Hydration and nutrition are often discussed as different subjects. In reality, they work together every day inside the digestive system.
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What a person drinks affects how well they handle fiber, how readily their intestines move, and how efficiently their digestive system functions. What they eat affects digestion, gas production, stool patterns, energy levels, and symptoms such as bloating or abdominal pain.
A simple way to think about this relationship is to imagine mixing materials in an entirely dry container.
Nothing moves correctly. The contents just sit there. Digestion operates in a similar manner: without enough fluid, the digestive system struggles to move food smoothly through the gut. Hydration provides the medium for digestion, whereas nutrition offers the elements that our bodies and gut microorganisms require to function.
But how can someone tell if their gut is responding properly to what they eat and drink? Surprisingly, one of the most useful signals might originate from something that is rarely considered: breath.
Breath as a “thermometer” for the gut
One of the reasons Dr. Jafar Jafari became interested in hydrogen-methane breath testing (HMBT) and eventually authored a book about it, The Essential Guide to Hydrogen and Methane Breath Testing, is that it provides unique insight into how the digestive system works.1
Human tissues do not create hydrogen or methane gas. These gases are produced when gut microorganisms ferment carbohydrates that escape digestion. Some of this gas enters the bloodstream and eventually makes its way to the lungs, where it is released as a person breathes.
Clinicians can learn about microbial fermentation patterns in the gut by analyzing hydrogen and methane levels in breath samples over time.
Breath testing works similarly to a thermometer for gut activity. Breath testing can disclose how gut microorganisms react to the food and drink we ingest, much as a thermometer shows how the body responds to infection or inflammation.
The interesting point is that this signal often appears much earlier than other markers of health.
Nutritional deficiencies, such as low iron or vitamin B12, can develop over months or even years before being recognized in blood testing. However, changes in microbial fermentation can occur much faster, reflecting the daily combination of nutrition, hydration, gut motility, and microbial activity.
In this regard, microbial fermentation may be one of the first indicators that a person’s digestive system is not working properly.
Why “healthy foods” sometimes cause symptoms
Often, people find themselves questioning why supposedly ‘healthy’ foods are affecting their gut. The solution is surprisingly straightforward: healthy does not always mean symptom-free.
Foods high in fiber and nutrients include fruits, beans, lentils, whole grains, and many vegetables.2 They promote long-term health and the proliferation of beneficial microorganisms in the stomach.
These foods, however, are fermentable. This fermentation produces gas, which, in those with slower gastrointestinal movement, can be difficult to tolerate, resulting in greater microbial activity in the small intestine or heightened gut sensitivity.1
This does not mean that those foods are inherently unhealthy or should be avoided entirely. It simply implies that digestion sometimes necessitates a more personalized approach, which includes considerations for portion size, timing, meal combinations, and underlying digestive function.
Breath testing can aid in understanding these patterns.
Hydration: The forgotten partner of fiber
Another common question is whether drinking extra water helps alleviate symptoms like constipation or bloating. Hydration is rarely a comprehensive remedy, but it is essential for maintaining regular digestive function.
Adequate fluid intake promotes healthy bowel movements and gut motility. When the body becomes dehydrated, the intestines absorb more water from the feces, making it harder to pass.
Hydration becomes more important when people consume more fiber: fiber and fluid work together. Rapidly increasing fiber intake without increasing fluid intake can worsen symptoms, particularly in people with sensitive digestive systems.
For most adults, the overall NHS recommendation is six to eight glasses of fluid per day,3 while individual needs vary depending on activity level, environment, and health status. Urine color is a simple daily indication of hydration; pale yellow suggests adequate hydration.3
Another good predictor of digestive health is the Bristol stool chart,4 which describes stool consistency. It is a simple but surprisingly helpful way of monitoring how well the digestive system is functioning.4
Daily observations such as hydration, stool pattern, and symptoms can serve as "thermometers" for gut health.
Breath testing is essentially a more advanced version of this thermometer, providing greater insight into microbial fermentation patterns when basic thermometers are no longer adequate.1
Why testing before treatment matters1
When people have persistent digestive issues, they may be inclined to experiment with probiotics, restrictive diets, or various supplements.
Trying something new can make one feel more proactive. However, the chosen technique may fail to treat the underlying problem and, on occasion, worsen the symptoms.
For example, if someone already has a high level of microbial activity in their small intestine, taking probiotics may not always be useful. Similarly, omitting large groups of foods without a good rationale might result in excessive dietary restrictions and poor nutritional balance.
Restrictive diets may temporarily alleviate symptoms, but they can also restrict the consumption of essential nutrients such as fiber, vitamins, and minerals, all of which promote long-term gut and overall health.
Extremely restrictive dietary behaviors may deny the gut of beneficial microorganisms and necessary nutrients for optimal activity and balance.
A more sensible strategy would be to test first, understand the underlying process, and then treat accordingly. Tools like hydrogen-methane breath tests can provide useful information in this situation.5,6
Breath testing should always be evaluated carefully in the clinical setting to achieve reliable results.5,6 It is the only clinically validated test for assessing microbial fermentation activity in the gut.1
Unlike many developing microbiome analyses–which are still not standardized or accurate enough to inform treatment decisions–breath testing, when applied correctly, can help reveal trends such as glucose malabsorption, aberrant fermentation, or potential bacterial overgrowth.
These findings can lead to more targeted management rather than trial-and-error techniques, particularly if you are experiencing symptoms.
A note on IBS, stress and unexplained symptoms7
Another important point is that conditions such as irritable bowel syndrome (IBS) must be considered as diagnoses of exclusion.
In practice, this means that if severe structural or inflammatory disorders have been totally ruled out, symptoms can be classified as IBS.
This does not, however, indicate that the symptoms are fictional or caused solely by stress. While stress can have an impact on gut health, attributing chronic symptoms only to psychological factors is often an oversimplification.
In reality, practically any symptom can be attributed to stress if we stop seeking other causes; similarly, many stomach problems can quickly and incorrectly be diagnosed as IBS.
In many cases, subtle physiological mechanisms, such as altered gut motility, microbial fermentation, gut hypersensitivity, or food intolerance, may be contributing to symptoms even when routine tests appear normal.1
Advances in gut physiology and microbiome research are gradually progressing the understanding of these mechanisms.
Practical advice for hydration and nutrition week
For Hydration and Nutrition Week, the message is straightforward.
- Drink fluids regularly throughout the day rather than all at once.3
Use urine color to determine hydration level.
- Include fiber in your diet. Make sure to increase it gradually to avoid developing symptoms.2
Use the stool pattern as a benchmark for fiber consumption.
- If symptoms persist despite healthy eating and drinking habits, consider whether there may be an underlying reason that deserves further investigation.1
Use the Hydrogen-Methane Breath Test as a verified medical-grade tool if symptoms persist, despite healthy eating and drinking habits.
Digestive issues rarely occur at random: there is always a reason the body does what it does.
In everyday life, we already use simple "thermometers" of gut health, such as urine color to indicate hydration and stool patterns to reflect gut motility. At a deeper level, hydrogen-methane breath testing can serve as a functional thermometer of microbial activity and nutrition in the gut.1
Sometimes the answers to persistent digestive complaints can be found not just in what someone eats and drinks, but also in what their breath reveals about our gut's inner workings.
People can start to take control of their gut health by hydrating and eating well. If someone has recurring gastrointestinal difficulties, they should contact their doctor about breath testing.
Hydrogen and methane breath testing (HMBT) with equipment such as the Gastrolyzer® family provides a non-invasive view of gut health and can aid in research or diagnosis in a clinical setting or from the comfort of your own home.
References
- Jafari J. 2025. The Essential Guide to Hydrogen and Methane Breath Testing: The Modern Non-Invasive Approach to Investigating Gastrointestinal Disorders. Bedfont Scientific Ltd, United Kingdom.
- NHS Choices (2019). How to get more fibre into your diet. NHS. Available at: https://www.nhs.uk/live-well/eat-well/how-to-get-more-fibre-into-your-diet/.
- NHS Choices (2019). Water, drinks and your health - Eat well. NHS. Available at: https://www.nhs.uk/live-well/eat-well/water-drinks-nutrition/.
- Yaseen, S. and Abuelass, F.K.M. (2026). Improving Documentation of Bowel Movements Using the Bristol Stool Chart: A Quality Improvement Project in a District General Hospital in the United Kingdom. Cureus. DOI: 10.7759/cureus.100902. https://www.cureus.com/articles/428450-improving-documentation-of-bowel-movements-using-the-bristol-stool-chart-a-quality-improvement-project-in-a-district-general-hospital-in-the-united-kingdom#!/.
- Rezaie, A., et al. (2017). Hydrogen and Methane-Based Breath Testing in Gastrointestinal Disorders: The North American Consensus. The American Journal of Gastroenterology, 112(5), pp.775–784. DOI: 10.1038/ajg.2017.46. https://journals.lww.com/ajg/abstract/2017/05000/hydrogen_and_methane_based_breath_testing_in.25.aspx.
- Pimentel, M., et al. (2020). ACG Clinical Guideline: Small Intestinal Bacterial Overgrowth. The American Journal of Gastroenterology, 115(2), pp.165–178. DOI: 10.14309/ajg.0000000000000501. https://journals.lww.com/ajg/fulltext/2020/02000/acg_clinical_guideline__small_intestinal_bacterial.9.aspx.
- NICE (2017). Overview | Irritable bowel syndrome in adults: diagnosis and management | Guidance | NICE. NICE. Available at: https://www.nice.org.uk/Guidance/CG61.
Acknowledgments
Produced using materials originally authored by Dr. Jafar Jafari from GI Cognition.
About Bedfont® Scientific Ltd
Bedfont® Scientific has specialised in the design and manufacture of exhaled breath and gas monitoring instruments since 1976.
For medical gas monitoring, their Medi-Gas Check medical pipeline testing range verifies not only the quantity but also quality of gas administered to patients.
Bedfont's breath analysers include carbon monoxide (CO) monitors such as the Smokerlyzer®, used for smoking cessation, and the ToxCO®, used by emergency services, to diagnose CO poisoning.
The NObreath® FeNO monitor provides accurate analysis of airway inflammation for the control of asthma, and the Gastrolyzer® range aids in the detection of gastrointestinal disorders and food intolerances. Quick and non-invasive, breath analysis is the new blood test.
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