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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-adjusteda rate ratio (95% CI) Multivariateb rate ratio (95% CI)
Total proteinc 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 = 0.04d P for trend = 0.12d
Animal proteine 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 = 0.08d P for trend = 0.14d
Vegetable proteine 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 = 0.89d P for trend = 0.36d
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 = 0.81d P for trend = 0.98d
Dietary irong 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 = 0.21d P for trend = 0.92d
Iron from supplementsg 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 = 0.27d P for trend = 0.68d
  1 0.80 mg/day 99 1.00 1.00
Heme iron intakeh 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 = 0.49d P for trend = 0.51d
  1. 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.