TABLE 4.

Age- and multivariate-adjusted HRs of incident kidney stones by quintile of caffeine intake¹

	Caffeine intake quintile
	First	Second	Third	Fourth	Fifth	P-trend
HPFS
Median caffeine intake, mg/d	7	58	152	357	627
Cases, n	330	390	322	285	254
Person-time, y	108,003	109,782	110,782	114,223	119,115
Age-adjusted HR (95% CI)	1.00 (Ref)	1.12 (0.96, 1.30)	0.92 (0.79, 1.08)	0.76 (0.65, 0.90)	0.64 (0.54, 0.76)	<0.001
Multivariate-adjusted HR (95% CI)	1.00 (Ref)	1.05 (0.91, 1.21)	0.89 (0.76, 1.03)	0.81 (0.69, 0.96)	0.74 (0.62, 0.88)	<0.001
NHS I
Median caffeine intake, mg/d	31	130	231	376	636
Cases, n	381	372	298	279	259
Person-time, y	246,555	251,522	254,650	260,933	258,978
Age-adjusted HR (95% CI)	1.00 (Ref)	0.94 (0.81, 1.08)	0.74 (0.64, 0.86)	0.68 (0.58, 0.79)		<0.001
Multivariate-adjusted HR (95% CI)	1.00 (Ref)	0.94 (0.81, 1.09)	0.78 (0.66, 0.91)	0.76 (0.64, 0.89)	0.71 (0.59, 0.85)	<0.001
NHS II
Median caffeine intake, mg/d	17	83	165	358	536
Cases, n	424	508	356	271	253
Person-time, y	239,435	238,667	238,963	240,811	240,323
Age-adjusted HR (95% CI)	1.00 (Ref)	1.20 (1.05, 1.36)	0.84 (0.73, 0.97)	0.65 (0.55, 0.75)	0.61 (0.52, 0.71)	<0.001
Multivariate-adjusted HR (95% CI)	1.00 (Ref)	1.13 (0.99, 1.29)	0.86 (0.74, 1.00)	0.75 (0.63, 0.88)	0.69 (0.58, 0.82)	<0.001

¹Multivariate Cox regression analysis adjusted for age, BMI, use of thiazides, use of calcium supplements, and intakes of calcium, phosphate, sodium, potassium, magnesium, fructose, oxalate, phytate, total fluid, alcohol, and vitamins B-6, C, and D. For illustrative purposes, quintile medians for caffeine were derived from responses to the 1986 (HPFS and NHS I) and 1991 (NHS II) dietary questionnaires. HPFS, Health Professionals Follow-Up Study; NHS, Nurses’ Health Study; Ref, reference.