trevelline said that using germ-free pigs or primates, for instance, would be difficult and expensive.

More questions to explore

Whether microbes in our food influence food cravings is “up for future research,” Dr. Trevelline told MNT.

“Our microbiomes are constantly influenced by our environment, our lifestyles, and the animals and others we interact with. All that has the potential to influence our microbiota. But whether it produces some significant behavioral consequences is still an outstanding question at this point.”
— Dr. Brian Trevelline

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