Explainer · Nutrition & Diet
Sweets, not ultra-processed food in general, tracked with kidney stones in 366 people in Tehran
Ultra-processed food gets blamed for most modern ailments. Split into its parts, only the sugary end of it lined up with calcium oxalate stones, and dairy went the other way.
- A small study comparing 183 people with confirmed stones against 183 matched people without.
- Total ultra-processed food intake showed no link to stones once other factors were accounted for.
- The highest intake of sweets and confectionery carried about twice the odds of a stone.
- People eating the most dairy had lower odds, which runs against a common worry about calcium.
- Diet was recalled after the diagnosis, so memory and hindsight could shape the answers.
Anyone who has passed a kidney stone will take dietary advice seriously for a while. The trouble is that the advice tends to arrive as a list of forbidden foods with no sense of which item on it actually matters, and ultra-processed food has recently been added to the list wholesale.
A study in Tehran took the category apart instead. Comparing 183 people with confirmed calcium oxalate stones against 183 matched people without, it asked which parts of a processed diet, if any, were doing the work. The answer was narrower than the headline category: sweets stood out, dairy went the other way, and ultra-processed food as a whole did nothing.
What a calcium oxalate stone is made of
Most kidney stones are this kind. Calcium in the urine binds to oxalate, a small molecule that comes partly from food and partly from the body’s own metabolism, and the two crystallize into something with edges.
MedlinePlus describes the result plainly: a stone may cause severe pain in the back or side that will not go away, along with blood in the urine, nausea and pain when passing urine. Once someone has had one, the chance of another is high, which is why the diet question comes up at all.
How the Tehran diet comparison was built
This was a case-control study, the design that starts with the outcome and works backwards. Researchers recruited 183 incident cases with confirmed calcium oxalate stones, meaning people newly diagnosed rather than long-term sufferers, alongside 183 age- and sex-matched controls, then asked both groups what they habitually ate.
Diet was assessed using a validated 147-item food frequency questionnaire, and food items were categorized based on the Nova classification system, a scheme that sorts food by how industrially processed it is rather than by nutrient. That allowed the team to look at the whole ultra-processed category and at its parts separately, which turned out to matter.
What the sweets and dairy numbers showed
Three results, and the least expected is the one about the category as a whole. Total ultra-processed food intake did not show a significant relationship with the odds of calcium oxalate kidney stones. Eating a lot of packaged, industrially formulated food was not, by itself, associated with having a stone.
Within the category, one group separated out. The highest intake of sweets and confectionery was associated with higher odds of calcium oxalate kidney stones compared with the lowest, at roughly double.
And one food went the other way. Kidney stones were less common in persons in the highest third of dairy product intake than in the lowest.
Why the dairy result runs opposite to kidney stone folklore
It reverses what most people assume, which is that calcium in food becomes calcium in stones. The physiology runs the other way. Calcium eaten with a meal binds oxalate inside the gut, so less oxalate reaches the bloodstream and less arrives in the urine to make trouble.
Cutting dairy to avoid stones is therefore a classic piece of intuitive advice that makes the problem worse, and this study lines up with the larger literature on that point. The authors are careful with it, noting that higher dairy intake may have a protective association, albeit with caution due to potential recall bias.
What a case-control study of 366 people cannot show
The design carries a specific weakness, and it is the reason to hold these numbers loosely. Everyone had already been diagnosed when they were asked what they ate, so the answers come through the filter of an illness people are trying to explain to themselves. A person who has decided sugar caused their stone may remember more of it.
The sample is also small and from one city, so the diets, the water supply and the climate are all local. Tehran is hot, and dehydration is the best-established driver of stones anywhere.
Finally, nothing here is a measurement of cause. The authors state it directly: these findings should be interpreted as associative rather than causal, and they call for prospective studies to confirm the observations.
What to actually do about kidney stone risk
The evidence-backed advice is duller than any food list and more effective: drink enough that your urine stays pale. Fluid is the intervention with the best trial evidence behind it, and it is the one most people under-use.
Beyond that, a stone worth preventing is worth analysing. Stones come in several types and the dietary advice differs sharply between them, which is why guessing from an article, including this one, is a poor substitute for the advice that follows a proper analysis. If there is a single thing to take from this study, it is not to cut the dairy.
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What did the study find?
After adjustment for confounders, the highest tertile of sweets and confectionery intake was associated with higher odds of calcium oxalate kidney stones compared with the lowest (OR 2.11, 95% CI 1.15-3.90, P-trend 0.017). Kidney stones were less common in people in the highest tertile of dairy intake (OR 0.54, 95% CI 0.29-0.97). Total ultra-processed food intake showed no significant relationship (OR 0.73, 95% CI 0.39-1.30).
What is a calcium oxalate stone?
The most common kind of kidney stone, formed when calcium binds to oxalate in the urine and crystallizes. Oxalate comes from some foods and is also made by the body.
Does eating calcium cause calcium stones?
Generally the opposite. Calcium eaten with a meal binds oxalate in the gut so less is absorbed, which is why dietary calcium is usually protective, and why the dairy result here is not the surprise it first looks.
Why did sweets stand out?
The study cannot say. Sugar increases how much calcium the kidneys excrete, and sugary drinks and confectionery tend to displace water and other foods, so several explanations fit and none was tested.
How reliable is a study like this?
It is the weakest of the common human designs. People were asked about their diet after they already had a stone, so what they remember and how they explain their illness can both distort the answer.
What lowers the chance of another stone?
Drinking enough fluid is the measure with the strongest evidence, along with advice specific to the stone type. Anyone who has had a stone should ask for that advice rather than guess. This is general information rather than medical advice.