The authors controlled for age, education level, body mass index, tea and coffee use, smoking status, hypertension, and diabetes mellitus
. They did not control for dehydration, immobilization, use of loop diuretics, corticosteroids, or vitamin D.
They found high-dose (1000 mg) vitamin C to be associated with a single new kidney stone per 680 high-dose users per year. They found no association between multivitamin use and kidney stone risk (relative risk, 0.86; 95% confidence interval, 0.