The Shocking Source of Your Cravings
Your gut microbiota brazenly gives you cravings to get what it wants.
Posted May 07, 2019
"If you're not constantly craving breakfast every second of every day, you're not a real American." –Rob Riggle
I have carefully honed my taste buds for years to appreciate the finer things in life like doughnuts and Cheetos. OK, I don't actually remember choosing those as primary food groups, but I must have, somewhere along the line. Right? Scientists, however, point out that our gut microbes—known as our microbiota—have a major vote in what we crave. How is that possible?
If you're happy with your weight and you have a craving for kale, you can skip this article. But if you're like the two-thirds of Americans who are trying to lose weight and craving anything with chocolate, cheese, or bacon, this post is for you.
Microbiota stories always seem to start with flies. That makes sense because flies are easy to experiment on and they have a simple microbiota. In fact, you can experiment on flies that have only a handful of different bacterial species, a radical improvement over studying the thousands of species jostling in our own guts.
One thing we know about flies is that they like yeast for its protein. It's like filet mignon to them, and they particularly like it when they have been denied protein. So it was interesting to Carlo Ribeiro and his colleagues when they found hungry flies that shunned yeast. These flies turned their tiny fly noses up, even though they needed the protein from the yeast. They discovered that these hungry flies had specific microbes that changed their appetite and altered their cravings, even though it was potentially bad for their health.
Cravings, it turns out, may be better for gut microbes than for their hosts. At least for flies.
After flies, the next best research subjects are mice, and here we have a creepy story. Normally, mice wisely scurry off when they smell cat urine, but there's a parasite called toxoplasma which alters that behavior. Mice infected with toxoplasma actually get turned on by cat pee. They crave it. As a consequence, they often end up as lunch for a cat, which is nourishing for the cat, essential to the life cycle of toxoplasma, and devastating for the mouse.
So, right off the bat, mouse studies demonstrate that cravings established by microbes can lead to a swift death. I think we can all agree that this is worse than following the scent of warm chocolate chip cookies, which may kill you, but slowly.
As another example, germ-free mice—which have no gut microbes—have altered taste receptors on their tongues. When they are given a microbiota, those receptors change, and the mice change their food preferences. So, for both flies and mice, microbes affect food choices.
Gastric-bypass patients offer a great window into microbial cravings. After a bypass, the microbiota undergoes a major shift. Along with that comes a big change in cravings. People who once kept Snickers in every pocket and drawer suddenly stop craving sweets. They lose their taste for fatty foods. Amazingly, researchers now suspect that the success of bypass surgery is due to these changes in cravings, not the decrease in stomach volume.
As another example, pregnancy brings about major changes in the mother's body. The hormonal shifts that drive pregnancy affect the composition of the mother’s microbiota. Vaginal microbes start to favor lactobacillus, which will provide a beneficial anointment as the baby comes out. Gut microbes are also affected and different cravings take over. Pregnancy, of course, is famous for crazy cravings. They are often completely different from the mother's normal preferences, and the subject of many amusing anecdotes.
So flies, mice, and humans all have manipulative microbes that can affect our eating behavior. That's more than a little humbling.
How Do Microbes Affect Cravings?
The fact is, we are being played. Gut microbes are not our "friends", they are associates that we share meals with but can turn on us at any moment. If the food runs out, they will be just as happy to digest us.
They each have their own appetites: yeasts crave sugar, Bacteroidetes enjoy fat, Prevotella love carbs and Bifidobacteria are fiber fiends. They each have their own method of asking for their favorite meal, using variations on two basic techniques to influence our food choices.
- Microbes can produce toxins and make us feel like crap if we don’t give them what they want. So they know how to make us miserable.
- Microbes increase our craving for food that they like by changing our taste buds, increasing opioid and cannabinoid receptors, and producing neurotransmitters such as dopamine and serotonin. So they also know how to make us happy.
That's right: microbes use a stick and carrot-cake approach to mold our cravings. They are very good at it, but their benefit is not always ours.
Cravings and Mood
Cravings may seem like a frivolous thing to worry about, but your food preferences are tightly linked to your mental state. As we've seen, your microbiota can create many of the same neuroactive compounds that are behind antidepressants, anxiety drugs, and recreational drugs. These are the powerful levers your gut microbes can pull to modify your eating behavior.
That microbial muscle-flexing can lead to several outcomes, not all of them good. A well-fed and balanced microbiota presents a healthy bulwark against the nasty pathogens of the world. It can reduce anxiety and lift your mood. On the other hand, crappy diets that unbalance your microbiota can lead to obesity or anorexia, dangerous syndromes that are strongly comorbid with depression and anxiety. An unbalanced gut may make us avoid healthy foods and instead seek out the junk food adored by some of our more selfish bacteria.
We've been under the mistaken perception that we make our own food choices, but ignorance is no longer an excuse: We need to manage our microbes before they make us sick. It's important, not just for our gut, but for our mood.
What to Do
Your gut microbiota turns over every half-hour or so because bacteria have a short life cycle. What you eat changes the composition of your microbiota; certain species thrive and others wither, depending on dinner. So, if you can override your bad cravings, you can effectively rebuild your microbiota in just a few days. Of course, that goes both ways—the lure of Oreos should not be underestimated, and you can be right back to where you started in a flash, a victory of microbe over prudence.
Still, all is not lost. You can build a better, healthier microbiota in no time at all. Here are some tips to deal with your junk-food crazed microbes:
- Increase the variety in your diet to increase the variety in your microbiota. Diversity prevents a bully microbe from dominating and pushing hard on your buttons.
- Eat more fiber. You can change your microbiota overnight with just a few good fiber-filled veggies like broccoli and artichokes – although to really make it stick you’ll need to eat these foods daily. They will help you crave healthier foods after a while.
- Substitute: when you are craving that donut, choose strawberries instead. We tend to forget how good fruit is when confronted with a glazed delicacy. You may complain for the first few days (OK, maybe weeks), but once you've established a healthier microbiota, it just gets easier and easier.
- Exercise, not to lose weight, but to balance your microbes. Researchers aren't sure why, but it works. Just walking for 15 minutes a day does your gut good.
- Befriend some lean foodies and see if you can catch their cravings for healthy food. Don't laugh: cravings may be contagious. 
We tend to identify with our cravings. We are chocolate lovers or meat-eaters. But knowing how much our microbes influence our cravings should make it easier to overcome them. After all, who do you want to be in charge of your diet: you or your selfish microbes?
 “Commensal Bacteria and Essential Amino Acids Control Food Choice Behavior and Reproduction.” Accessed April 30, 2019.
 Behary, Preeshila, and Alexander D. Miras. “Food Preferences and Underlying Mechanisms after Bariatric Surgery.” The Proceedings of the Nutrition Society 74, no. 4 (November 2015): 419–25.
 Alcock, Joe, Carlo C Maley, and C Athena Aktipis. “Is Eating Behavior Manipulated by the Gastrointestinal Microbiota? Evolutionary Pressures and Potential Mechanisms.” Bioessays 36, no. 10 (October 2014): 940–49.
 Temko, Jamie E., Sofia Bouhlal, Mehdi Farokhnia, Mary R. Lee, John F. Cryan, and Lorenzo Leggio. “The Microbiota, the Gut and the Brain in Eating and Alcohol Use Disorders: A ‘Ménage à Trois’?” Alcohol and Alcoholism 52, no. 4 (July 1, 2017): 403–13.
 Hill, Alison L., David G. Rand, Martin A. Nowak, and Nicholas A. Christakis. “Infectious Disease Modeling of Social Contagion in Networks.” PLOS Computational Biology 6, no. 11 (November 4, 2010): e1000968.