Null Results in Brief

Dietary Iron and Heme Iron Intake and Risk ofBreast Cancer: A Prospective Cohort Study

Geoffrey C. Kabat,1 Anthony B. Miller,2 Meera Jain,2 and Thomas E. Rohan1

1Department of Epidemiology and Population Health, Albert Einstein College of Medicine, Bronx, New York and2Department of Public Health Sciences, University of Toronto, Toronto, Ontario, Canada

Abstract

Recent studies suggest that elevated body iron levels maycontribute to breast carcinogenesis; however, epidemiologicevidence is lacking. We used data from a large cohort study ofCanadian women to assess breast cancer in association withtotal iron and heme iron intake. Among 49,654 women ages40 to 59 followed for an average of 16.4 years, we identified2,545 incident breast cancer cases. Data from a food frequencyquestionnaire administered at baseline were used to calculatetotal dietary iron and heme iron intake. Using Cox propor-

tional hazards models, we found no association of iron orheme iron intake with risk of breast cancer overall, in womenconsuming 30+ g of alcohol per day, or in women who hadever used hormone replacement therapy. The present studyoffers no support for an association of iron or heme iron intakewith breast cancer risk or for a modification by iron of theeffect of alcohol or estrogen. However, further cohort studieswith repeated measurement of iron intake are warranted.(Cancer Epidemiol Biomarkers Prev 2007;16(6):1306–8)

Introduction

Free iron is associated with oxidative DNA damage and lipidperoxidation (1-4), both of which are thought to contribute tobreast carcinogenesis (5-11). Iron has been shown to enhancechemically initiated mammary carcinogenesis (12, 13) andmay potentiate the effects of two established risk factors forbreast cancer (i.e., alcohol and exogenous estrogens). Bothalcohol and estrogen are capable of inducing oxidative DNAdamage, and both may disturb iron homeostasis by releasingiron from its bound form, further contributing to the produc-tion of reactive oxygen species (2, 14). Intake of red meat, animportant source of the more bioavailable heme iron, hasshown an inconsistent association with breast cancer (15).However, to date, only one report (16) has assessed intake ofheme iron in relation to breast cancer risk, reporting a positiveassociation of iron and heme iron intake with postmenopausalbreast cancer in women consuming 20+ g of alcohol per daybut no overall association. Given the limited epidemiologicevidence currently available, we used data from a large cohortstudy of Canadian women to assess breast cancer risk inassociation with total iron intake and heme iron intake.

Materials and Methods

Our study was conducted in the Canadian National BreastScreening Study, a randomized controlled trial of screeningfor breast cancer involving 89,835 women ages 40 to 59 atbaseline enrolled between 1980 and 1985 (17, 18). Starting in1982, dietary information was obtained by means of a foodfrequency questionnaire completed by women attendingparticipating screening centers (19). The food frequency

questionnaire elicited information on usual portion size andconsumption of 86 food items and included photographs ofportion sizes to assist respondents in quantifying intake. Atotal of 49,654 dietary questionnaires was returned. Data fromthe food frequency questionnaire were used to calculate totaldietary iron intake using a database described elsewhere (19).The values for iron intake presented here are for dietarysources alone because data on iron supplements were notcollected. Total intake of meat iron was calculated from thereported intake of 22 meat items and 2 mixed dishescontaining meat. Heme iron intake was computed by twodifferent methods using different proportions for heme ironfrom different types of meat: 69% for beef; 39% for pork, ham,bacon, pork-based luncheon meats, and veal; 26% for chickenand fish; and 21% for liver (20); and, alternatively, using 40%as the average proportion of heme iron in all meats (21).Results were similar for both methods, and we present datausing the first approach. Total iron and heme iron intake werecalorie adjusted using the residuals method (22). In addition,we assessed meat iron intake and red meat intake.During an average follow-up of 16.4 years through the end

of December 2000, 2,545 breast cancer cases were identifiedby means of record linkage to the Canadian Cancer Registry.We excluded 947 women with extreme energy intake values(<730 or >6,485 kcal/d) and women whose body mass indexwas <15 kg/m2 or >50 kg/m2 or who were missinginformation on body mass index. In addition, 4 women withprevalent breast cancer were excluded, leaving 48,662 women,including 2,491 cases, available for analysis.Cox proportional hazards models (using age as the time

scale) were used to estimate hazard ratios (HR) and 95%confidence intervals (95% CI) for the association between ironintake and breast cancer risk. Quintiles and deciles of iron-related variables were derived based on their distribution inthe total population. All multivariate models included cova-riates shown in the footnotes to Tables 1 and 2. We alsoexamined the associations within strata of alcohol consump-tion (nondrinker versus drinkers of 30+ g/d) and hormonereplacement therapy (HRT) use (ever versus never), and wetested for interactions using likelihood ratio tests. To test fortrends in risk with increasing levels of exposure, we assigned

1306

Cancer Epidemiol Biomarkers Prev 2007;16(6). June 2007

Received 1/30/07; revised 2/20/07; accepted 4/6/07.

Grant support: National Cancer Institute of Canada.

The costs of publication of this article were defrayed in part by the payment of page charges.This article must therefore be hereby marked advertisement in accordance with 18 U.S.C.Section 1734 solely to indicate this fact.

Requests for reprints: Geoffrey C. Kabat, Department of Epidemiology and PopulationHealth, Albert Einstein College of Medicine, 1300 Morris Park Avenue, Room 1301, Bronx, NY10461. Phone: 718-430-3038; Fax: 718-430-8653. E-mail: gkabat@aecom.yu.edu

Copyright D 2007 American Association for Cancer Research.

doi:10.1158/1055-9965.EPI-07-0086

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the median value for each quintile (or decile) and then fittedthe medians as a continuous variable in the risk models. Wethen evaluated the statistical significance of the correspondingcoefficient using the Wald test (23). All analyses were doneusing Statistical Analysis System version 9 (SAS Institute). Useof the lifetest procedure in Statistical Analysis System showedthat the proportional hazards assumption was met in this dataset. All tests of statistical significance were two sided.

Results

In multivariate models, quintiles of total iron intake, heme ironintake, iron intake from meat, and red meat intake showed noassociation with breast cancer risk in premenopausal women,postmenopausal women, or all women combined (Table 1).Non–heme iron intake also was not associated with breastcancer risk (data not shown). Furthermore, no associations or

Table 2. Multivariate-adjusted HRs and 95% CIs for the association between heme iron intake and breast cancer, stratifiedby alcohol consumption and use of HRT

Stratification variables HR* (95% CI) HR* (95% CI)

Alcohol intake Nondrinker (n cases = 524) >30 g/d (n cases = 193)

Heme iron intake<1.58 1.00 (reference) 1.00 (reference)1.58 to <1.99 0.94 (0.71-1.26) 1.13 (0.76-1.69)1.99 to <2.40 1.07 (0.81-1.42) 1.38 (0.92-2.08)2.40 to <2.95 1.06 (0.81-1.40) 1.32 (0.84-2.06)>2.95 0.99 (0.76-1.30) 0.80 (0.45-1.43)P for trend 0.82 0.78

HRT Never (n cases = 1,831) Ever (n cases = 660)Heme iron intake<1.58 1.00 (reference) 1.00 (reference)1.58 to <1.99 1.02 (0.87-1.18) 1.10 (0.86-1.43)1.99 to <2.40 1.05 (0.90-1.22) 1.25 (0.98-1.61)2.40 to <2.95 1.13 (0.98-1.32) 1.23 (0.95-1.58)>2.95 1.06 (0.91-1.24) 0.96 (0.73-1.25)P for trend 0.18 0.92

*Adjusted for the following variables (except the stratification variable): age (time to event variable), body mass index (kg/m2; quintiles), menopausal status(premenopausal, perimenopausal, postmenopausal), parity (parous/nulliparous), age at menarche (continuous), family history of breast cancer in a first-degreerelative (yes/no), history of benign breast disease (yes/no), oral contraceptive use (ever/never), HRT (ever/never), total calorie intake (continuous), alcohol intake(continuous), education (three levels), study center, and randomization group (intervention or control in the original clinical trial).

Table 1. Multivariate-adjusted HRs and 95% CIs for the association of four measures of dietary iron or meat intake andincident breast cancer

Factor Multivariate HR* (95% CI)

Premenopausal (n cases = 1,171) Postmenopausalc(n cases = 993) All women (n cases = 2,491)

Total iron (mg/d)<11.90 1.00 (reference) 1.00 (reference) 1.00 (reference)11.90 to <12.90 1.06 (0.88-1.27) 0.90 (0.73-1.10) 0.98 (0.86-1.11)12.90 to <13.82 1.04 (0.86-1.25) 0.88 (0.72-1.07) 0.96 (0.84-1.09)13.82 to <14.99 0.97 (0.80-1.17) 0.93 (0.76-1.14) 0.97 (0.85-1.10)z14.99 1.07 (0.89-1.30) 0.87 (0.71-1.06) 0.97 (0.85-1.10)P for trend 0.82 0.28 0.63

Heme iron (mg/d)<1.58 1.00 (reference) 1.00 (reference) 1.00 (reference)1.58 to <1.99 1.06 (0.88-1.27) 1.00 (0.82-1.23) 1.03 (0.91-1.18)1.99 to <2.40 1.08 (0.89-1.30) 1.07 (0.87-1.30) 1.10 (0.97-1.25)2.40 to <2.95 1.14 (0.94-1.37) 1.15 (0.94-1.41) 1.15 (1.01-1.31)>2.95 1.03 (0.84-1.25) 0.97 (0.78-1.20) 1.03 (0.90-1.18)P for trend 0.56 0.71 0.25

Meat iron (mg/d)<3.40 1.00 (reference) 1.00 (reference) 1.00 (reference)3.40 to <4.23 1.17 (0.97-1.40) 1.05 (0.86-1.29) 1.09 (0.96-1.23)4.23 to <5.02 0.91 (0.74-1.58) 1.15 (0.94-1.41) 1.02 (0.90-1.17)5.02 to <6.11 1.13 (0.94-1.37) 1.08 (0.88-1.32) 1.09 (0.96-1.24)z6.11 1.13 (0.93-1.37) 1.03 (0.83-1.27) 1.09 (0.96-1.24)P for trend 0.35 0.73 0.26

Red meat (g/d)<14.25 1.00 (reference) 1.00 (reference) 1.00 (reference)14.25 to <21.02 1.10 (0.91-1.34) 0.99 (0.81-1.19) 0.98 (0.87-1.11)21.02 to <28.74 1.29 (1.07-1.58) 0.87 (0.71-1.06) 1.04 (0.92-1.16)28.74 to <40.30 1.18 (0.97-1.43) 0.89 (0.72-1.09) 1.03 (0.90-1.18)40.30+ 1.14 (0.94-1.39) 0.89 (0.72-1.09) 0.98 (0.86-1.12)P for trend 0.16 0.13 0.91

*Adjusted for age (time to event variable), body mass index (kg/m2; quintiles), menopausal status (premenopausal, perimenopausal, postmenopausal), parity(parous/nulliparous), age at menarche (continuous), family history of breast cancer in a first-degree relative (yes/no), history of benign breast disease (yes, no), oralcontraceptive use (ever/never), HRT (ever/never), total calorie intake (continuous), alcohol intake (never drinker, >0 to <5 g/d, 5 to <10 g/d, 10 to <20 g/d, 20 to <30g/d, 30 to <40 g/d, 40+ g/d), education (three levels), study center, and randomization group (intervention or control in the original clinical trial).cThree hundred and twenty-seven cases and 6,365 noncases were classified as perimenopausal and therefore excluded.

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trends were seen when iron-related variables were categorizedby deciles or treated as continuous variables (data not shown).In this study population, alcohol was associated with

increased risk of breast cancer (HR for an intake of z40 g/d,1.20; 95% CI, 0.96-1.51; P for trend = 0.04), whereas HRT usewas not (HR, 0.98; 95% CI, 0.89-1.09). There was no associationbetween iron intake or heme iron intake and breast cancer riskwithin strata defined by alcohol consumption or HRT use(Table 2). Tests for heterogeneity in the association betweensubgroups were not statistically significant.

Discussion

Consistent with the cohort data of Lee et al. (16), we found nooverall association between dietary iron or heme iron intakeand breast cancer risk. However, the prior report did find asignificant association of both iron intake and heme iron intakein women consuming 20+ g of alcohol per day. With f2,500incident breast cancer cases ascertained over 16 years offollow-up, our study had excellent statistical power to detect amain effect of iron and heme iron on breast cancer. Specifically,we had 87% power to detect a HR of 1.2 for extreme quintileswith a two-sided a-level of 5%. In the subgroup with thesmallest number of breast cancer cases (women who reporteddrinking 30+ g/d of alcohol at baseline), we had 86% power todetect a HR of 1.8 for extreme quintiles. Two limitations of thisstudy should be mentioned. First, information on use of ironsupplements was not available. However, in their studies onheme iron, Lee et al. al. (21, 24) seem to have based theirestimates on dietary data only. Second, our dietary data werelimited to intake reported at baseline, and changes in dietaryintake over the long follow-up period could have led tomisclassification of exposure, thereby reducing our ability todetect an association.Considering the biological plausibility of an association

between iron and breast cancer risk, further investigation iswarranted given the limited data currently available. Ofparticular importance are cohort studies with repeatedmeasurement of iron intake. If elevated iron intake is foundto contribute to breast carcinogenesis, preventive strategiesmight include dietary modification, avoidance of iron supple-ments, and chelation (25).

AcknowledgmentsWe thank Statistics Canada, the provincial and territorial Registrars ofVital Statistics, and the Cancer Registry directors for their assistance inmaking the cancer incidence and mortality data available.

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2007;16:1306-1308. Cancer Epidemiol Biomarkers Prev   Geoffrey C. Kabat, Anthony B. Miller, Meera Jain, et al.   Cancer: A Prospective Cohort StudyDietary Iron and Heme Iron Intake and Risk of Breast

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