Table 4. Distribution and HRsa with 95% CIs for breast cancer risk according to iron and iron and haem iron from meat.

Characteristic	Q1	Q2	Q3	Q4	Q5	P trendb
Total iron c
Cases/Person-years	216/57 408	247/58 320	242/57 471	246/57 486	254/57 564
Median (range) (mg per day)	9.6 (⩽11.4)	13.4 (>11.4–16.8)	25.5 (>16.8–28.0)	29.5 (>28.0–31.2)	33.7 (>31.2–68.6)
HR (95% CI)	1.00	1.09 (0.90–1.31)	1.07 (0.89–1.29)	1.05 (0.87–1.27)	1.08 (0.90–1.30)	0.58
Dietary iron
Cases/Person-years	216/56 980	240/57 697	228/57 697	247/57 853	274/58 021
Median (range) (mg per 1000 kcal)	6.2 (⩽6.9)	7.4 (>6.9–7.8)	8.3 (>7.8–8.8)	9.5 (>8.8–10.3)	11.6 (>10.3–37.5)
HR (95% CI)	1.00	1.09 (0.90–1.31)	1.03 (0.85–1.25)	1.12 (0.92–1.36)	1.25 (1.02–1.52)	0.03
Iron from meat d
Cases/Person-years	218/58 422	256/58 105	258/57 650	237/57 174	236/56 898
Median (range) (mg per 1000 kcal)	0.13 (⩽0.20)	0.26 (>0.20–0.31)	0.37 (>0.31–0.44)	0.51 (>0.44–0.61)	0.78 (>0.61–5.00)
HR (95% CI)	1.00	1.19 (0.99–1.43)	1.21 (1.00–1.45)	1.14 (0.94–1.38)	1.16 (0.95–1.42)	0.37
Haem iron from meat
Cases/Person-years	216/58 523	259/58 136	254/57 492	250/57 157	226/56 940
Median (range) (mg per 1000 kcal)	0.05 (⩽0.07)	0.09 (>0.07–0.11)	0.14 (>0.11–0.16)	0.19 (>0.16–0.23)	0.29 (>0.23–1.49)
HR (95% CI)	1.00	1.22 (1.02–1.47)	1.21 (1.01–1.46)	1.22 (1.01–1.47)	1.12 (0.92–1.38)	0.59
Supplemental iron e
Cases/Person-years	463/113 215	696/163 714	46/11 320
Range (mg per day)	0	0.3–18	21.4–39.4
HR (95% CI)	1.00	0.99 (0.88–1.12)	1.00 (0.74–1.35)			0.94

Abbreviations: CI=confidence interval; HR=hazard ratio; Q=quintile.

^aAdjusted for age (continuous), race (non-Hispanic white, non-Hispanic black, Hispanic, Asian, other), education (⩽11 years, 12 years or high school graduate, post high school training, some college, college graduate or postgraduate education, missing), study centre, randomisation group, family history of breast cancer (yes, no, missing), age at menarche (<10, 10–11, 12–13, 14–15, ⩾16 years, missing), age at menopause (<40, 40–44, 45–49, 50–54, ⩾55 years, missing), age at first birth and number of live births (nulliparous; <20 years and 1, 2, or ⩾3 births; 20–29 years and 1, 2, or ⩾3 births; ⩾30 years and 1, 2, or ⩾3 births; missing), history of benign breast disease (yes, no, missing), number of mammograms during past 3 years (0, 1, >1, missing), menopausal hormone therapy use (never, ⩽1, 2–5, 6–9, ⩾10 years), body mass index (18.5–24.9, 25–29.9, ⩾30 kg m⁻²), alcohol intake (<5, 5–14.9, ⩾15 g per day), total fat intake (continuous, g per 1000 kcal), and total energy intake (continuous, kcal per day).

^bP trend calculated using the median of each quintile.

^cDietary iron (residual energy adjusted) plus iron from supplements. Nutrients in this model were residual energy adjusted.

^dLimited to only those meats in the haem iron database.

^eOwing to the small number of people taking supplements containing iron, iron from supplements was broken into three levels.