CORRESPONDENCE

Re: Population Attributable Risk for Breast Cancer: Diet, Nutrition, and Physical Exercise

Geir Egil Eide, Ivar Heuch, Grethe Albrektsen

Affiliations of authors: G. E. Eide (Centre of Clinical Research, Haukeland Hospital and Section for Medical Statistics), I. Heuch (Department of Mathematics), G. Albrektsen (Section for Medical Statistics, Department of Public Health and Primary Health Care), University of Bergen, Norway.

Correspondence to: Geir Egil Eide, M.Sc., Haukeland Hospital and Section for Medical Statistics, University of Bergen, NHH, N-5045 Bergen, Norway (e-mail: Geir.Eide{at}NHH.No).

Mezzetti et al. (1) reported population attributable risks (PARs) for breast cancer in Italy with respect to lifestyle factors. As a multivariate analysis of PARs for modifiable risk factors, their study was especially interesting. We want to raise some questions regarding the interpretation of the PARs in general and for breast cancer in particular.

Some major findings in the study by Mezzetti et al. (1) are summarized in Fig. 1Go, using the graphical approach of Eide and Gefeller (2). The combined PAR percentage for any set of three out of the four risk factors analyzed was approximately 30%, with indications that ß-carotene and physical activity were the most important risk factors. For an overall assessment, the combined PAR for all four variables would be very interesting, but such data were not provided by Mezzetti et al. (1).



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Fig. 1. Population attributable risk percents of breast cancer with respect to lifestyle factors in an Italian population, in order of preventive strategies [see Table 3 of Mezzetti et al. (1)].

 
All PARs reported by Mezzetti et al. (1) were also adjusted for age, center, educational level, calorie intake, and menopausal status. Tavani et al. (3) found from the same study sample that approximately 50% of the breast cancer risk could be attributed to the reproductive factors, as acknowledged in (1). From these results (1,3), one might easily be misled to believe that 80% of the breast cancer risk can be attributed to the combined effect of lifestyle and reproductive factors. However, even if the sets of lifestyle and reproductive factors were statistically independent, PARs cannot be summed to give the combined PAR for all factors (4,5). Furthermore, a potential confounding effect of reproductive factors on the association between breast cancer risk and certain lifestyle factors might change the PARs. Only analyzing all factors jointly can answer these questions. Are there technical reasons why this was not done?

Reproductive factors are not easily modifiable, and corresponding PARs constitute less useful measures for preventive purposes. However, since nulliparous women constitute a group at high risk for breast cancer, it would be of interest to calculate the PAR for modifiable risk factors, such as lifestyle factors, within this group. We encourage the authors to extend their analyses on this point.

Whether occupational physical activity at age 30–39 years is a valid measure of general physical activity is questionable. Did other measures of physical activity yield similar results for the PARs?

Mezzetti et al. (1) stated, "The PAR for each factor describes the theoretic percent of cases that would be prevented if all subjects were moved into the exposure level associated with the lowest risk for that factor." However, in their Table 3 (1), PARs were reported for the highest risk level of each lifestyle factor only. What is the correct interpretation when each factor has three levels? If only two levels were considered, are the PARs underestimated in relation to the models underlying Table 2 (1)?

PARs are increasingly getting more attention in the evaluation of preventive strategies. It is important to realize that the context is crucial for their interpretation. PARs are reported sometimes for nonmodifiable factors (3) and sometimes for modifiable ones (1). Strategies for prevention can be rationally discussed only by using correct methodology (6,7). Furthermore, concepts like sequential and average attributable risks (2) may be of substantial help in discussing implications of PAR estimates.

REFERENCES

1 Mezzetti M, La Vecchia C, Decarli A, Boyle P, Talamini R, Franceschi S. Population attributable risk for breast cancer: diet, nutrition, and physical exercise. J Natl Cancer Inst 1998;90:389–94.[Abstract/Free Full Text]

2 Eide GE, Gefeller O. Sequential and average attributable fractions as aids in the selection of preventive strategies. J Clin Epidemiol 1995;48:645–55.[Medline]

3 Tavani A, Braga C, La Vecchia C, Negri E, Russo E, Franceschi S. Attributable risks for breast cancer in Italy: education, family history and reproductive and hormonal factors. Int J Cancer 1997;70:159–63.[Medline]

4 Walter SD. Effects of interaction, confounding and observational error on attributable risk estimation. Am J Epidemiol 1983;117:598–604.[Abstract]

5 Gefeller O, Eide GE. The attributable fraction of the combined effect of two risk factors [letter]. Int J Epidemiol 1992;21:819–20[Medline]

6 Eide GE, Gefeller O. Re: Attributable risk in practice [letter]. Am J Epidemiol 2000;151: 208–9.[Medline]

7 Walter SD. Reply: Re: Attributable risk in practice [letter]. Am J Epidemiol 2000;151: 209–10.



             
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