Ultra-Processed Foods and Kidney Stones: What a 166,000-Person Study Found
2025 — Ultra-processed foods (UPF) — sodas, packaged snacks, reconstituted meats, sweetened cereals — are already linked to obesity, diabetes, and cardiovascular disease. A 2025 study adds kidney stones to that list, with a strikingly large dataset behind the claim.
The Study at a Glance
Published in European Journal of Nutrition, researchers analyzed 166,427 middle-aged UK Biobank participants, following them for 1,510,300 person-years. Higher UPF consumption was positively associated with incident kidney stones (trend p<0.01). Compared with the lowest third of intake, the highest third had a 24% higher risk (hazard ratio 1.24). When yogurt and cereals were excluded, the association strengthened (HR 1.32). In a joint analysis, people with both high UPF intake and high genetic predisposition had the highest risk (HR 1.95). The authors framed UPF as a modifiable dietary target for stone prevention, while noting intervention studies are still needed to confirm causality.
The scale matters. With over 166,000 participants and more than 1.5 million person-years of follow-up, this is among the largest examinations of diet and stone risk to date, giving the finding considerable weight even before interventional confirmation.
Why Might UPF Raise Stone Risk?
Several pathways are plausible. Many UPFs are high in added sodium (raising urinary calcium), added sugars including fructose (independently linked to stones in umbrella reviews), and low in protective nutrients. They also tend to displace minimally processed foods — fruits, vegetables, dairy — that support stone-friendly urine chemistry. The study could not prove mechanism, but the nutritional profile of typical UPF fits known lithogenic patterns. Fructose in particular has been repeatedly associated with higher stone risk independent of other sugars.
Reading the Oxalate Load of Common Items
| Food (serving) | Oxalate | Verdict | Type |
|---|---|---|---|
| All-Bran cereal (1 cup) | 103 mg | Avoid | Processed grain |
| Dark chocolate (1 oz) | 65 mg | Caution | Processed sweet |
| Sweet potato chips (1 oz) | 37 mg | Caution | Processed snack |
| Potato, Russet baked (1/2 cup) | 29 mg | Caution | Processed potato |
| Corn chips (1 oz) | 9 mg | Safe | Processed snack |
| Apple (1 medium) | 2 mg | Safe | Minimally processed |
| Yogurt, plain (1 cup) | 0 mg | Safe | Minimally processed |
| Milk, cow (1 cup) | 1 mg | Safe | Minimally processed |
All oxalate values: Harvard T.H. Chan School of Public Health Oxalate Database (2024), lab-measured via ion chromatography. Verdicts: Safe <25 mg/serving, Caution 25–99 mg, Avoid ≥100 mg.
Oxalate Is Only Part of the Story
Notice that oxalate alone does not separate "good" from "bad" here — corn chips are low-oxalate yet still processed. The UPF-stone link is driven more by sodium, sugar, and nutrient displacement than by oxalate per se. So a low-oxalate processed snack is not automatically stone-safe. The healthier swap is toward minimally processed foods: fruit, plain dairy, whole grains, and home-prepared meals. This is why a plain yogurt (0 mg oxalate, minimally processed) is a better default than a sweetened breakfast bar even if both happen to be low-oxalate.
Fact vs. Inference
The UK Biobank study demonstrates a clear association between UPF intake and higher incident stone risk in a very large cohort — that is strong evidence. It does not prove that any single processed snack causes a stone in an individual, nor that all processing is equal. The reasonable takeaway: reducing UPF and favoring whole foods aligns with stone prevention, especially for those with a genetic predisposition.
Frequently Asked Questions
What counts as ultra-processed food?
UPF is defined by the NOVA system: formulations with many added ingredients (sugars, oils, preservatives, additives) and little intact food — sodas, packaged snacks, instant noodles, sweetened cereals, reconstituted meats.
Do all processed foods cause stones?
No. The 2025 study distinguished UPF from minimally processed foods. Plain yogurt, milk, fruit, and home-cooked meals are minimally processed and stone-friendly regardless of low oxalate.
Is the risk from oxalate or something else?
Mostly something else — sodium, added sugars (including fructose), and displacement of protective foods. Oxalate alone does not explain the UPF-stone link.
If I already eat little UPF, do I need to change anything?
If your diet is already whole-food based, this study mostly confirms what you are doing. The actionable group is habitual UPF consumers, especially those with a stone or family history.
Is all processing bad for stones?
No. Minimally processed foods — plain yogurt, milk, frozen vegetables, whole grains — are stone-friendly. The risk is specific to ultra-processed formulations high in sodium, added sugar, and additives, not to processing in general.
Which UPF should stone formers avoid most?
Sugary sodas and sweetened drinks top the list because of fructose and empty calories; salty packaged snacks raise urinary calcium; and sweetened breakfast cereals can be surprisingly high in oxalate (some bran cereals exceed 100 mg per cup).
Does the 24% risk mean UPF causes stones?
Not directly. The UK Biobank study shows a strong association in a very large cohort, but association is not proof of causation. The mechanism is plausible via sodium, sugar, and nutrient displacement, yet intervention trials are still needed.
Are whole-grain processed foods safer?
More so than refined UPF, but read labels. Some 'whole grain' breakfast cereals are still ultra-processed and high in sugar or oxalate. Intact grains cooked at home are the safer choice.
Can cutting UPF lower my stone risk?
Likely yes, as part of a broader plan. Replacing UPF with minimally processed foods tends to lower sodium and added sugar and raise protective nutrients — all of which support a less lithogenic urine profile.
Are 'healthy' packaged snacks like protein or granola bars a problem?
Some are. Many bars are ultra-processed and can be surprisingly high in sodium, added sugar, or oxalate (certain bran and chocolate variants). Check the label: short ingredient lists and low added sugar are better signals than a 'wellness' claim on the front.
How do I spot ultra-processed food on a label?
Look for long ingredient lists with additives, flavorings, emulsifiers, and sugars you would not use at home. The NOVA system classifies these as UPF. If the product is mostly intact food with minimal added ingredients, it is more likely minimally processed.
Quick Takeaways
- A 166,427-person study linked high UPF intake to 24% higher stone risk.
- The driver is sodium, added sugar, and nutrient displacement — not oxalate alone.
- Minimize sodas, salty snacks, and sweetened cereals; favor whole foods.
- Plain yogurt, milk, fruit, and home-cooked meals are minimally processed and stone-friendly.
- Cutting UPF is a low-risk, high-upside move, especially with a genetic predisposition.
Bottom line: The 2025 evidence gives stone formers one more reason to limit ultra-processed foods — not because of oxalate alone, but because the sodium, sugar, and nutrient displacement in UPF fit the lithogenic profile. Swapping toward minimally processed foods is a low-risk, high-upside move.