Drinking a regular can of soda does not dehydrate you. Despite decades of popular belief that the caffeine in cola and similar drinks pulls water out of your body faster than the beverage puts it in, the research consistently shows that soda contributes to your daily fluid intake much like water does. That said, the full picture involves a few caveats worth knowing, especially around sugar concentration, physical activity, and long-term consumption patterns.
Where the Caffeine Myth Comes From
Caffeine is a mild diuretic, meaning it can increase urine production. Early research on this effect, dating back over 80 years, found that caffeine does push the kidneys to produce more urine under certain conditions. That finding made its way into public health messaging, and the advice to “not count caffeinated drinks toward your daily fluid intake” became widespread. The problem is that the conditions required for caffeine to meaningfully increase urine output are fairly specific and don’t reflect how most people actually drink soda.
A review of the available research found that caffeine needs to be consumed in large doses, at least 250 to 300 milligrams, before it triggers a noticeable short-term increase in urine output. That’s the equivalent of two to three cups of brewed coffee. A typical 12-ounce can of cola contains roughly 30 to 45 milligrams of caffeine, well below that threshold. The same review concluded that caffeine at the doses normally found in standard servings of carbonated soft drinks appears to have no diuretic action at all.1PubMed. Caffeine ingestion and fluid balance: a review
There’s also a tolerance effect. Even in people who are sensitive to caffeine’s diuretic properties, regular intake over four or five consecutive days dramatically blunts the response. So someone who drinks a soda every day is unlikely to experience any measurable increase in urine output from the caffeine alone. An 11-day controlled study found that varying caffeine doses had no detectable effect on any hydration marker, including urine volume, urine concentration, blood sodium levels, and body mass.2International Journal of Sport Nutrition and Exercise Metabolism. Fluid, Electrolyte, and Renal Indices of Hydration during 11 Days of Controlled Caffeine Consumption
How Soda Actually Stacks Up Against Water
Researchers have directly tested how different beverages affect hydration in head-to-head comparisons. One well-known trial developed a “beverage hydration index” by measuring how much urine people produced over four hours after drinking various beverages compared to water. Cola, diet cola, hot tea, iced tea, coffee, lager, orange juice, sparkling water, and a sports drink all produced urine volumes that were statistically indistinguishable from water.3PubMed. A randomized trial to assess the potential of different beverages to affect hydration status: development of a beverage hydration index In other words, your body retains about the same amount of fluid from a cola as from a glass of plain water.
A separate study looked at hydration over a full 24-hour period in men who drank different combinations of beverages, including caffeinated and non-caffeinated options, caloric and non-caloric. There were no differences in hydration status regardless of which combination the participants consumed.4PubMed. The effect of caffeinated, non-caffeinated, caloric and non-caloric beverages on hydration A follow-up trial that specifically varied the proportion of water versus other beverages in participants’ daily fluid intake found the same result: whether people got most of their fluids from water or from a mix that included sodas and other drinks, their total body water, serum concentration, and urine markers all remained in normal range.5PubMed. Hydration Status over 24-H Is Not Affected by Ingested Beverage Composition
Even a thorough review of the “drink eight glasses of water a day” recommendation acknowledged that caffeinated beverages can be counted toward daily fluid intake, supported by the body’s own regulatory system for maintaining water balance.6PubMed. “Drink at least eight glasses of water a day.” Really? Is there scientific evidence for “8 x 8”?
What Sugar Does to Fluid Absorption
While caffeine isn’t the issue people think it is, the sugar in regular soda introduces a different wrinkle. How quickly your body absorbs water from a beverage depends partly on its osmolality, which is a measure of how concentrated the dissolved particles in the drink are relative to your blood. A regular soda is hypertonic, meaning it has a higher concentration of dissolved sugar than your blood does.
In the small intestine, fluid absorption works best when glucose and sodium are present at moderate concentrations. Glucose and sodium are taken up together by a shared transport system in the intestinal lining, and their absorption creates a gradient that pulls water along.7PubMed. The effects of consuming carbohydrate-electrolyte beverages on gastric emptying and fluid absorption during and following exercise This is why oral rehydration solutions and sports drinks work well: they have just enough sugar and salt to drive absorption without overdoing it.
Regular soda overshoots that sweet spot. A systematic meta-analysis found that hypertonic drinks cause water to move from the body into the intestinal space rather than the other way around, producing a net negative effect on water absorption and plasma volume. The more hypertonic fluid consumed, the worse the outcome for hydration of the blood.8PubMed Central. The Hydrating Effects of Hypertonic, Isotonic and Hypotonic Sports Drinks and Waters on Central Hydration During Continuous Exercise: A Systematic Meta-Analysis and Perspective High osmolality also slows gastric emptying, meaning the fluid sits in your stomach longer before it even reaches your intestines.
This doesn’t mean a can of soda dehydrates you in any practical sense. When you’re sitting at your desk or watching television, the temporary slowdown in absorption barely matters because your body has time to process the fluid at its own pace. The beverage hydration index study described above confirmed that cola still hydrated like water over four hours. But in situations where you need rapid rehydration, like during intense exercise, this delay becomes much more meaningful.
Exercise in the Heat Changes the Equation
The one scenario where soda performs genuinely poorly compared to water is during physical exertion in hot conditions. A trial that had participants exercise in the heat and then rehydrate with either a soft drink or water found strikingly different outcomes. About three-quarters of the participants in the soft drink group showed signs of Stage 1 acute kidney injury afterward, compared to fewer than one in ten in the water group. Markers of kidney stress were elevated not just immediately but also overnight. The researchers attributed this to vasopressin-mediated mechanisms: the body’s antidiuretic hormone response was significantly higher after soft drink consumption, along with increases in serum uric acid.9PubMed. Soft drink consumption during and following exercise in the heat elevates biomarkers of acute kidney injury
The combination of sugar, caffeine, heat stress, and dehydration from sweating appears to strain the kidneys in ways that water alone does not. This is a niche scenario, but it’s a real one for outdoor workers, athletes, and anyone doing manual labor in summer conditions. Under those circumstances, water or a properly formulated electrolyte drink is a clearly better choice than soda. Research into optimal rehydration drinks has consistently found that beverages should contain carbohydrate and sodium to stimulate fluid absorption and retention, but that the sugar content and osmolality need to be kept relatively low.10ScienceDirect (Elsevier). Review Functional drinks for athletes
How Carbonation Affects Thirst Perception
An interesting side angle is what the bubbles in soda do to your sense of thirst. Carbonation creates an oral irritation sensation that your brain interprets as part of drinking water. Research on thirsty adults found that carbonation and coldness both enhanced the feeling of thirst quenching, reducing additional water intake by up to half compared to still, room-temperature water. Participants who drank carbonated beverages also overestimated how much liquid they had consumed by about 22 percent.11PubMed Central. Oral Cooling and Carbonation Increase the Perception of Drinking and Thirst Quenching in Thirsty Adults
This cuts both ways. On one hand, the enhanced satisfaction from carbonation could help someone feel hydrated with less volume, which is fine if you’re already getting enough fluids overall. On the other hand, if you’re relying on thirst to tell you when to drink more, the carbonation in soda might shut off that signal prematurely. Someone doing yard work on a hot day who reaches for a cold soda might feel satisfied and stop drinking sooner than they should.
Temperature plays a role too. Cold beverages, the way most people drink soda, empty from the stomach more slowly than body-temperature liquids. A study measuring gastric emptying of drinks at different temperatures found that cold drinks (around 4°C) left the stomach significantly more slowly than drinks at body temperature.12PubMed Central. Effect of meal temperature on gastric emptying of liquids in man Combined with the high sugar content slowing things down further, an ice-cold regular soda takes longer to deliver its water to your bloodstream than room-temperature plain water would. Again, this is only practically relevant when speed of absorption matters.
Diet Soda Is a Non-Issue for Hydration
If the sugar in regular soda is what creates the osmolality problem, diet soda largely sidesteps it. Artificial sweeteners contribute virtually no osmotic load, so diet sodas are much closer to water in terms of concentration. The beverage hydration index study found no difference between diet cola and water for urine output over four hours.3PubMed. A randomized trial to assess the potential of different beverages to affect hydration status: development of a beverage hydration index And the 24-hour hydration study that tested various beverage combinations, including non-caloric options, confirmed that hydration status did not differ based on what people drank.5PubMed. Hydration Status over 24-H Is Not Affected by Ingested Beverage Composition The caffeine content in diet soda is the same as regular soda, and as covered earlier, that amount is too low to matter.
Diet soda has its own set of health debates around metabolic effects, gut bacteria, and appetite signaling, but dehydration is not one of the credible concerns.
Heavy Soda Drinking and Your Kidneys Over Time
While a single soda won’t dehydrate you, heavy long-term consumption of sugar-sweetened soda raises concerns that go beyond any single drinking session. An analysis using national health survey data found that drinking more than one soda per day was associated with higher odds of elevated uric acid levels and chronic kidney disease. Among heavy soda drinkers who also had very high serum uric acid, the odds of kidney disease were roughly two and a half times higher.13PubMed Central. Sugar-sweetened soda consumption, hyperuricemia, and kidney disease
The mechanism likely involves fructose, which is abundant in both high-fructose corn syrup and sucrose. When the body metabolizes fructose, it produces uric acid as a byproduct. Chronically elevated uric acid can damage kidney tubules and contribute to the kind of progressive injury that eventually impairs the kidneys’ ability to concentrate urine and regulate fluid balance. In that sense, very heavy soda consumption could indirectly compromise hydration over years by degrading the organ responsible for maintaining it.
This isn’t a reason to worry that the occasional soda is harming your kidneys. But people who drink multiple sugar-sweetened sodas daily are putting their kidneys under a biochemical load that plain water obviously does not impose.
Older Adults Face Different Risks
Hydration gets more complicated with age. Older adults are generally more vulnerable to dehydration because of changes in kidney function, a blunted thirst sensation, medications like diuretics, and chronic conditions that alter fluid balance.14PubMed Central. Hydration Status in Older Adults: Current Knowledge and Future Challenges For this population, the question of whether soda counts toward hydration is somewhat beside the point. Any fluid counts, but the priority should be on reliable fluid intake of any kind rather than debating beverage rankings.
The carbonation effect on thirst perception is potentially more concerning here. If an older adult’s thirst drive is already diminished and a carbonated drink makes them feel even more satisfied with less fluid, the combination could contribute to under-drinking. For elderly people living alone or in care settings, where fluid intake is already a chronic challenge, encouraging plain water or low-sugar beverages is a safer default than relying on soda as a hydration source.
What Soda Does to Your Mouth
One area where soda’s composition matters in a tangible way isn’t hydration itself but what happens in your mouth. Research measuring salivary changes after consuming sugary beverages found that salivary pH drops immediately after drinking them, meaning the mouth becomes more acidic.15PubMed Central. Effect of Various Sugary Beverages on Salivary pH, Flow Rate, and Oral Clearance Rate amongst Adults Both the sugar and the phosphoric or citric acid in soda contribute to this acidic environment, which promotes enamel erosion and creates conditions favorable for cavity-forming bacteria.
Salivary flow rate does increase after drinking soda, which is the mouth’s natural response to wash away the acid. But frequent soda consumption throughout the day gives the mouth less recovery time between acid exposures. This is a different kind of “drying out” than systemic dehydration, but it’s worth noting because people who drink a lot of soda sometimes complain of a dry or sticky mouth. That sensation is more about the oral environment than about whole-body hydration.
Adolescents and Soda as a Fluid Source
Survey data on drinking patterns show that adolescents tend to be the heaviest soda consumers. A study of fluid intake across age groups in France found that teenagers consumed the most soda, averaging about 169 milliliters per day, which represented roughly 15 percent of their total daily fluid intake. Children also drank soda regularly, while adults and older adults consumed relatively little.16European Journal of Clinical Nutrition. A study of fluid intake from beverages in a sample of healthy French children, adolescents and adults
For teenagers, soda functioning as a hydration source isn’t the health concern. The concern is the sugar load, the caloric displacement of more nutritious beverages like milk, and the dental effects of regular acid exposure. From a pure hydration standpoint, a teenager drinking a couple of sodas a day is still getting fluid from them. But framing soda as a legitimate hydration strategy misses the forest for the trees. The fluid “counts,” but the package it comes in carries enough other downsides that encouraging water, milk, or other low-sugar options makes far more sense for habitual consumption.