Regular soda consumption is associated with higher rates of constipation, but the relationship is not as straightforward as “fizzy drink equals backed up.” A large cross-sectional study using U.S. national health data found that people in the highest tiers of sugar-sweetened beverage intake had roughly 40 to 57 percent greater odds of being constipated compared to the lowest consumers. The culprit is less the carbonation and more the cocktail of sugar, caffeine, and additives inside the can. In fact, plain carbonated water appears to do the opposite, easing constipation in some studies.
What the Large-Scale Data Actually Shows
Two separate studies paint a consistent picture. An analysis of U.S. adults using nationally representative survey data found a significant positive association between sugar-sweetened beverage consumption and constipation, with the highest intake group showing about 57 percent higher odds of constipation after adjusting for other lifestyle and dietary factors.1PubMed Central. The impact of sugar-sweetened beverages consumption on constipation: evidence from NHANES A separate study focused on cola drinkers found a statistically strong link between cola consumption and constipation, alongside higher rates of obesity and gastritis.2Mediterranean Journal of Nutrition and Metabolism. Soft drink consumption: Do we know what we drink and its implication on health? Neither study can prove soda directly causes constipation on its own. People who drink a lot of soda tend to eat fewer fruits, vegetables, and whole grains, and those dietary patterns independently raise constipation risk. But even after statistical adjustments for diet and lifestyle, the soda-constipation link persisted, suggesting something about the beverages themselves contributes.
How Sugar Affects Your Gut’s Fluid Balance
A regular 12-ounce can of soda typically contains around 39 grams of sugar, much of it as high-fructose corn syrup. That concentration makes the liquid hyperosmolar, meaning it has a higher concentration of dissolved particles than your blood. When hyperosmolar fluid enters your small intestine, your body pulls water from circulation into the gut to dilute it. In theory, extra water in the intestines should make stool softer and easier to pass. But fructose is absorbed slowly through the entire length of the small intestine, and while it lingers, it traps that extra water in the upper digestive tract rather than letting it reach the colon where it would do the most good.3PubMed Central. Arterial hypertension due to fructose ingestion: model based on intermittent osmotic fluid trapping in the small bowel
This matters because of what happens downstream. If the water gets locked up in the small bowel for hours and triggers hormonal responses that promote water retention elsewhere in the body, less fluid ends up in the colon. And the colon is where stool consistency is determined. When the colon absorbs too much water from stool, you get hard, difficult-to-pass stools. So the paradox of sugary drinks is that they pull water into the gut initially but may not deliver that water to the part of the digestive system where constipation actually develops.
Compare this to a genuinely osmotic agent like sorbitol, the sugar alcohol found in sugar-free gum and some fruits. Sorbitol reaches the colon largely unabsorbed and measurably speeds up colonic transit.4PubMed. Effects of an osmotically active agent on colonic transit Fructose and sucrose do not behave the same way because they are absorbed higher up. The sugar in soda hydrates the wrong part of your digestive system.
The Caffeine Paradox
Many people reach for coffee when they feel backed up, and caffeinated sodas contain a meaningful dose of caffeine, typically 30 to 50 milligrams per can. Caffeine does stimulate the gut. One study found that caffeine intake lowered the rectal sensory threshold for the urge to defecate, meaning it took less rectal distension to trigger the feeling that you need to go.5PubMed. Effects of caffeine on anorectal manometric findings That sounds like it should help. But the same study found that caffeine also significantly increased the resting pressure of the anal sphincter, which could make it harder for stool to actually pass. Those two effects pull in opposite directions: your brain gets the signal to go sooner, but the exit is tighter.
The dose-response curve adds another wrinkle. A population-based study found a U-shaped relationship between caffeine intake and chronic constipation. At moderate levels, caffeine was associated with lower odds of constipation, with about an 18 percent reduction in risk per unit increase. But past a certain threshold, the relationship reversed, and higher caffeine intake was linked to slightly increased constipation risk.6Dove Medical Press (J Multidiscip Healthc). Association Between Caffeine Intake and Bowel Habits and Inflammatory Bowel Disease: A Population-Based Study In practical terms, a single caffeinated soda might give your bowels a gentle nudge. Drinking several cans a day, especially combined with the dehydrating potential of caffeine, may tip the balance the other way.
Plain Carbonated Water Tells a Different Story
If carbonation itself caused constipation, sparkling water would be a problem too. The evidence suggests the opposite. In a randomized trial of people with both dyspepsia and constipation, those assigned to drink carbonated water for 15 days saw their constipation scores drop significantly, while the tap water group showed no improvement.7PubMed. Effects of carbonated water on functional dyspepsia and constipation A study in elderly stroke patients, who are particularly prone to constipation due to immobility and medication, found that carbonated water intake increased the frequency of bowel movements and reduced constipation symptoms.8PubMed. Effects of carbonated water intake on constipation in elderly patients following a cerebrovascular accident
The mechanism is not entirely clear. Carbonation does change how your stomach handles food, pushing more of the meal into the upper part of the stomach, though it does not seem to change the overall speed of gastric emptying.9PubMed. Effect of carbonated water on gastric emptying and intragastric meal distribution Some researchers speculate that dissolved carbon dioxide stimulates receptors in the stomach lining that trigger a cascade of nerve signals promoting gut motility further down the tract. Whatever the mechanism, the pattern is remarkably consistent: plain carbonated water helps constipation, while sugar-loaded carbonated soda does not. The bubbles are not the issue. It is everything else in the drink.
Diet Soda and Artificial Sweeteners
Switching to diet soda sidesteps the sugar problem but introduces a different set of questions. Most diet sodas contain artificial sweeteners like aspartame, sucralose, or acesulfame potassium. A systematic review aimed at gastroenterologists found that multiple animal studies showed these sweeteners exert measurable effects on the gut microbiome, though the specific changes were conflicting across studies, and human data at the time was limited to a single published trial.10PubMed Central. Artificial Sweeteners: A Systematic Review and Primer for Gastroenterologists A more recent critical review spanning in vitro experiments, animal models, and clinical trials confirmed that artificial sweeteners can alter the composition and abundance of gut microbes, raising concerns about their potential to affect gut health and contribute to gastrointestinal disorders.11PubMed. A critical review on effects of artificial sweeteners on gut microbiota and gastrointestinal health
The gut microbiome is closely involved in regulating how quickly food moves through your intestines. Certain bacterial populations produce short-chain fatty acids that influence colonic motility and water absorption. If sweeteners shift those populations, the downstream effects on stool consistency and transit time are plausible. But “plausible” is doing a lot of work here. The evidence in humans is still thin, and the doses used in animal studies often far exceed what you would get from a few cans of diet soda. The honest take is that diet soda has not been convincingly shown to cause constipation, but it has not been cleared of suspicion either, and the microbiome angle is the one to watch as more human data accumulates.
When You Already Have a Sensitive Gut
For people with existing digestive conditions like irritable bowel syndrome, gastroesophageal reflux, or functional dyspepsia, soda can make things worse in ways that go beyond constipation. A narrative review evaluating 85 studies found that higher intakes of soft drinks and carbonated beverages were associated with aggravated symptoms in all three of those conditions.12Caspian Journal of Clinical Cases. Fluid Intake and Functional Gastrointestinal Disease: A Narrative Review The combination of carbonation, acidity, sugar, and caffeine can trigger bloating, gas, and abdominal discomfort that makes you feel more backed up even if stool frequency has not technically changed.
The acid profile of cola specifically deserves mention. Coca-Cola’s phosphoric and citric acids lower duodenal pH significantly, increasing periods of acidification in the upper small intestine.13Gut. Duodenal pH in health and duodenal ulcer disease: effect of a meal, Coca-Cola, smoking, and cimetidine While increased acidity is sometimes used therapeutically to dissolve certain types of gastric obstructions, habitual exposure can irritate the lining of the gut, potentially altering motility patterns over time. If you have a sensitive digestive system and feel worse after soda, the acid load is a real contributor, not just the sugar or caffeine.
The BPA Question Nobody Asks About
Most conversations about soda and digestion focus on what is dissolved in the liquid, but the container matters too. Many canned sodas have linings that contain bisphenol A (BPA), a chemical that leaches into acidic beverages. Research on BPA’s effects on the gut, while still largely in animal models, is concerning. A rat study found that both single-dose and 28-day exposure to BPA-containing food slowed gastrointestinal transit, the kind of change that leads to constipation.14PubMed Central. The Effects of Short- and Long-Term Ingestion of Plastic Toxin Bisphenol A on Gastrointestinal Transit Time in Rats Separate research showed that BPA, by binding to estrogen receptors in the gut, alters epithelial barrier function and heightens the visceral pain response.15Journal of Medical Evidence. Effects of Plastic Chemical Bisphenol a on Gut Functions
Before you throw out every can in your fridge, keep some perspective. The doses used in these animal studies are generally higher than what a person would absorb from drinking canned soda, and many manufacturers have been shifting to BPA-free linings over the past decade. Still, it is a variable that rarely enters the “does soda cause constipation” discussion but probably should, especially for heavy consumers. A couple of cans a week is unlikely to matter. Multiple cans a day, every day, over years, adds up to a meaningful cumulative exposure.
Sodium Benzoate and Other Preservatives
Soda also contains preservatives, most commonly sodium benzoate, which prevents mold and bacteria growth in acidic drinks. Since gut motility partly depends on a healthy microbiome, any additive that shifts microbial populations is worth examining. A recent study tested sodium benzoate at a dose ten times the accepted daily intake limit and found that it did change some bacterial populations, including a drop in one major bacterial phylum and a shift in the production of certain short-chain fatty acids. However, overall microbial diversity was barely affected, leading the researchers to conclude the effect was minimal even at that exaggerated dose.16PubMed Central. The Effect of Sodium Benzoate on the Gut Microbiome Across Age Groups
This is reassuring for the average soda drinker. The preservative that keeps your drink shelf-stable is unlikely to be the thing causing digestive problems, at least not at the amounts present in a few servings. When people blame “chemicals in soda” for their constipation, the sugar and caffeine are far more likely suspects than the trace preservatives.
Practical Takeaways for Your Gut
If you are dealing with constipation and you drink soda regularly, the evidence points to a few straightforward adjustments. Cutting back on sugar-sweetened soda is the highest-yield change, because the sugar-driven fluid trapping in the small intestine and the displacement of fiber-rich foods from your diet both push your bowels in the wrong direction. Switching to plain sparkling water, based on the constipation-relief data, could actually help rather than hurt. If you prefer some flavor, sparkling water with a squeeze of fruit avoids the sugar load entirely.
Caffeinated soda in moderation, say one can per day, is unlikely to cause problems and might even give your gut a slight nudge. But stacking multiple caffeinated drinks pushes you past the inflection point where caffeine’s constipating effects begin to outweigh its stimulating ones. Diet soda sits in a gray zone: better than regular soda from a sugar perspective, but the sweetener-microbiome question remains open. If you have switched to diet soda and noticed worsening digestive symptoms, it is worth trying a period without it to see if things improve.
One thing that gets lost in the soda debate is displacement. Every ounce of soda you drink is an ounce of water, herbal tea, or another hydrating fluid you did not drink instead. And more broadly, heavy soda consumption tracks with dietary patterns that are already constipation-promoting: less fiber, fewer whole foods, more processed meals. Soda may contribute to constipation directly through its ingredients, but it also contributes indirectly by shaping the rest of what you eat and drink.