Table 5.

HRs of incident kidney stones by categories of supplemental intake of vitamin C in NHS I and II and HPFS cohorts

	Supplemental Vitamin C Intake				P for trend
	No use	<500 mg/d	500–999 mg/d	≥1,000 mg/d
NHS I and II
Median supp. vitamin C intake (mg/d)
NHS I	0	90	560	1,060
NHS II	0	60	545	1,060
Cases	1,856	1,891	363	282
Person-time (y)	988,407	1,086,075	228,542	191,764
Age-adjusted HR (95% CI)	1.00 (reference)	0.90 (0.84–0.96)	0.88 (0.79–0.99)	0.79 (0.70–0.89)	0.002
Multivariable HR (95% CI)	1.00 (reference)	0.98 (0.90–1.06)	1.01 (0.90–1.15)	0.90 (0.79–1.04)	0.5
HPFS
Median supp. vitamin C intake (mg/d)	0	125	560	1,120
Cases	645	737	203	268
Person-time (years)	189,156	271,029	74,713	80,129
Age-adjusted HR (95% CI)	1.00 (reference)	0.83 (0.74–0.93)	0.82 (0.70–0.99)	0.98 (0.84–1.13)	0.3
Multivariable HR (95% CI)	1.00 (reference)	0.90 (0.80–1.02)	0.98 (0.82–1.16)	1.19 (1.01–1.40)	0.001

Note: Multivariable analysis adjusted for age; body mass index; thiazide use; supplemental calcium intake; intake of dietary vitamin C, calcium, sodium, potassium, magnesium, fructose, oxalate, phytate, animal protein, and total fluid; and alcohol intake. For illustrative purposes, medians for supplemental vitamin C were derived from responses to the 1986 (NHS I, HPFS) and 1991 (NHS II) dietary questionnaires. However, supplemental vitamin C intake was updated throughout the study. CI, confidence interval; HPFS, Health Professionals Follow-up Study; HR, hazard ratio; NHS, Nurses’ Health Study; supp, supplemental