Is Soda Hydrating at All? The Science Explained

Soda does hydrate you. A can of cola, diet cola, or other carbonated soft drink is mostly water, and your body absorbs that water in much the same way it absorbs plain water. In controlled trials measuring urine output over several hours, cola performed no differently from still water as a hydrating beverage. That finding surprises people who assume the caffeine, sugar, or carbonation in soda must somehow undo the hydration, but the research consistently says otherwise. The full picture, though, involves some real caveats about what “hydrating” means versus what is actually healthy to drink.

How Soda Stacks Up Against Water in Hydration Trials

The most direct evidence comes from a study designed to create a “beverage hydration index,” which measured how much urine people produced in the four hours after drinking various beverages compared to water. Researchers gave participants equal volumes of different drinks and tracked what came out. Cola, diet cola, hot tea, iced tea, coffee, lager beer, orange juice, sparkling water, and a sports drink all produced urine volumes that were statistically indistinguishable from water’s.¹ In other words, your body retained about the same amount of fluid from a cola as it did from a glass of water over a half-day window.

A separate trial looked at 24-hour hydration status across different beverage patterns and reached the same conclusion. When researchers assessed urine concentration and total urine volume over a full day, hydration status did not differ based on what people had been drinking.² The body is remarkably good at pulling water out of whatever liquid you give it, regardless of what else is dissolved in that liquid.

The Caffeine Question

The most common reason people assume soda is dehydrating is the caffeine. Caffeine is a mild diuretic, meaning it can push your kidneys to produce a bit more urine. But the amount of caffeine in a typical soft drink is modest, and the research shows that at those levels, the diuretic effect is essentially negligible.

A review of the available evidence found that caffeine doses equivalent to what you’d find in standard servings of tea, coffee, and carbonated soft drinks “appear to have no diuretic action.”³ You need to consume at least 250 to 300 milligrams of caffeine before the diuretic effect becomes meaningful, and that’s roughly the amount in two to three cups of brewed coffee. A 12-ounce can of cola contains somewhere around 30 to 45 milligrams of caffeine, which is a fraction of that threshold. Even when caffeine does cause a short bump in urine production, the effect is temporary and much weaker in people who consume caffeine regularly, because tolerance builds quickly.

A meta-analysis pooling data from multiple studies confirmed this conclusion, finding that caffeine produced only a “minor diuretic effect” overall and that even that small effect disappeared when people exercised.⁴ So if you drink a caffeinated soda before or during physical activity, the caffeine is even less of a factor in your fluid balance.

What Sugar Does to Fluid Absorption

Regular soda contains a substantial amount of sugar, typically around 35 to 40 grams in a 12-ounce can. This creates a drink that is quite concentrated compared to what your small intestine handles most efficiently. Research on oral rehydration has long shown that the intestine absorbs water fastest from solutions that are slightly less concentrated than body fluids, with a combination of some glucose and some sodium pulling water across the gut wall.⁵ Soda is essentially the opposite formulation: high sugar, minimal sodium, and an overall concentration that’s higher than what the gut prefers.

Does this mean soda absorbs more slowly? Probably somewhat, yes. The intestine has to dilute a concentrated sugar solution before it can absorb the water efficiently, and that process takes a little longer than absorbing plain water or a well-formulated rehydration drink. But “slower absorption” is not the same as “no absorption.” The water in soda still gets absorbed. It just may take a bit more time for the full benefit to arrive, and in the meantime, some of that concentrated sugar can pull water into the intestinal lumen rather than out of it. For everyday hydration when you are not dehydrated or exercising hard, this difference is clinically meaningless. For someone who is severely dehydrated or trying to recover fluid losses quickly, it matters more.

The sugar in soda also means additional metabolic processing. Fructose, which makes up roughly half of the sugar in most sodas (whether from high-fructose corn syrup or sucrose), is metabolized primarily in the liver through a pathway that differs from glucose. This pathway can increase fat production in the liver and raise uric acid levels.⁶ None of this affects whether the water in soda hydrates you right now, but it matters for long-term health in ways that go well beyond the hydration question.

Diet Soda Versus Regular

If sugar slows absorption slightly and adds metabolic baggage, does diet soda hydrate better? From a pure fluid-balance perspective, the evidence suggests the difference between regular and diet soda is negligible. The beverage hydration index study tested both cola and diet cola and found neither differed from water in cumulative urine output at four hours.¹ And the 24-hour trial measuring hydration markers across different beverage compositions found no differences either.²

Diet soda does remove the sugar-related osmolality issue: without 35-plus grams of dissolved sugar, diet soda is much closer in concentration to plain water, so it likely absorbs a bit more quickly in the gut. It also eliminates the caloric load and the fructose metabolism concerns. But in terms of whether the water in the drink ends up hydrating your body, the trials that have measured this directly find no meaningful gap between the two.

Does the Fizz Affect Anything?

Carbonation adds dissolved carbon dioxide, which creates carbonic acid and gives the drink a slightly lower pH. Some people worry this somehow interferes with hydration. It doesn’t, based on the available evidence, but carbonation does affect how comfortable it is to drink large volumes quickly.

When people drink a carbonated beverage, the gas temporarily inflates the stomach. One study found that after 300 milliliters of a carbonated drink, total gastric volume increased significantly compared to the same drink without bubbles.⁷ This bloating sensation can make you feel full sooner and might discourage you from drinking as much. For someone who is mildly dehydrated and trying to take in fluid, the carbonation could subtly slow the pace of intake. On the other hand, many people find carbonated drinks more palatable than flat water, which can lead them to drink more overall. The net effect depends entirely on the individual.

Research on gastrointestinal symptoms from carbonated beverages found that discomfort from the fizz tends to appear only above about 300 milliliters in a single sitting.⁸ Below that amount, most people experience no meaningful stomach issues. So a single can of soda is right around the threshold where some people will feel the bloat and others won’t notice it at all.

Soda for Post-Exercise Rehydration

Where soda genuinely falls short is after hard exercise. When you sweat heavily, you lose both water and electrolytes, especially sodium. Getting those back efficiently requires a drink that contains sodium and a modest amount of carbohydrate, both of which help the gut pull water into the bloodstream faster. This is why oral rehydration solutions and sports drinks exist.

A study comparing water, a sports drink, and an oral rehydration solution after exercise-induced dehydration found that athletes retained roughly 74 to 77 percent of the fluid from the sports drink and ORS, compared to only about 58 percent from plain water, over 3.5 hours of recovery.⁹ The sodium in those drinks signals the kidneys to hold on to more fluid rather than dumping it as urine.

Soda sits in an awkward middle ground here. It has some carbohydrate (too much, actually, for optimal absorption) and almost no sodium. It doesn’t trigger the same kidney-level fluid retention that an electrolyte-containing beverage does. After a tough workout or on a hot day when you’ve been sweating for hours, soda is a worse rehydration choice than a proper sports drink or even just water with a pinch of salt. It will still hydrate you to some degree, but the recovery will be slower and less complete.

Kidney Stones and the Long Game

The acute hydration story and the chronic health story are very different for soda. A drink can hydrate you today and still cause problems if you rely on it for years. Kidney stones are one area where this distinction is particularly clear.

A large prospective study found that people who consumed the most sugar-sweetened cola had a 23 percent higher risk of developing kidney stones compared to those who drank the least. For sugar-sweetened noncola sodas, the risk was even higher at 33 percent. Interestingly, coffee, tea, beer, wine, and orange juice were all associated with lower kidney stone risk.¹⁰ The mechanism isn’t entirely settled, but it likely involves the way large amounts of fructose increase uric acid production and alter the composition of urine, making stone formation more likely over time.

This creates a paradox worth understanding. In any given moment, a soda hydrates you about as well as water, and staying hydrated is one of the most important ways to prevent kidney stones. But if soda is your primary source of hydration day after day, the sugar and other compounds in it can increase your stone risk despite the fluid itself being protective. The hydration benefit is real, but it’s offset by the metabolic costs of how that hydration was delivered.

The Phosphoric Acid Factor in Cola

Colas specifically contain phosphoric acid, which gives them their characteristic sharp tang. This ingredient has drawn attention because of its potential to interfere with calcium metabolism. The concern is straightforward: phosphoric acid introduces a load of phosphorus that can bind calcium in the gut and shift the balance of calcium and phosphorus in the blood.

Animal research has shown that soft drink intake can cause significant increases in calcium and phosphorus excretion in urine. In younger animals, the effects were more pronounced, including drops in ionized calcium and active vitamin D levels.¹¹ In human studies, the most striking findings involve populations already vulnerable to calcium imbalances. A study of postmenopausal women found that consuming one or more bottles of cola per day was independently associated with low blood calcium.¹² And a case report documented a patient with impaired parathyroid function whose calcium levels stayed dangerously low despite high-dose calcium supplements, because daily cola consumption was undermining absorption.¹³

This is a cola-specific issue, not a soda-in-general issue. Lemon-lime sodas, ginger ale, and most other non-cola soft drinks use citric acid instead of phosphoric acid and don’t carry the same risk. But for people who drink cola as their go-to beverage throughout the day, the phosphoric acid load is something to be aware of, particularly for older adults and anyone with conditions affecting calcium or parathyroid hormone.

What Soda Does in Your Mouth

There’s a smaller but still interesting dimension of hydration that involves the mouth itself. One reason people reach for soda is that it feels refreshing, and part of that sensation is real physiology rather than just perception. Carbonated and sugary drinks stimulate a burst of saliva production. Research measuring salivary flow rates found that after drinking a carbonated cola, flow peaked immediately and returned to baseline within about 15 minutes.¹⁴ Acidic beverages in general trigger a sharp spike in parotid saliva flow that falls off within a few minutes.¹⁵

That saliva burst contributes to the sensation of quenching thirst, and it helps clear sugar and acid from tooth surfaces. But it’s temporary. In the long run, the acid and sugar in soda promote enamel erosion and cavities, which is among the most well-documented downsides of regular soda consumption. If the erosion becomes severe enough to make drinking uncomfortable, it could paradoxically discourage adequate fluid intake. This is an unlikely scenario for most adults, but for children or people with already-compromised dental health, it’s worth noting that the drink that “feels hydrating” in the moment can damage the structures that make comfortable drinking possible.

When Soda Is Your Only Option

Given everything above, the practical takeaway is context-dependent. If you are mildly thirsty and soda is available, it will hydrate you. If the choice is between a cola and nothing at all, drink the cola. Dehydration is a more immediate threat than the sugar or phosphoric acid in a single serving, and your body will absorb most of the water in that drink just fine.

Where the calculus shifts is when soda becomes a habitual, primary fluid source. The acute hydration benefit is real every single time, but the accumulated sugar intake, the fructose metabolism burden on the liver, the increased kidney stone risk, and the phosphoric acid effects on calcium balance all pile up over months and years. The hydration trials confirm that your body doesn’t care much about what the water was dissolved with when it comes to absorbing fluid. But the rest of your biology very much cares about the 40 grams of sugar, the acid load, and the metabolic byproducts that came along for the ride.

For athletes and people rehydrating after heavy sweating, soda is a poor choice compared to drinks formulated with electrolytes. For everyday sipping, it hydrates about as well as water but with significant nutritional downsides. For someone who simply doesn’t enjoy plain water and would otherwise drink very little, the fact that soda hydrates meaningfully is genuinely useful information. Staying hydrated with soda is better than staying dehydrated without it.