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★★☆Single study — solid evidence, such as one good trial or a large observational dataset, but not replicated at the top tier.
Three days of a gram a day raised oxalate in the urine
In healthy volunteers with no stone history, urinary oxalate went from 25 to 39 mg over three days, and calcium oxalate crystallised more readily in every group.
What the study found
A gram of vitamin C a day pushed urinary oxalate from 31 to 50 mg
Three days is all it takes. The same regimen moved healthy volunteers with no stone history from 25 to 39 milligrams, and raised an index of how readily calcium oxalate crystallises out of urine in every group. Urine acidity did not move.
The caveat · Nobody counted a single stone
Nobody counted a stone here, and three days is nowhere near long enough to. Everyone took the vitamin and everyone was measured before and after, with no placebo group to compare against. The 2 gram group rose less than the 1 gram group, so dose and excretion do not line up cleanly.