Why you are bloated, and what the trials actually measured
Researchers put people on two identical lunches and measured the hydrogen in their breath. One meal produced more than double the reflux events and nearly double the bloating score. Here is the mechanism, and here is what removing it is worth.
Bloating has a measurable cause in most people, and it is not water retention. Every number below comes from a randomised trial or a pooled analysis of them, with the weak results labelled as weak.
The short answer
Most bloating is gas, and most of that gas is made by bacteria fermenting carbohydrates you did not absorb. In a crossover trial, swapping a wheat lunch for a rice lunch cut bloating scores from 7.0 to 3.1 and reflux events from 5.0 to 1.9, with breath hydrogen tracking the difference. Removing the fermentable load is the lever with the most evidence behind it.
In this article
Bloating is gas, and the gas has a factory
The word bloating covers two different sensations that get talked about as one. Distension is your abdomen visibly getting bigger. Bloating is the feeling of pressure, whether or not anything visibly changed. They often travel together, and in most cases the thing driving both is volume of gas in the intestine.
You do not make that gas. Bacteria do. When a carbohydrate reaches the colon without having been absorbed in the small intestine, resident bacteria ferment it, and fermentation produces hydrogen and methane. This is not a malfunction. It is the normal end of the digestive process, and it is the same fermentation that produces the short chain fatty acids the colon lining runs on.
Which sets up the tension that runs through this entire subject. The material that feeds your gut also inflates it. Every honest answer to bloating is a negotiation between those two facts, and anybody offering you a clean win has skipped one of them.
The trial that makes the mechanism visible
The single most useful study for understanding your own bloating is small and specific. In a randomised crossover study published in Nutrients, eight patients were given rice noodle meals or wheat noodle meals for breakfast and lunch, and researchers measured breath hydrogen and methane, reflux events and symptom scores.1
Wheat produced significantly more transient lower oesophageal sphincter relaxations after lunch than rice, 5.0 against 1.9 in two hours. Breath hydrogen and methane were significantly higher after wheat, and the hydrogen curve correlated with the number of relaxation events. Symptom scores separated sharply: bloating 7.0 against 3.1, satiety 7.7 against 3.5, regurgitation 2.9 against 0.4, belching 3.8 against 1.1.1
Figure 01
Wheat lunch against rice lunch, same people
Mean symptom scores after each meal in a randomised crossover study. Higher is worse.
Eight patients with overlapping reflux and irritable bowel syndrome. A small study, and a crossover design, meaning every person served as their own control. The breath gas measurements are what turn this from a symptom diary into a mechanism.
Source: Plaidum S, Patcharatrakul T, Promjampa W, Gonlachanvit S. Nutrients, 2022;14(9):1755, retrieved via PubMed.
Two things make this study worth more than its sample size. It is a crossover, so each person is compared against themselves rather than against a different group of people. And it measured the gas, not just the complaint. When the breath hydrogen curve tracks the reflux events, you are watching cause and effect rather than a coincidence of feelings.
The same lunch, twice, in the same eight people. The only thing that changed was what the bacteria were given to work with.
What FODMAP actually means
FODMAP is an acronym for fermentable oligosaccharides, disaccharides, monosaccharides and polyols. Strip the chemistry and it means: short chain carbohydrates that are poorly absorbed in the small intestine, so they arrive in the colon intact and get fermented. Wheat is high in them. Rice is not. That is the entire reason the meals above behaved differently.
Removing them systematically is the low FODMAP diet, and it has been tested. A systematic review and meta analysis in Frontiers in Nutrition pooled 10 randomised controlled trials covering 511 participants with irritable bowel syndrome. The diet was associated with improvement in global symptoms with a risk ratio of 1.54, and with improved stool consistency.2
The same review is careful about what did not move. Quality of life, anxiety score, depression score and body mass index showed no statistically significant change. The quality of evidence was rated moderate for global symptom improvement and stool consistency, and low or very low for the other outcomes.2 Symptoms improved. Life did not automatically improve with them, which is a distinction worth carrying into any decision about a restrictive diet.
Figure 02
What removing fermentable carbohydrates is worth
Outcomes from pooled randomised trials, with the quality grade the reviewers assigned.
Swipe to see the whole table
| Outcome | Result | Evidence quality |
|---|---|---|
| Global symptom improvement | Risk ratio 1.54 | Moderate |
| Stool consistency | Improved | Moderate |
| Stool frequency | Trend only, not significant | Low |
| Quality of life | No significant change | Low |
| Anxiety and depression scores | No significant change | Very low |
| Body mass index | No significant change | Low |
Source: Wang J, Yang P, Zhang L, Hou X. Front Nutr, 2021;8:683191, retrieved via PubMed.
A 2026 randomised crossover trial in Alimentary Pharmacology and Therapeutics adds a number that is easier to hold on to. In 66 children with irritable bowel syndrome, treatment success was achieved in 57.6 percent on the low FODMAP diet against 30.3 percent on a dietitian guided habitual diet, with the greatest benefit in those with the most severe baseline pain, and with significant reductions in symptom severity and bloating.3
Notice the control arm. Thirty percent succeeded on a habitual diet guided by a dietitian. Roughly a third of people improve on attention and structure alone, which is worth knowing before you conclude that the specific food you removed was the culprit.
Where fibre fits, and why it can make this worse
If you arrived here from constipation advice, this is the collision. Fibre works by giving bacteria more to ferment, and fermentation makes gas. The trials measured that directly rather than leaving it to anecdote.
In a meta analysis of 16 randomised trials in adults with chronic constipation, flatulence was significantly higher in the fibre groups than the control groups.4 An earlier meta analysis found the same thing.5 This is a documented cost of a documented benefit, and it is the reason so many people abandon fibre in the second week.
There is a route through it. In the US comparative effectiveness trial of kiwifruit, prunes and psyllium in 79 patients with chronic constipation, patients in the kiwifruit group reported significant improvement in bloating scores, and kiwifruit had the lowest rate of adverse events of the three.6 Constipation improved and bloating improved at the same time, which the fibre supplement did not manage. The full constipation picture is in what forty years of research says about not being able to go.
Bloating that is not about food
Three other causes are common enough that they should be ruled in or out before you rebuild your diet around a food you have decided to blame.
- Constipation itself. Stool sitting in the colon is volume, and it is also more time for fermentation. Bloating that improves after a bowel movement is usually this, and treating the bloating without treating the constipation gets you nowhere.
- Swallowed air. Eating fast, drinking carbonated drinks, chewing gum and talking through meals all put gas in from the top rather than making it below. Belching dominates rather than lower abdominal pressure.
- Gut motility and sensitivity. Irritable bowel syndrome is defined in part by heightened sensitivity to normal volumes. The gas may be ordinary while the perception of it is not, which is why two people eating the same lunch can report completely different experiences.
And the boundary that matters most: bloating that is persistent and new, especially in women over 45, alongside weight loss, early fullness, change in bowel habit or pelvic pain, is a reason to see a doctor rather than to remove wheat. Persistent bloating is on the referral lists for good reason. That sentence is the most useful one in this article.
Frequently asked questions
What actually causes bloating?
In most cases, gas produced when gut bacteria ferment carbohydrates that were not absorbed in the small intestine. In a crossover study, a wheat meal produced significantly higher breath hydrogen and methane than a rice meal in the same people, along with more than double the bloating score. Swallowed air, constipation and heightened gut sensitivity are the other common contributors.
Is wheat the problem?
Wheat is high in fermentable short chain carbohydrates, which is why it separated so clearly from rice in the crossover study. That is a carbohydrate effect rather than a gluten effect, and it is not the same thing as coeliac disease, which is a diagnosed immune condition with a specific test. Do not remove gluten long term before being tested, because testing needs you to be eating it.
Does the low FODMAP diet work?
For symptoms, yes, moderately. A meta analysis of 10 randomised trials in 511 adults found improvement in global symptoms with a risk ratio of 1.54 and improved stool consistency, both graded moderate quality. Quality of life, anxiety, depression and body mass index showed no significant change. It is a restrictive diet and is intended to be time limited and reintroduced from.
Why does fibre make me bloated?
Because fibre works by feeding fermentation, and fermentation produces gas. Two separate meta analyses of randomised trials in chronic constipation both found significantly more flatulence in the fibre groups than in the control groups. It is highest early on while the bacterial population adjusts, which is exactly when most people quit.
When is bloating something to get checked?
Persistent, new bloating that does not come and go with meals, particularly in women over 45, and particularly alongside weight loss, feeling full very quickly, a change in bowel habit or pelvic pain, should be assessed by a doctor rather than managed with a diet. Persistent bloating appears on referral guidance for exactly this reason.
Sources
- Plaidum S, Patcharatrakul T, Promjampa W, Gonlachanvit S. The Effect of FODMAP Meals on Transient Lower Esophageal Relaxations in GERD Patients with Overlapping Irritable Bowel Syndrome. Nutrients, 2022;14(9):1755. doi.org/10.3390/nu14091755
- Wang J, Yang P, Zhang L, Hou X. A Low-FODMAP Diet Improves the Global Symptoms and Bowel Habits of Adult IBS Patients: A Systematic Review and Meta-Analysis. Frontiers in Nutrition, 2021;8:683191. doi.org/10.3389/fnut.2021.683191
- Cristofori F, Celano G, Dargenio VN, Apa CA, Amendolara M, Stella M, et al. Clinical trial: Efficacy of a Low FODMAP Diet in Paediatric Irritable Bowel Syndrome, A Randomised Crossover Trial With Exploratory Metabolomic Analysis. Alimentary Pharmacology and Therapeutics, 2026;63(12):1592 to 1604. doi.org/10.1111/apt.70718
- van der Schoot A, Drysdale C, Whelan K, Dimidi E. The Effect of Fiber Supplementation on Chronic Constipation in Adults: An Updated Systematic Review and Meta-Analysis of Randomized Controlled Trials. The American Journal of Clinical Nutrition, 2022;116(4):953 to 969. doi.org/10.1093/ajcn/nqac184
- Christodoulides S, Dimidi E, Fragkos KC, Farmer AD, Whelan K, Scott SM. Systematic review with meta-analysis: effect of fibre supplementation on chronic idiopathic constipation in adults. Alimentary Pharmacology and Therapeutics, 2016;44(2):103 to 116. doi.org/10.1111/apt.13662
- Chey SW, Chey WD, Jackson K, Eswaran S. Exploratory Comparative Effectiveness Trial of Green Kiwifruit, Psyllium, or Prunes in US Patients With Chronic Constipation. The American Journal of Gastroenterology, 2021;116(6):1304 to 1312. doi.org/10.14309/ajg.0000000000001149
All references retrieved via PubMed.
