Table 4. Supplemental vitamin C intake and the RR of incident kidney stonesa

Supplemental Vitamin C Intake (mg/d)P for Trend
None1–99 mg100–499 mg500–999 mg≥1000 mg
aFor illustrative purposes, category medians for intake of vitamin C were derived from responses to the 1994 dietary questionnaire. However, the period-specific quintile values were used for the 1986 to 2000 analyses. RR are for the risk for stone formation compared with the group that had the lowest intake of vitamin C.
bResults are adjusted for age; body mass index (six categories); use of thiazide diuretics (yes or no); volume intake (in quintiles); alcohol use (seven categories); calcium supplement use (four categories); and dietary intake of animal protein, calcium, potassium, sodium, and magnesium (all in quintiles).
Category median (mg/d)0602455601120
Cases618298208161188
Person-years182,448109,38976,80551,49057,569
Age-adjusted RR (95% CI)1.00.91 (0.78–1.06)0.85 (0.73–1.00)1.01 (0.85–1.20)1.02 (0.86–1.20)0.58
Multivariate RRb (95% CI)1.00.95 (0.81–1.12)0.91 (0.78–1.07)1.11 (0.93–1.34)1.16 (0.97–1.39)0.01