Your Breath Might Know You’re Sick Before You Do

What if a single exhale could reveal that the gut microbiome is out of balance? A growing body of research suggests that natural compounds in the breath may help signal whether there is a serious underlying health issue.

A new study in Cell Metabolism adds more weight to that idea. Researchers ran separate experiments with mice and with 27 children, measuring the levels of tiny molecules in their exhaled breath. They then compared those breath samples with stool samples to see whether there was a clear link between the two. The same process was repeated with the mice.

After analysing the data, the team found that breath contains volatile organic compounds (VOCs) produced by gut microbes. These VOCs may give scientists a non-invasive window into what is happening in the gut microbiome, metabolism and overall health. The researchers even found that exhaled breath could detect high levels of Eubacterium siraeum, a type of gut bacteria that acts as a biomarker for asthma in children.

What might your breath say about your health?

“The compounds in your breath contain important information about your health and some of those compounds come from the microbes in your gut,” says Dr Audrey R Odom John.

According to Dr John and other specialists, there are two main ways breath can flag that something is not quite right:

Odor

Using breath for formal diagnostic tests is still an evolving area of medicine, but many doctors already pay attention to how a patient’s breath smells. A quick “whiff” can help decide whether further testing is needed for certain conditions, says Odom John. These may include:

• Type 1 and type 2 diabetes. “Breath has been used historically to diagnose conditions like diabetes because of characteristic ketones in the breath,” Dr John says. Ketones are chemicals produced by the liver when the body breaks down fat and they can make breath smell fruity or like acetone.
• Liver disease. A distinct musty odour known as fetor hepaticus or “breath of the dead” can signal severe liver disease or liver failure, Dr John explains.
• Dry mouth. Dry mouth (also called xerostomia) happens when there is not enough saliva. Without saliva to wash away bacteria and food debris, breath can develop a strong sulphur or “rotten egg” smell.

Bacteria and viruses

Of course, a doctor noticing an unusual smell is different from an actual diagnostic test that confirms a specific illness. “There are only a couple of breath-based diagnostic tests that are clinically used,” Dr John says.

One widely used example is the breath test for Helicobacter pylori infection, a bacterium that can cause stomach ulcers. “There is also a COVID-19 breath test that received emergency authorisation from the FDA,” she notes. This type of test looks for a pattern of volatile organic compounds linked with the virus and can return results in just a few minutes—much faster than standard nasal swabs.

Why might breath tests be helpful?

Breath testing may give doctors an early signal to investigate a particular diagnosis and could make it easier to spot other conditions. “Clinicians may soon be able to tell when your gut is healthy based on breath testing alone, which is much more convenient and faster than genetic sequencing of stool,” says Dr John.

These tests are usually quick, simple and painless. “The value is that a non-invasive test can offer a lot of information by sampling the molecules in a person’s breath,” says Dr Amesh A. Adalja. “These molecules can result from an underlying disease process and/or the interaction of human physiology with a microorganism.” He adds that breath tests “can be very rapid,” often delivering answers as quickly as an alcohol breathalyser.

“Breath testing has been clinically validated in very few uses,” notes Dr Aditya Sreenivasan, “It is a largely studied area due to its non-invasive nature and because it may provide insight into the gut microbiome, which is a relatively new area of research that may revolutionise healthcare.”

For now, though, this field is still evolving. “Right now, applying these experimental results to clinical practice can be tricky given lack of evidence on how to interpret results,” she says.

What this means for future testing

The latest research suggests that compounds in exhaled air could one day underpin new, non-invasive tests that reveal more about the gut microbiome and help clinicians make more accurate diagnoses. While a handful of breath tests are already available, scientists are working to widen the possibilities.

“I can imagine that breath testing will be used in the future to diagnose a number of diseases and to give additional information about the health of the body, including metabolic diseases such as diabetes, liver and kidney disease and gut health,” Dr John says. “This work is an important step to realise this dream, as understanding where individual breath components come from is key to understanding what they are telling us about the body.”

When to talk to a doctor about your breath

Any ongoing change in the way your breath smells is worth mentioning to a health professional. Dr Sreenivasan recommends seeing a doctor promptly if there is a sudden, unexplained shift in odour, especially when dental issues have been ruled out.

“It can be a sign of a wide range of things,” she says. “From an H. pylori infection, sinus issues, acid reflux, slow gastric emptying, diabetes, gastric cancer, bacterial overgrowth, or oesophageal diverticulum, further testing can be done by your GI doctor to help determine if there is a serious issue.”