Can Carbonation Cause Diarrhea?

Carbonation itself, meaning the dissolved carbon dioxide gas in a fizzy drink, is unlikely to cause diarrhea. Studies comparing carbonated water to still water consistently find that the bubbles do not speed food or liquid through your digestive tract. The trouble usually comes from what else is in the glass: sugars, sugar alcohols, caffeine, and sweeteners are far more likely culprits. Sorting out which ingredient is actually responsible matters, because people often blame the fizz and then avoid sparkling water for no real reason.

What Carbon Dioxide Actually Does in Your Stomach

When you drink something carbonated, dissolved CO2 begins releasing gas almost immediately in the warm, acidic environment of your stomach. That gas stretches the upper part of the stomach, a sensation most people recognize as bloating or fullness. In a controlled study comparing meals consumed with carbonated water versus still water, the carbonated version held a significantly larger share of solids and liquids in the upper stomach (about 74% of solids versus 56% with still water). But the overall rate at which the stomach emptied its contents into the small intestine was identical for both drinks.¹ In other words, carbonation changed where food sat inside the stomach, not how fast it moved through.

A separate study looked at carbonated versus flat drinks during exercise and reached the same conclusion: carbonation had no effect on gastric emptying rate.² Even during prolonged treadmill running in heat, carbonated and noncarbonated sports drinks emptied at similar rates, with no meaningful differences in gastric residual volume.³ The consistent finding across these studies is that CO2 gas makes your stomach feel fuller and can cause belching, but it does not push food through faster in a way that would trigger loose stools.

Why CO2 Might Still Make Your Gut React

Even though carbonation doesn’t accelerate digestion in a measurable way, some people genuinely feel increased gut activity after drinking fizzy water. Part of the explanation may involve sensory nerve pathways. CO2 can diffuse into cells and lower the local pH, and researchers have found that a specific ion channel called TRPA1, present on sensory neurons, is directly activated by CO2 through this intracellular acidification.⁴ TRPA1 is the same channel that responds to mustard oil and cinnamaldehyde, so it is essentially a pain and irritation sensor. When it fires in the gut lining, it could contribute to cramping sensations, urgency, or the feeling that you need to use the bathroom. But feeling like you need to go is not the same as diarrhea in a clinical sense. For most people, this amounts to temporary discomfort rather than actual loose stools.

The bloating and distension that carbonation causes in the upper stomach can also trigger the gastrocolic reflex, an involuntary increase in colon activity that happens whenever the stomach stretches. This is the same reflex that sometimes sends you to the bathroom shortly after a big meal. The reflex is a normal part of digestion and varies a lot from person to person. If your gut is already sensitive, as with irritable bowel syndrome, even a modest stretch of the stomach from trapped gas may set off an outsized response downstream.

The Ingredients That Actually Cause Diarrhea

When someone says a carbonated drink gave them diarrhea, there is almost always something besides bubbles at work. Most carbonated beverages are not just water and CO2. They contain sugars, high-fructose corn syrup, sugar alcohols, caffeine, acids, or artificial sweeteners, each of which has a stronger and more direct connection to loose stools than carbonation does.

Sugar, especially fructose, is poorly absorbed in the small intestine when consumed in large amounts or without accompanying glucose. Unabsorbed fructose draws water into the intestinal lumen by osmosis. That extra water loosens stool and can accelerate transit through the colon. A single can of regular soda can contain upward of 40 grams of sugar, much of it as high-fructose corn syrup. People who are especially sensitive to fructose can develop bloating, gas, and watery stool from amounts well below that threshold. This is a well-recognized malabsorption issue, and it has nothing to do with bubbles.

Sugar Alcohols and the Osmotic Trap

Diet and “sugar-free” carbonated drinks often replace sugar with sugar alcohols like sorbitol, mannitol, or xylitol. These are widely used because they taste sweet but contribute fewer calories. The trade-off is that the small intestine absorbs them slowly and incompletely. What remains unabsorbed acts as an osmotic agent, pulling water into the gut.

Research on sorbitol found that most people experienced mild symptoms like gas and bloating after consuming about 10 grams, and severe symptoms including cramps and diarrhea after about 20 grams.⁵ Twenty grams is not a hard amount to reach if you drink multiple diet sodas or also chew sugar-free gum throughout the day. Because the label usually just says “contains sugar alcohols” or lists a polyol deep in the ingredient list, many people do not realize how much they are consuming or that the sweetener, rather than the carbonation, is the source of their distress.

Caffeine and Your Colon

Many of the most popular carbonated drinks contain caffeine: colas, energy drinks, and some citrus-flavored sodas. Caffeine is a known stimulant of colonic motor activity. Research has shown that caffeinated coffee triggers stronger muscle contractions in the descending colon compared to decaffeinated coffee, and that caffeine lowers the rectal sensory threshold for the urge to defecate, meaning you feel the need to go sooner.⁶ Tea at lower caffeine doses has a similar, if weaker, effect.

The caffeine content in a can of cola is modest, roughly 30 to 45 milligrams. But energy drinks commonly deliver 80 to 200 milligrams or more per can. If you are sensitive to caffeine, drinking a large caffeinated carbonated drink on an empty stomach gives your colon a double signal: stomach distension from the gas plus direct colonic stimulation from the caffeine. Blaming the fizz is easy, but removing the caffeine alone often solves the problem.

Artificial Sweeteners and Gut Bacteria

Non-nutritive sweeteners like sucralose, aspartame, and acesulfame potassium are common in diet sodas. Whether they cause digestive symptoms is a topic researchers have gone back and forth on for years, and the evidence remains genuinely unsettled.

One study found that ten weeks of sucralose consumption at typical dietary levels altered the composition of gut bacteria in healthy young adults, tripling the abundance of one bacterial group while reducing another by about a third.⁷ In theory, shifts in the microbial population could affect how the gut processes food and could contribute to symptoms like gas or diarrhea. However, a separate randomized crossover trial that tested both aspartame and sucralose at doses reflecting typical high consumption found minimal effects on gut microbiota composition and no changes in short-chain fatty acid production.⁸

The discrepancy likely comes down to differences in dose, duration, individual gut ecology, and how the studies measured changes. For most people drinking a diet soda or two a day, artificial sweeteners are probably not the primary driver of diarrhea. But in people who consume large quantities, or who already have a disrupted gut microbiome, the picture could be different. The honest answer is that the science here is thinner than either the “sweeteners are harmless” camp or the “sweeteners wreck your gut” camp would like to admit.

When Sparkling Water Might Actually Help

Here is an irony worth knowing: plain carbonated water, far from causing diarrhea, may actually improve slow-moving bowels. A trial involving people with functional dyspepsia and constipation found that drinking carbonated water for two weeks significantly reduced constipation scores, while tap water did not produce a meaningful change.⁹ The mechanism likely involves the CO2-driven distension of the stomach stimulating the gastrocolic reflex, encouraging the colon to contract and move things along. For someone dealing with constipation, a glass of sparkling water may be more helpful than harmful.

This finding also underscores the difference between plain carbonated water and a carbonated soft drink. The former contains nothing but water and dissolved gas. The latter is a chemical cocktail of sugars, acids, caffeine, sweeteners, preservatives, and flavorings, each with its own potential to irritate the gut. Lumping them together when you are trying to troubleshoot digestive problems makes it nearly impossible to identify what is actually bothering you.

How to Figure Out What Is Actually Bothering You

If you suspect carbonated drinks are giving you diarrhea, the most practical step is an elimination approach. Start by switching to plain sparkling water for a week or two. If the symptoms stop, the carbonation was not the problem; something else in the soda was. From there you can narrow it down:

Caffeine: Switch to a caffeine-free carbonated drink. If the symptoms disappear, caffeine was the likely trigger.
Sugar alcohols: Check ingredient labels for sorbitol, mannitol, xylitol, or erythritol. Even small amounts add up across multiple products in a day.
Fructose: If you drink regular (non-diet) sodas, the sheer sugar load could be overwhelming your small intestine’s absorption capacity, especially on an empty stomach.
Artificial sweeteners: Try alternating between drinks with and without sucralose or aspartame for a couple of weeks each. Individual responses vary enough that self-experimentation is more useful than population-level studies here.

Keeping a simple log of what you drank and when symptoms appeared makes patterns much easier to spot. People are surprisingly bad at remembering what they consumed two days ago, and gut symptoms often lag behind the trigger by several hours.

People With IBS and Other Functional Gut Disorders

For people with irritable bowel syndrome, particularly the diarrhea-predominant subtype, carbonated beverages deserve extra caution. The gastrocolic reflex that barely registers in a healthy gut can trigger urgent, painful bowel movements in someone with visceral hypersensitivity. Even plain sparkling water could conceivably push someone with IBS over the threshold from manageable to miserable, not because the CO2 is toxic to the gut, but because their gut overreacts to normal mechanical signals like distension.

Many IBS management approaches, including low-FODMAP diets, already recommend limiting carbonated drinks. This advice exists less because of rigorous trial data showing that CO2 worsens IBS and more because clinicians consistently observe that their patients report improvement when they cut fizzy drinks. Given that many carbonated beverages also contain FODMAPs (fructose, polyols), it can be genuinely hard to separate the effect of the gas from the effect of the sweetener even in a careful elimination diet.

People with inflammatory bowel disease, celiac disease, or other conditions that damage the intestinal lining may also have lower tolerance for the distension and acidity that carbonation brings. If your gut is already inflamed, adding a mechanical stretching force and a mild acid load is unlikely to help, even if it would be perfectly benign in a healthy intestine.

Cold Temperature and Drinking Speed

Two factors that rarely make it into the carbonation conversation are temperature and speed of consumption. Most carbonated drinks are consumed cold, and cold liquids can independently stimulate gut contractions. A cold drink consumed quickly stretches the stomach more abruptly than a slowly sipped warm one, amplifying both the gastrocolic reflex and the sensation of bloating. People tend to drink carbonated beverages faster than flat ones, partly because the fizz makes them feel refreshing. This faster intake means more gas entering the stomach at once, more distension, and a stronger reflex arc.

If you enjoy sparkling water but find it occasionally triggers gut urgency, sipping it slowly at closer to room temperature can reduce the mechanical stimulation considerably. It will not taste as satisfying, admittedly, but it is a low-effort test for whether speed and temperature, rather than the CO2 itself, are contributing to your symptoms.

Carbonation in Children

Children’s digestive systems are smaller, and their intestinal absorption capacity relative to what they consume is lower than in adults. A child who drinks a large soda gets a proportionally bigger hit of sugar, caffeine, and additives relative to body weight. Fructose malabsorption is more common in young children, and their lower threshold for osmotic overload means even moderate sugar intake from a single soda can provoke loose stools.

Pediatric dietary guidance in many countries discourages carbonated soft drinks for children, but the reasoning centers on sugar, acid, and caffeine rather than on the gas itself. A child who tolerates plain sparkling water fine but gets diarrhea from cola is telling you something about sugar and caffeine, not about bubbles. That said, very young children may swallow more air with carbonated drinks and experience more discomfort from trapped gas, which can mimic or worsen the sensation of an upset stomach even when the stool itself is normal.

The Role of Acidity

Dissolving CO2 in water creates carbonic acid, giving sparkling water a pH of roughly 3 to 4. Commercial sodas are often even more acidic because they contain phosphoric acid or citric acid. While this acidity is well below the level needed to damage healthy intestinal lining, it does create a mildly irritating environment in the stomach, which could compound the effects of distension. For most people this is a non-issue, but anyone with existing gastritis, an ulcer, or esophageal reflux may find that the acidity of carbonated drinks worsens their upper-gut symptoms, leading them to assume the drink is also responsible for any lower-gut irregularity they experience the same day.

Acid-related discomfort in the upper gut and diarrhea in the lower gut are different problems with different mechanisms, but they often get blamed on the same beverage. Sorting out which symptom is actually occurring helps direct the response. If the main complaint is heartburn and bloating, the carbonation and acidity may genuinely be part of the problem. If the complaint is loose, watery stool, the search for a cause should shift to sugars, sweeteners, and caffeine.