Regular Coke does appear to raise kidney stone risk, though the sugar it contains is a bigger culprit than the carbonation or the phosphoric acid that most people blame. A large prospective study following over 194,000 people found a 23% higher risk of developing kidney stones among those who drank the most sugar-sweetened cola compared with those who drank the least.1PubMed Central. Soda and other beverages and the risk of kidney stones The story gets more interesting when you pull the pieces apart, because the ingredients in a can of Coke each do something slightly different inside your kidneys, and not all of them are harmful.
What the Biggest Study Actually Found
The most cited research on this question pooled data from three large cohorts tracked over eight or more years. People who drank the most sugar-sweetened cola had that 23% bump in stone risk, but sugar-sweetened noncola sodas were actually worse, with a 33% higher risk. Artificially sweetened noncola drinks showed a borderline increase as well.1PubMed Central. Soda and other beverages and the risk of kidney stones The fact that noncola sodas, which contain citric acid instead of phosphoric acid, carried a higher risk than cola is a strong hint that the sugar load matters more than the specific acid in the drink.
This study also looked at other beverages. Coffee, tea, beer, wine, and orange juice were all associated with lower risk. The pattern is consistent: drinks that are sugar-heavy push risk up, while drinks that boost hydration or add protective compounds like citrate tend to pull risk down.
Why Sugar Is the Bigger Problem
A standard 12-ounce can of Coke contains about 39 grams of sugar, most of it from high-fructose corn syrup. Fructose has a direct connection to kidney stones that goes beyond just adding calories. When your body metabolizes fructose, it produces uric acid as a byproduct. That uric acid can end up in your urine, and high urinary uric acid is one of the recognized risk factors for stone formation.2PubMed Central. Fructose increases risk for kidney stones: potential role in metabolic syndrome and heat stress
Fructose also appears to increase the urinary excretion of calcium and oxalate, both key ingredients in the most common type of kidney stone, the calcium oxalate stone.3PubMed. Fructose consumption and the risk of kidney stones So fructose is doing two things at once: raising uric acid levels and pushing more of the raw materials for stone formation into your urine. When you consider that many heavy soda drinkers consume multiple cans a day, the cumulative fructose load becomes substantial.
This mechanism also explains why sugar-sweetened noncola sodas carry similar or even higher risk than Coke itself. The sugar content in many lemon-lime or fruit-flavored sodas is comparable to or higher than Coke, and they deliver the same fructose hit to your kidneys regardless of which acid flavors the drink.
The Phosphoric Acid Myth
Phosphoric acid is the ingredient most people point to when they claim Coke is uniquely bad for kidneys. The reasoning sounds logical: phosphoric acid could bind calcium in the gut, disrupt calcium balance, and eventually lead to more calcium being dumped into urine. A theoretical pathway exists where high dietary phosphorus paired with low calcium stimulates parathyroid hormone, which in turn causes the body to pull calcium from bones and excrete it.4The American Journal of Clinical Nutrition. Cola beverages and bone mineral density in older women and men: The Framingham Osteoporosis Study
But actual experiments testing this idea have been surprisingly disappointing for the phosphoric-acid-is-the-villain narrative. A controlled crossover study that compared several carbonated beverages head to head found that phosphoric acid without caffeine did not increase calcium in urine. The bump in urinary calcium seen with caffeinated colas matched what caffeine alone would produce.5The American Journal of Clinical Nutrition. Carbonated beverages and urinary calcium excretion In other words, the phosphoric acid was not contributing any extra calcium loss beyond what caffeine was already doing.
That doesn’t entirely let phosphoric acid off the hook. A small study where participants drank large quantities of cola daily found that urinary citrate dropped by an average of 122 milligrams and urinary oxalate increased by about 8 milligrams.6Urology. Changes in urinary magnesium citrate, and oxalate levels due to cola consumption Citrate is one of the body’s natural defenses against stone formation because it binds calcium in urine and keeps it from crystallizing. A drop in citrate combined with a rise in oxalate would shift the balance toward stone formation. However, that study involved only three people who managed to drink about three quarts of cola per day, which is an extreme intake. A separate study also found that cola consumption supported calcium oxalate crystallization in urine to a greater extent than control conditions, and the researchers concluded that patients should consider avoiding cola when trying to increase their fluid intake.7PubMed. Effect of cola consumption on urinary biochemical and physicochemical risk factors associated with calcium oxalate urolithiasis
So phosphoric acid likely plays some role, particularly by lowering protective citrate levels, but the evidence that it’s the primary driver of stone risk in cola drinkers is weaker than most internet health advice suggests. Sugar and caffeine appear to be doing more of the heavy lifting.
Caffeine and Calcium in Your Urine
A can of Coke contains about 34 milligrams of caffeine, modest compared to coffee, but still enough to have a measurable effect on your kidneys. One study found that high-dose caffeine increased the kidney’s clearance of calcium by 77%, and this effect appeared linked to how caffeine interferes with sodium reabsorption in the kidney.8PubMed. The effect of high-dose, short-term caffeine intake on the renal clearance of calcium, sodium and creatinine in healthy adults More calcium leaving through your urine means more calcium available to crystallize with oxalate.
The catch is that most of these caffeine studies use doses well above what a single soda delivers. Coffee drinkers consume far more caffeine than cola drinkers, yet the large epidemiological studies consistently find coffee associated with lower stone risk. The likely explanation is that coffee provides a large volume of fluid that dilutes urine, and this dilution effect overwhelms the modest caffeine-driven increase in calcium. A can of Coke offers less fluid and far more sugar per unit of caffeine, which is a much less favorable trade-off.
An earlier study specifically looking at carbonated beverage consumption in healthy women found no significant difference in urinary calcium excretion during a high-carbonated-beverage diet compared with a control diet.9JAMA Internal Medicine. A Preliminary Report of the Short-term Effect of Carbonated Beverage Consumption on Calcium Metabolism in Normal Women The mixed results suggest that moderate consumption may not move the needle much, while heavy daily intake could start to matter.
Is Diet Coke Any Better?
If sugar is the main problem, then cutting to Diet Coke should eliminate most of the risk, and the evidence mostly supports that. A study that had participants drink either caffeine-free Diet Coke, Fresca, or bottled water found no statistically significant differences in any urinary stone risk parameters between the three groups. Urine volumes went up across all groups, and calcium oxalate supersaturation actually dropped.10PubMed Central. Effect of soda consumption on urinary stone risk parameters The researchers concluded there was no increased risk from consuming those sugar-free sodas compared with plain water when it came to stone formation.
This is an underappreciated finding. A lot of stone-prevention advice lumps all sodas together, telling patients to avoid carbonated drinks entirely. But if the carbonation and acid themselves are neutral for stone risk when sugar is removed, that guidance is probably too broad. A person who genuinely enjoys diet cola and would otherwise drink less fluid might be better off having the diet soda than drinking nothing. Hydration itself is one of the strongest protective factors against stone formation, and any fluid that keeps urine dilute has value.
That said, the large cohort data did show a marginally significant increase in stone risk for artificially sweetened noncola drinks.1PubMed Central. Soda and other beverages and the risk of kidney stones The borderline nature of that result makes it hard to draw firm conclusions, and the researchers did not find the same trend for artificially sweetened cola specifically. So diet cola appears to sit in a fairly neutral zone, neither clearly protective nor clearly harmful.
Drinks That Actually Help Prevent Stones
If you’re actively trying to prevent stones, some beverages go beyond mere hydration and provide compounds that actively work against stone formation. Citrate is the big one. It binds with calcium in urine, preventing it from pairing up with oxalate to form crystals. This is actually the same mechanism behind potassium citrate, one of the most commonly prescribed medications for recurrent stone formers.
A review of clinical studies found that orange juice, grapefruit juice, and lemon juice all increased urinary citrate levels, which is exactly what you want. Orange juice and grapefruit juice also had an alkalinizing effect on urine, making it harder for uric acid stones to form. Lemon juice raised citrate but did not significantly change urine pH.11PubMed Central. Role of Citrus Fruit Juices in Prevention of Kidney Stone Disease (KSD): A Narrative Review There’s a trade-off with orange juice, though: it also raised urinary oxalate levels and comes with a high carbohydrate load, which could partially offset its benefits for people managing their weight or blood sugar.
A comparison of orange juice and lemonade found that orange juice had a greater alkalinizing and citrate-boosting effect, despite the two having similar citrate content. Orange juice consumption was associated with lower calculated calcium oxalate supersaturation and lower undissociated uric acid in urine.12PubMed. Comparative value of orange juice versus lemonade in reducing stone-forming risk For people who find plain water boring and want something that tastes good while also lowering stone risk, diluted citrus juices or homemade lemonade (not the sugar-loaded commercial kind) are solid options.
Plain water remains the gold standard. The simplest advice for preventing kidney stones is to drink enough fluid to produce at least two liters of urine per day, and water is the easiest way to get there without introducing sugar, extra calories, or potential complications. But it doesn’t have to be the only thing you drink.
Children, Teens, and the Rising Stone Rate
Kidney stones were once considered an adult problem, but their prevalence in children and adolescents appears to be climbing in developed countries. Several factors may be driving this trend, including rising rates of childhood obesity, shifting dietary patterns, and possibly even climate change leading to more dehydration.13PubMed Central. The increasing pediatric stone disease problem The evidence base for pediatric kidney stones is much thinner than for adults, with very few large epidemiological studies available.
What makes this relevant to the soda question is that children and teenagers are among the heaviest consumers of sugar-sweetened beverages. The fructose-driven mechanisms that raise stone risk in adults presumably apply to younger people as well, and kids who drink large amounts of soda may be displacing water and other fluids that would otherwise dilute their urine. The concern is less about any single can of soda and more about a sustained dietary pattern where sweetened drinks are the primary source of hydration.
How Much Is Too Much?
One of the frustrations with the research on this topic is that most studies compare the highest consumption category against the lowest, without pinpointing a specific threshold where risk jumps. There is no magic number of Cokes per week that flips a switch. The 23% increased risk seen in the large cohort study was for people in the top consumption bracket, which generally means daily or near-daily intake of sugar-sweetened cola.
An occasional Coke at a barbecue is almost certainly not going to give you a kidney stone. The concern is really about habitual, heavy consumption, the person who drinks one or two cans every day and may be simultaneously under-hydrating with plain water. If you have a personal or family history of calcium oxalate stones, cutting back on regular sugary soda is a straightforward and evidence-supported step. Switching to diet versions, sparkling water, or citrus-based drinks addresses most of the known risk factors while still letting you enjoy something besides tap water.
It’s also worth remembering that kidney stone formation is influenced by dozens of factors. Dietary oxalate from foods like spinach and nuts, overall calcium intake, sodium consumption, body weight, genetics, and how much you sweat all play roles. Soda is one modifiable piece of the puzzle, and not even the biggest one for most people. Adequate hydration across all beverage types matters more than fixating on whether one specific drink is “bad.” But if you’re reaching for a drink specifically to protect your kidneys, a glass of water or diluted orange juice will always beat a Coke.