Protein, iron, and meat consumption and risk for rheumatoid arthritis: a prospective cohort study

Benito-Garcia, Elizabeth; Feskanich, Diane; Hu, Frank B; Mandl, Lisa A; Karlson, Elizabeth W

doi:10.1186/ar2123

Arthritis Research & Therapy

Table 2 Rate ratios of rheumatoid arthritis by protein and iron intakes among women in the Nurses' Health Study, 1980 to 2002

From: Protein, iron, and meat consumption and risk for rheumatoid arthritis: a prospective cohort study

Nutrients	Quintile	Median intake	Number of cases	Age-adjusted^a rate ratio (95% CI)	Multivariate^b rate ratio (95% CI)
Total protein^c	1	60.8 g/day	95	1.00	1.00
	2	68.4 g/day	109	1.12 (0.86–1.48)	1.11 (0.84, 1.47)
	3	73.9 g/day	89	0.90 (0.67–1.20)	0.90 (0.67, 1.21)
	4	79.5 g/day	133	1.35 (1.04–1.76)	1.34 (1.03, 1.76)
	5	89.0 g/day	120	1.23 (0.94–1,61)	1.17 (0.88, 1.55)
				P for trend = 0.04^d	P for trend = 0.12^d
Animal protein^e	1	42.5 g/day	101	1.00	1.00
	2	50.4 g/day	99	0.97 (0.74–1.28)	0.93 (0.71–1.24)
	3	56.1 g/day	104	1.02 (0.78–1.34)	1.03 (0.78–1.36)
	4	62.3 g/day	125	1.23 (0.95–1.60)	1.21 (0.92–1.59)
	5	72.9 g/day	117	1.13 (0.87–1.49)	1.13 (0.84–1.50)
				P for trend = 0.08^d	P for trend = 0.14^d
Vegetable protein^e	1	12.2 g/day	111	1.00	1.00
	2	15.5 g/day	95	0.81 (0.61–1.06)	0.83 (0.62–1.11)
	3	17.5 g/day	101	0.82 (0.62–1.08)	0.89 (0.66–1.21)
	4	19.3 g/day	127	1.04 (0.79–1.36)	1.14 (0.84–1.55)
	5	22.2 g/day	112	0.90 (0.67–1.20)	1.02 (0.74–1.41)
				P for trend = 0.89^d	P for trend = 0.36^d
Total iron intake (diet and supplements)^f	1	8.00 mg/day	101	1.00	1.00
	2	10.0 mg/day	110	1.01 (0.76–1.34)	1.01 (0.76–1.35)
	3	11.8 mg/day	111	1.00 (0.75–1.33)	0.99 (0.74–1.33)
	4	15.2 mg/day	117	1.12 (0.84–1.49)	1.10 (0.81–1.48)
	5	24.4 mg/day	107	1.00 (0.75–1.34)	1.00 (0.74–1.36)
				P for trend = 0.81^d	P for trend = 0.98^d
Dietary iron^g	1	8.10 mg/day	117	1.00	1.00
	2	9.78 mg/day	80	0.90 (0.67–1.20)	1.01 (0.76–1.35)
	3	10.6 mg/day	113	1.01 (0.77–1.34)	0.99 (0.74–1.33)
	4	11.9 mg/day	114	1.10 (0.83–1.44)	1.10 (0.81–1.48)
	5	14.4 mg/day	122	1.20 (0.92–1.56)	1.00 (0.74–1.36)
				P for trend = 0.21^d	P for trend = 0.92^d
Iron from supplements^g	1	0.00 mg/day	371	1.00	1.00
	2	2.08 mg/day	41	0.73 (0.52–1.03)	0.79 (0.56–1.11)
	3	10.0 mg/day	134	0.92 (0.75–1.12)	0.94 (0.77–1.16)
				P for trend = 0.27^d	P for trend = 0.68^d
	1	0.80 mg/day	99	1.00	1.00
Heme iron intake^h	2	1.04 mg/day	121	1.21 (0.92–1.57)	1.16 (0.88–1.52)
	3	1.23 mg/day	106	1.08 (0.82–1.42)	0.99 (0.74–1.31)
	4	1.46 mg/day	113	1.08 (0.82–1.43)	0.99 (0.74–1.32)
	5	1.90 mg/day	107	1.18 (0.89–1.57)	0.96 (0.70–1.31)
				P for trend = 0.49^d	P for trend = 0.51^d

In all, 82,063 women were included and there were 546 incident cases of rheumatoid arthritis (RA). Diet was assessed in 1980, 1984, 1986, 1990, 1994, and 1998. Nutrient intakes were cumulatively updated over follow up. Total intakes include multivitamins and supplements. Dietary intakes are from food sources only. ^aThe age-adjusted models were adjusted for the total energy intake as well as age. ^bThe multivariate models were adjusted for age, body mass index, smoking, and total lifetime breastfeeding. ^cProtein was adjusted for total iron as well as for the same variables adjusted for in all the multivariate models. ^dAll P values for trend were calculated with median intake of each nutrient in each quintile as a continuous variable. ^eAnimal and vegetable protein were adjusted for each other, total iron, and for the same variables adjusted for in all the multivariate models. ^fTotal iron was adjusted for total protein as well as for the same variables adjusted for in all the multivariate models. ^gDietary and supplemental iron were adjusted for each other, total protein, and the same variables adjusted for in all of the multivariate models; supplemental iron is given in tertiles. ^hHeme iron models are additionally adjusted for total protein. CI, confidence interval.

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