blue intestines

Demystifying Dysbiosis


Written by: CDHF

Updated: January 24th, 2023

Dysbiosis is a useful concept but only has medical relevance when linked to a health outcome.

Living in the dark depths of your intestines is a diverse community of microorganisms of all shapes and sizes—the bacteria as well as fungi and archaea (ancient microorganisms) that make up your gut microbiome. At any given moment, some of the bugs swim around frantically while others hide out. Some bugs fight their neighbors while others devour food sources and replicate, in a continual circle of life.

One day, say some unwelcome visitors arrive in the intestine: a population of nasty, sickness-causing microbes called pathogens. These microbes release toxic chemicals and start to gobble up all the energy sources, starving out the local population of microbes. Things get even more chaotic as the immune system kicks into high gear to try to control the pathogens.

Sometimes after an event like this, the original community of microbes bounces right back. But other times, many of the members remain missing as the community scrabbles back together. The gut microbiome may end up less diverse overall while some of the bugs that did not previously thrive, such as oxygen-loving bacteria, suddenly find the perfect conditions for growth.

Such a situation is often called ‘dysbiosis’—the gut microbiome shifting away from its normal state. This shift can be brought about not just by pathogen infections, but also by low-fibre diets, over-use of antibiotics, and several other factors.

The basic concept of dysbiosis is easy enough to grasp: a disrupted or depleted microbiome.

But how do scientists measure it? And what does dysbiosis mean for health?

Digging into the science, here are the top four things you need to know about the concept of dysbiosis in the gut microbiome.

Dysbiosis must be linked to a health outcome.

It’s possible to suffer from a gastrointestinal infection and recover perfectly, with no lasting health consequences. So any gut microbiome disruption that occurs in this scenario would be medically meaningless.

Dysbiosis is not an ‘end’ in itself—it only becomes important when it can signal current or future health problems. Thus, many microbiome scientists are currently working to identify what sort of disruptions in the gut microbiome have actual health consequences, such as gastrointestinal symptoms or even chronic digestive disease. And knowing that the gut microbiome is intricately linked to aspects of human health, they are working to establish whether the gut microbiome is the direct cause of any health problems that crop up after a microbiome disruption.

Measuring dysbiosis depends on the ability to define what’s normal.

For every body parameter—from blood pressure to urinary metabolites—scientists need to know the range of what’s normal before they know if something is abnormal.

One of the first large-scale microbiome projects in the world, the Human Microbiome Project, set out to define a normal or healthy microbiome. But the scientists ran into a problem: healthy people have such a wide range of gut microbial communities that ‘red flags’ signaling current or future health problems were exceedingly difficult to find.

Since then, advanced mathematics and machine learning methods have helped scientists move closer to measuring dysbiosis in a meaningful way. One company has proposed a “microbiome health index” that computes the relationship between four bacterial groups deemed important for predisposing individuals to disease. And in 2020, a group of scientists proposed an index that classified people as ‘healthy’ or ‘nonhealthy’ based on 50 species of microbes in their guts. Neither of these, however, has been sufficiently tested to become an accepted marker of dysbiosis.

In fact, the latest science adds another layer of complexity. A 2022 study cast doubt on the common understanding that the gut microbiome in adulthood is stable. The gut microbiome was shown to vary significantly over time in the same individual—so any reliable measurement of dysbiosis will have to take these normal variations into account.

Likely, many varieties of dysbiosis exist.

Some scientists have proposed the key to dysbiosis could come from the first line of the famous Tolstoy novel, Anna Karenina:

Happy families are all alike; every unhappy family is unhappy in its own way.

Similarly, every ‘happy’ gut microbiome carries out the necessary functions for health, but every dysbiosis looks different and has a different health implication. So after a major gastrointestinal infection, for example, one person might end up with a disrupted gut microbiome that predisposes them to irritable bowel syndrome, while another person’s disrupted gut microbiome (in combination with a poor diet, perhaps) may set the stage for colorectal cancer.

This means scientists have to study diseases one by one, in a way that enables them to systematically link a disease with a certain measurable dysbiosis.

The future will hold specific solutions for dysbiosis.

In the future, we are likely to have specific, personalized ways to steer away from dysbiosis states: for example, tailored antibiotics, probiotics, or blends of different fibres; perhaps even ‘bugs as drugs’ that are tested and shown to prevent a disease by acting through the gut microbiome.

But until that future arrives, we can use more general strategies to reduce the chances of having a disrupted gut microbiome. Practicing good hygiene and food safety to reduce the chance of a gastrointestinal infection is a winning strategy. Using antibiotics wisely also goes a long way. Maintaining your daily recommended fibre intake (subject to your dietitian’s guidance if you have a digestive disease) is a must; you might even consider a daily dose of prebiotics. The chaotic microbial world in your intestines is still full of mysteries. Dysbiosis is a useful concept to describe a disruption in the gut microbiome—but scientists are still working on being able to measure it and link it to specific diseases. With more knowledge about specific types of dysbiosis, we’ll be able to find solutions to maintain health with the help of our gut microbiome.

About the Author: Kristina Campbell, a science and biotechnology writer, is author of The Well-Fed Microbiome Cookbook and co-author of the academic textbook Gut Microbiota: Interactive Effects on Nutrition and Health (which has a second edition forthcoming). Kristina specializes in translating microbiome science and helping the general public sort fact from fiction when it comes to gut health, probiotics, and prebiotics.She writes about microbiome science for online and print media throughout Europe and North America, and enjoys participating in science communication projects for diverse audiences. She holds degrees from University of Toronto and University of British Columbia. View Kristina’s work here.

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