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1 University Department of Medical Oncology, Christie Hospital, Manchester, United Kingdom; 2 Mayo Clinic Cancer Center, Rochester, Minnesota; 3 H. Lee Moffitt Cancer Center and Research Institute, Tampa, Florida; and 4 University of Minnesota, Minneapolis, Minnesota
Requests for reprints: Thomas A. Sellers, Cancer Prevention and Control Division, H. Lee Moffitt Cancer Center and Research Institute, 12902 Magnolia Drive, Tampa, FL 33612. Phone: 813-632-1315; Fax: 813-632-1334. E-mail: sellerta{at}moffitt.usf.edu
| Abstract |
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| Introduction |
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Several studies suggest that adult weight gain may be a better predictor of postmenopausal breast cancer than weight or body mass index (BMI) measured at a single point in time (4-6). However, this observation is not entirely consistent, as greater weight in the earlier premenopausal years (up to age 25 years) has been linked to a reduced risk of postmenopausal breast cancer (5, 7, 8). Whether the association between adult weight gain and risk of postmenopausal breast cancer is evident throughout the premenopausal and postmenopausal years or only at critical time periods of adult life is not known. We therefore investigated the association of changes in weight (loss or gain in excess of 5% of initial body weight) in the time intervals of age 18 to 30 years, age 30 years to menopause, and after the menopause on subsequent risk of postmenopausal breast cancer in a large prospective cohort of postmenopausal women (9).
| Materials and Methods |
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Risk Factor Assessment
The questionnaire solicited information on factors known or suspected to be relevant to breast cancer risk, including family history of breast cancer, pregnancy history, menstrual history, physical activity, and smoking history. We used the 127-item Willett semiquantitative food frequency questionnaire to assess baseline alcohol and energy consumption in 1986; its use for this cohort has been shown (11). Average daily alcohol intake over the previous year was calculated from summing across all alcoholic beverages the product of the frequency of consumption of each of red wine, white wine, beer, and liquor by the nutrient content of each item's serving size. Women reported current height and weight at baseline as well as weight at age 18, 30, 40, and 50 years. From this list of weights, we defined weight at menopause as a woman's weight at the age closest to her reported age at menopause. A paper tape measure and written instructions were enclosed so that a friend could measure the circumference of the waist (1 inch above the umbilicus) and hips (maximal protrusion) according to a validated protocol (12). Anthropometric measures were used to derive the BMI [weight (kg)/height (m)2] and the ratio of the waist circumference to that of the hips (waist-to-hip ratio).
Follow-up
We mailed follow-up questionnaires in 1987, 1989, 1992, and 1997 to establish vital status and change of address. Through linkage with the National Death Index, we identified nonrespondents who were deceased. We ascertained additional deaths and cancer incidence through the State Health Registry of Iowa, a part of the National Cancer Institute's Surveillance, Epidemiology and End Results Program (13). Annually, we matched by computer a list of cohort members and the records of Iowans with incident cancer in the Health Registry using combinations of first, last, and maiden name; zip code; birth date; and Social Security number.
Exclusion Criteria
For the analyses presented here, we excluded women at baseline if they were not postmenopausal (n = 569), had had a mastectomy or partial breast removal (n = 1,870), had any cancer other than skin cancer at baseline (n = 2,292), or were missing weight information (n = 1,871). We also excluded an additional 1,574 women with onset of menopause at age <35 years, because we were studying weight change between age 30 years and menopause in both analyses. These exclusions left a total of 33,660 women eligible for analysis.
Analytic Methods
The length of follow-up for each individual in the study was calculated as the time from completion of the baseline questionnaire to the date of breast cancer diagnosis, date of move from Iowa, or date of death. If none of these events applied, the woman was assumed to be cancer free and living in Iowa through December 31, 2000. Women continued to be followed and contributed additional person-years if diagnosed with a cancer other than breast cancer.
Risk ratios (RR) and 95% confidence intervals (95% CI) were calculated using Cox proportional hazards regression analysis. Survival was modeled as a function of age, because age is a better predictor of breast cancer risk in this cohort than length of follow-up time (14). We defined two primary risk factor models: (a) combinations of weight change from the time intervals age 18 to 30 years and from age 30 years to menopause and (b) combinations of weight change from the time intervals age 30 years to menopause and from menopause to baseline. Weight change was categorized as weight gain (increase of at least 5% body weight from beginning to end of interval), weight loss (decrease of at least 5% body weight), or no change (change in body weight of <5%). This cut point was selected based on the distribution of weight change variables before any analyses of breast cancer. To avoid groups with sparse cells, the initial weight change categories combined the no change and weight loss groups. In all analyses, women who gained weight during both the first and the second time intervals within each risk factor model were defined as the reference group, as this was the most common category and provided the most stable baseline rates on which to show comparison. Two sets of regression models were fitted: one adjusting only for age and one adjusting for both age and potential confounding variables associated with the development of breast cancer: BMI at age 18 years, age at menopause, education, age at menarche, oral contraceptive use, use of hormone replacement therapy (HRT), number of live births, age at first live birth, smoking status, and alcohol consumption. All statistical tests were two sided and all analyses were carried out using the SAS (SAS Institute, Inc., Cary, NC) and Splus (Insightful, Inc., Seattle, WA) software systems.
| Results |
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Compared with the reference category of women who gained weight in both intervals in the first risk factor model, weight gain from age 18 to 30 years was not associated with an increased risk of breast cancer when subsequent weight from age 30 years to menopause was either unchanged (RR, 0.76; 95% CI, 0.66-0.88) or decreased (RR, 0.62; 95% CI, 0.47-0.82). These relative risks were similar to that among women whose weight remained stable in both time intervals (RR, 0.71; 95% CI, 0.61-0.82). Likewise, compared with the reference category of women who gained weight in both intervals in the second risk factor model, weight gain from age 30 years to menopause was not associated with increased risk of breast cancer if subsequent weight decreased after the menopause (RR, 0.78; 95% CI, 0.65-0.94). This relative risk was slightly higher than that for women whose weight remained stable in both time intervals (RR, 0.62; 95% CI, 0.54-0.72).
Although we were interested in whether weight gain at different times of adult life was associated with different magnitudes of subsequent breast cancer risk, a complicating factor was that the average weight gain between age 30 years and menopause (15 pounds) was greater than the average weight gain between age 18 to 30 years (10 pounds) and after the menopause (6 pounds). Thus, we did additional analyses in which combinations of weight change were treated as continuous rather than categorical variables. For a given combination, models were fit that simultaneously included weight change in the first interval, weight change in the second interval, and the corresponding interaction as well as the same potential confounding variables mentioned previously. The resulting relative risk functions are shown in Fig. 2. Results seem to suggest that weight gain between age 30 years and menopause was associated with greater risk than weight gain between age 18 and 30 years.
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| Discussion |
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Several previous studies have examined whether weight gain is a risk factor for breast cancer. Far less is known whether the timing of such weight gain during adult life is important. Greater weight in early adulthood (age 18-20 years; refs. 4, 5, 15) and at first full-term pregnancy (8) seems to decrease subsequent risk of both premenopausal and postmenopausal breast cancer. The apparent greater risk associated with weight gain in the later premenopausal years (i.e., from age 30 years to menopause) in our analysis is consistent with the findings of Trentham-Dietz et al. (16) who associated weight gain after age 30 years with a slightly greater risk of breast cancer [odds ratio (OR), 1.15; 95% CI, 1.08-1.23] than weight gained between age 21 and 30 years (OR, 1.09; 95% CI, 1.05-1.11). Zeigler et al. (7) reported women who had gained
11 pounds in the fourth decade to have 2.3 times the risk of developing breast cancer in their 50s compared with women in the bottom quintile (RR, 2.6; 95% CI, 1.21-4.21). Two other case-control studies have observed that both excess weight at age 25 years (17) and at age 30 years (18) and that weight gain since age 25 and 30 years, respectively, increased breast cancer risk.
The biological effect of weight gain may in part depend on the influence of other hormone-related variables, such as puberty, pregnancy, lactation, and menopause. For example, weight gain during pubescent years is distributed primarily on the hips and buttocks (gynoid), whereas weight gain during adult years (including during pregnancy and menopause) accumulates preferentially around the waist (android; refs. 3, 19). Several studies have reported that abdominal adiposity (a high waist-to-hip ratio) is a risk factor for postmenopausal breast cancer (4, 20), although the data are not entirely consistent (21).
Given the overwhelming evidence that obesity is a risk factor for breast cancer, it would seem to be important to explore any possible benefit of weight loss. However, there are no direct clinical trial data and few observational data on the subject. Previous reports have linked weight loss after age 35 years (ref. 22; OR, 0.8; 95% CI, 0.69-0.94), between ages 22 and 44 years (OR with >10 kg weight loss, 0.6; 95% CI, 0.3-1.3; ref. 23), and before but not after age 45 years (ref. 16; OR per 5 kg weight loss, 0.9; 95% CI, 0.84-0.98) with reduction in risk of postmenopausal breast cancer. A study among 490 American Asian women in their early 50s provided suggestive evidence that weight loss in the previous decade of life may reduce risk of postmenopausal breast cancer, but the 95% CIs were wide and thus consistent with no effect (RR, 0.69; 95% CI, 0.29-1.66; ref. 7). A further study reported an inverse association between weight loss after age 45 years and risk of postmenopausal breast cancer (OR for weight loss of >10 kg, 0.6; 95% CI, 0.3-1.2; ref. 23), but the 95% CIs included the null value of 1.0.
The benefits of weight loss compared with weight maintenance on breast cancer risk were consistent regardless of the period of initial or subsequent weight gain. That is, the relative risks and incidence rates were always lower for the "loss" category than that for the "no change" category. However, it is important to note that only for model 2 and among women who maintained weight between age 18 and 30 years were the differences statistically significant (RR, 0.36; 95% CI, 0.22-0.60 versus RR, 0.73; 95% CI, 0.64-0.84). Previous studies have also failed to show the benefits of weight loss compared with weight maintenance on breast cancer risk (8, 16, 22, 24-27). Some studies have even reported a weak positive association between weight loss, in comparison with weight maintenance, on breast cancer risk (4, 17).
Loss or gain of weight after the menopause was not associated with a material difference in risk of breast cancer in this analysis compared with the apparent effect of weight loss before menopause. Previous cohort studies have associated weight gain after the menopause with nonsignificant increases in risk (4). A recent analysis from the Women's Health Initiative (15) found that weight loss after the menopause was associated with lower breast cancer risk compared with the more usual situation of weight gain over this time. However, the association seemed to be restricted to the subset of women who had never taken HRT (15). One possible explanation for this observation is that HRT may obscure a potential relationship between weight change and breast cancer risk by elevating levels of estrogen among both lean and overweight postmenopausal women (28). This did not seem to be the case in the current study, as there was no differential association between weight change after the menopause and breast cancer within strata of HRT use.
The apparent benefits of weight loss were contingent on maintenance of such weight losses. Weight loss maintenance is notoriously difficult to achieve. A recent survey however reported 25% of successful dieters managed to maintain their weight loss for at least 5 years (29) and should refute the notion that all attempts at weight loss are ill fated. At worst, attempts to lose weight are known to limit the unremitting trend to gain weight over adult life (30). Weight maintenance over adult life (from age 18 years to postmenopausal years) has been associated with a 30% to 60% lower risk of postmenopausal breast cancer compared with the largest weight gainers within previous studies (4, 6, 22-25, 27, 31-33).
The current study has several notable strengths. The large sample size and extensive data collection at baseline allowed for adjustment for many potential confounders. The data on recalled weight permitted systematic assessment of weight change in different periods of adult life. Validation studies have shown good correlations between self-reported and actual measurements of current weight within this cohort (12). Others have reported good conciliation between recall and existing records of weight at age 20 years (correlation of 0.87; ref. 34). The potential to underreport weight and weight change among overweight subjects (35) would, if anything, have weakened the associations shown here.
Despite these strengths, several limitations should be considered when interpreting these results. Although we were interested in whether weight gain at different times of adult life was associated with different magnitudes of subsequent breast cancer risk, adult weight gain is also associated with changes in the distribution of body fat, with some evidence that peripheral obesity is particularly increased (36). Because central fat seems to be a risk factor for breast cancer, it might have been more important to examine changes in fat distribution. Unfortunately, the design of the study precluded the ability to do so. Weight at menopause was not ascertained from direct questioning but was defined as one of the self-reported weights at either age 30, 40 or 50 years, which was closest to her age at menopause. This may mean that any changes in weight occurring around the time of the menopause may not necessarily be accounted for in our analysis. Finally, the issue of generalizability must be considered. The Iowa population is primarily Caucasian; thus, the findings in this study may not be generalizable to other racial ethnic groups. However, the observed patterns of weight gain over the premenopausal and postmenopausal years in this cohort are representative of those seen among the general population in the United States (2).
The major public health message is that avoiding adult weight gain, or arresting the unremitting weight gain over adult life, should be encouraged. For women who have gained weight, successful weight loss may lower breast cancer risk. A 5% weight loss represents a realistic goal that has been associated with significant reductions in cardiovascular disease (37) and type II diabetes (38). Given the increasing prevalence of overweight among women in western societies, prevention of weight gain and successful weight loss programs could bring about significant reductions in breast cancer rates.
| Footnotes |
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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.
Received 6/28/04; revised 9/15/04; accepted 10/ 8/04.
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