As you read this, trillions of bacteria and other microorganisms are living in the warmth of your gut.
Collectively known as your gut microbiome, these microscopic beings are vital for good health.
And conversely, your health can be affected if these bacteria aren’t in a state of balance. This imbalance is known as dysbiosis.
Below, we’ll describe what dysbiosis is. We’ll also explain why it might happen, how it relates to specific health conditions, and how it's treated.
What is dysbiosis?
In a nutshell, dysbiosis means an unbalanced microbiome. That could be the microbiome of your skin, mouth, or anywhere else bacteria live.
In this article, we’ll focus on gut microbiome dysbiosis.
We should mention that there’s no official criteria for dysbiosis. Because everyone’s gut microbiome is different, scientists are still figuring out what’s “good” and “bad.”
Still, we have a lot to learn about our essential bacterial guests and what happens when they’re out of balance.
What changes in dysbiosis?
Most people always have “good” and “bad” bacteria in their guts, and maintaining a peaceful balance is important for health.
A healthy balance is called eubiosis, and when that balance is upset in some way, it’s dysbiosis.
The healthy order in your gut can be upset to produce dysbiosis in three main ways:
a loss of “good” gut bacteria
too many “bad” gut bugs
Often, two or three of these can happen at the same time.
What can cause dysbiosis?
Scientists are still getting to the bottom of it, but they have identified some factors that can lead to this imbalance. We’ll cover a few below.
The first and perhaps least surprising factor is what you eat.
The Western diet
Researchers have shown that people with a Western diet have very different gut microbiomes from people who live off the land in more traditional societies.
People with a Western diet have fewer gut bacteria that produce short-chain fatty acids (SCFAs).
SCFAs play important roles throughout your body, and one of their top jobs is nourishing the lining of your gut. We’ll meet them a few times throughout this article.
One SCFA called butyrate crops up particularly frequently. It seems to block “bad” gut bacteria from getting a foothold, while supporting the growth of “good” bacteria.
So, butyrate might be important in preventing dysbiosis and stabilizing the gut microbiome.
One of your gut microbiome’s favorite nutrients is fiber. It’s the fuel that these bacteria need to thrive.
Scientists have shown that people who eat more fiber have more diverse gut bacteria, which is a good sign of a healthy gut microbiome. They also have more species that produce butyrate.
On the other hand, people who eat the least fiber are likelier to have a less diverse population of gut bacteria.
And this links back to the Western diet, which is particularly low in fiber, compared with more traditional diets.
There’s evidence that genes play a part in dysbiosis, too. Many that scientists have identified so far are associated with inflammatory bowel disease (IBD).
IBD is an umbrella term for bowel conditions that involve chronic inflammation.
The two most common are Crohn's disease and ulcerative colitis. And we’ll talk more about these conditions later.
One gene called NOD2 plays a part in your immune response. Among other things, it helps prevent “bad” bugs from settling in your gut.
In this way, it helps maintain stability and protect against dysbiosis.
However, some people have variants of the NOD2 gene that simply don’t work.
People with these versions have an increased risk of developing dysbiosis. Once dysbiosis begins, it causes inflammation, which might then lead to Crohn’s disease.
Because antibiotics are excellent at killing bacteria, they've saved countless lives. But for the same reason, they can cause havoc in your gut microbiome.
For instance, broad-spectrum antibiotics are useful because they kill a wide range of bacteria.
This is great for treating infections when doctors don't know which microbe is to blame. But this broad action means that many of your “good” gut bacteria are also in the firing line.
Dysbiosis associated with antibiotics can involve reduced diversity and a reduced ability to fight off “bad” gut bugs.
However, if you’re taking antibiotics, make sure you take them as your doctor prescribed. They’re still essential, lifesaving medicines.
We don’t know all of the risk factors for dysbiosis, but a list is slowly forming. And some of the relationships are complex.
Returning to what we eat, evidence from animal studies suggests that a diet high in meat proteins and fat might be associated with dysbiosis.
Plus, diets high in some forms of carbohydrates are linked to dysbiosis, as are artificial sweeteners.
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Finally, environmental chemicals, like pesticide residues, can upset your gut microbiome’s state of balance. After all, many of these compounds are designed to kill bacteria.
Thanks to the complex relationship between your gut bugs and you, there are probably many other factors waiting to be discovered.
Next, we’ll look at some health conditions that have intimate ties to dysbiosis.
Over the years, scientists have hunted for a bacterial cause of IBD.
Scientists now think that rather than just one species causing IBD, a more general dysbiosis might be to blame.
A review of dysbiosis studies identified specific changes in the gut microbiomes of people with IBD.
Microbiome changes in IBD
There’s often a loss of gut microbiome diversity in people with IBD. This particularly involves species of Firmicutes and Bacteroides, which are usually the most abundant bacteria in your gut.
Scientists have also found increased numbers of Ruminococcus gnavus, one of the 15 “bad” bugs that ZOE’s research identified.
And they've noted decreases in certain species, like Faecalibacterium prausnitzii, which we’ll return to in a moment.
As it stands, experts are still trying to figure out whether dysbiosis causes IBD or whether IBD causes dysbiosis. The old chicken-or-egg question.
However, growing evidence suggests that dysbiosis might directly cause IBD. And scientists have some theories about how it might trigger the condition.
As we mentioned, studies have shown that people with IBD have reductions in F. prausnitzii bacteria. These bacteria produce an SCFA called butyrate, which is vital for maintaining a healthy gut wall.
So, if the bacteria become scarce and the gut wall becomes less stable, that might play a part in triggering IBD symptoms.
Scientists still need to determine whether IBD causes dysbiosis or vice versa.
And the relationship could run both ways: Dysbiosis might cause IBD, while other factors associated with IBD might drive changes in the gut microbiome.
A role in obesity?
People with obesity tend to have less diverse gut microbiomes — one sign of dysbiosis. They also tend to have less Bacteroidetes and more Firmicutes bacteria.
Experts are still figuring out whether dysbiosis might cause obesity or whether factors associated with obesity, like changes in diet, might cause dysbiosis.
There's some evidence that dysbiosis might cause obesity.
For example, scientists have shown that when you transfer the gut microbiome of an obese mouse into a mouse with a moderate weight, it becomes obese.
Other studies looked at a similar relationship in humans. Researchers took gut bacteria from "lean" participants and transplanted the bacteria into participants with obesity and insulin resistance.
After 6 weeks, the recipients’ insulin sensitivity improved.
Back to SCFAs
Experts have some theories about how dysbiosis might cause obesity. As with IBD, one theory involves SCFAs.
People with obesity tend to have fewer gut bacteria that produce SCFAs, like butyrate. And SCFAs might stop fat from accumulating as adipose tissue, which we also call body fat.
So, if fewer gut bugs are producing SCFAs, more fat might accumulate, contributing to weight gain.
Risk factors for colon cancer include IBD, obesity, and a diet high in fat and protein.
As these factors are also associated with dysbiosis, some experts think that there may be a link between gut microbiome dysbiosis and colon cancer.
Researchers have found a wide range of gut microbiome changes associated with colon cancer. Among these changes is a reduction in species that produce the SCFA butyrate.
And you might remember that butyrate helps nourish the lining of your gut — the tissue that turns cancerous in cases of colon cancer.
One animal study helps demonstrate the complexity of the links between cancer, dysbiosis, and other factors.
The researchers used mice with a particular susceptibility to colon cancer. When the team fed them a high-fat diet, their gut microbiomes changed. This shift in gut bacteria was linked to a weakened gut immune response and increased tumor growth.
When they gave butyrate to these animals, their immune systems recovered, and tumor growth slowed. So once again, butyrate seems important.
Next, the scientists took poop samples from the high-fat diet mice that had developed tumors. Then, they transferred the poop into healthy mice. These mice then started to grow tumors.
As with IBD and obesity, the links are complex. Scientists are figuring it out, but there’s likely an interaction between dysbiosis, diet, the immune system, cancer, and other factors.
The best approach depends on the cause of the imbalance. For instance, if it's caused by a medication, your doctor might ask you to stop the treatment for a while or try a different drug.
Meanwhile, there’s growing interest in fecal transplants. In this procedure, doctors transfer gut bacteria from a healthy donor into a patient’s colon.
Research has shown that this technique is effective for treating C. difficile infections and restoring diversity to the gut microbiome.
And experts believe that this type of transplant might help address gut dysbiosis from other causes.
However, understanding in which situations fecal transplants are helpful and safe requires more research.
Dysbiosis is a general imbalance of your gut microbiome. It can involve a loss of diversity, increased numbers of “bad” gut bacteria, and reduced numbers of “good” bacteria.
We know some of the factors that can help generate dysbiosis, including diet, genetics, stress, and medications. But there are probably many others.
Dysbiosis is linked with a range of conditions, including IBD, obesity, and colon cancer. But we don’t fully understand how these relationships work.
As for treatment, fecal transplants seem promising, but many questions remain.
If you’re worried that you may have dysbiosis, speak with a doctor. And if you’re interested in supporting your gut microbiome, read our guide to improving gut health.
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