Physical Activity and Breast Cancer Risk in Hispanic and Non-Hispanic White Women

Frank D. Gilliland1,2, Yu-Fen Li1, Kathy Baumgartner2, Diane Crumley2 and Jonathan M. Samet3

1 Division of Occupational and Environmental Health, Department of Preventive Medicine, and Norris Comprehensive Cancer Center, University of Southern California Health Sciences Center, Los Angeles, CA.
2 New Mexico Tumor Registry, Cancer Research and Treatment Center, University of New Mexico Health Sciences Center, Albuquerque, NM.
3 Department of Epidemiology, Bloomberg School of Public Health, Johns Hopkins University, Baltimore, MD.


    ABSTRACT
 TOP
 ABSTRACT
 INTRODUCTION
 MATERIALS AND METHODS
 Identification of case subjects
 Identification of controls
 Data collection
 Physical activity, body mass...
 Covariates
 Statistical analysis
 RESULTS
 DISCUSSION
 REFERENCES
 
To investigate breast cancer risk in Hispanic and non-Hispanic White women, the authors conducted a population-based case-control study of New Mexican women during 1992–1994 using incident breast cancer cases aged 35–74 years and frequency-matched controls selected using random digit dialing. Activity type and weekly duration of usual nonoccupational physical activity were used to calculate weekly metabolic equivalent (MET)-hours of total and vigorous physical activity (>=5 METs). Conditional logistic regression models were fitted to estimate the relative risk of breast cancer for levels of physical activity and to assess the difference in effects by ethnicity, body mass index, energy intake, and menopausal status. Vigorous physical activity was associated with reduced breast cancer risk in both Hispanic and non-Hispanic White women. Women in the highest category of vigorous activity had lower risk of breast cancer (adjusted odds ratio = 0.34, 95% confidence interval: 0.22, 0.51 for Hispanic; adjusted odds ratio = 0.60, 95% confidence interval: 0.41, 0.89 for non-Hispanic White women) compared with women reporting no vigorous physical activity. Both pre- and postmenopausal Hispanic women showed decreasing risk with increasing level of activity. Physical activity was protective only among postmenopausal non-Hispanic White women. The effects of physical activity were independent from reproductive factors, usual body mass index, body mass index at age 18, adult weight gain, and total energy intake.

body mass index; breast neoplasms; energy intake; exercise; Hispanic Americans

Abbreviations: MET, metabolic equivalent


    INTRODUCTION
 TOP
 ABSTRACT
 INTRODUCTION
 MATERIALS AND METHODS
 Identification of case subjects
 Identification of controls
 Data collection
 Physical activity, body mass...
 Covariates
 Statistical analysis
 RESULTS
 DISCUSSION
 REFERENCES
 
The Hispanic population in the United States is currently estimated to exceed 20 million by the year 2030 and is projected to become the largest minority population (1Go). Breast cancer is the most common cancer among Hispanic women and a leading cause of cancer mortality (2GoGoGo–5Go). Moreover, both incidence and mortality rates are increasing at an alarming pace among Hispanic women (3Go, 4Go). Yet while published studies on the risk factors for breast cancer are extensive for non-Hispanic women in the United States and other countries, little information is available for Hispanic women (5GoGoGoGoGoGoGoGo–13Go).

Differences in reproductive patterns were thought to underlie the ethnic variation and rising cancer incidence rates among Hispanic women; however, we have shown that reproductive risk factors, such as age at first full-term birth, parity, and breastfeeding, account for only a small portion of the ethnic differences in risk or temporal trends (8Go). Results from studies of other established risk factors, such as a family history of breast cancer and body mass index, have been inconsistent (5GoGoGoGoGoGoGoGo–13Go). Because reproductive factors do not account for the differences in breast cancer occurrence between Hispanic and non-Hispanic White women, the contribution of other potential determinants of breast cancer risk, especially modifiable lifestyle factors, is of interest.

A growing body of evidence indicates that women who are the most physically active have substantially reduced breast cancer risk, especially women who participate in vigorous physical activities (14GoGoGo–17Go). Studies of the relation between physical activity and breast cancer risk have largely included non-Hispanic White women, resulting in a paucity of data about the effects of physical activity on breast cancer risk among Hispanic and other ethnic minority women (18Go).

To investigate the relations of physical activity with breast cancer risk in Hispanic and non-Hispanic White women, we examined data collected in the New Mexico Women's Health Study, a statewide population-based case-control study. We assessed the associations of usual level of nonoccupational physical activity, adjusted for selected aspects of reproductive history, usual energy intake, and body mass index, with breast cancer risk in pre- and postmenopausal Hispanic and non-Hispanic White women aged 30–74 years.


    MATERIALS AND METHODS
 TOP
 ABSTRACT
 INTRODUCTION
 MATERIALS AND METHODS
 Identification of case subjects
 Identification of controls
 Data collection
 Physical activity, body mass...
 Covariates
 Statistical analysis
 RESULTS
 DISCUSSION
 REFERENCES
 
The New Mexico Women's Health Study is a statewide population-based case-control study of breast cancer in Hispanic and non-Hispanic White women. Results from this study have been reported previously for reproductive breast cancer risk factors (8Go). Women newly diagnosed with an invasive or in situ breast carcinoma during the period from January 1, 1992, through December 31, 1994, who were residents of the state and aged between 30 and 74 years at diagnosis were eligible for the study.


    Identification of case subjects
 TOP
 ABSTRACT
 INTRODUCTION
 MATERIALS AND METHODS
 Identification of case subjects
 Identification of controls
 Data collection
 Physical activity, body mass...
 Covariates
 Statistical analysis
 RESULTS
 DISCUSSION
 REFERENCES
 
Women with new primary breast cancer were ascertained by the New Mexico Tumor Registry, a population-based tumor registry and a member of the Surveillance, Epidemiology, and End Results Program of the National Cancer Institute (19Go). All Hispanic cases were eligible for the study. Because the overall expected number of breast cancer cases for the study period was approximately three times higher for non-Hispanic Whites than for Hispanics, we randomly selected approximately 33 percent of non-Hispanic White cases while including all Hispanic cases to maximize our power to examine ethnic differences in risk with the available sample size and funding. The sampling strata for non-Hispanic White cases were age group (30–39, 40–64, 65–74 years) and geographic region defined by seven state health-planning districts. The sampling fraction for non-Hispanic Whites in each of these 21 strata was chosen to give a distribution similar to the age and geographic distribution of Hispanic cases ascertained by the New Mexico Tumor Registry in the 3-year period 1988 through 1990. A total of 491 eligible Hispanic breast cancer cases were ascertained. The stratified random selection of non-Hispanic White women resulted in ascertainment of 493 cases. Of the eligible cases, 332 Hispanics, 68 percent, and 380 non-Hispanic Whites, 77 percent, completed interviews. Reasons for nonparticipation have been reported elsewhere (8Go).


    Identification of controls
 TOP
 ABSTRACT
 INTRODUCTION
 MATERIALS AND METHODS
 Identification of case subjects
 Identification of controls
 Data collection
 Physical activity, body mass...
 Covariates
 Statistical analysis
 RESULTS
 DISCUSSION
 REFERENCES
 
Controls were ascertained using random digit dialing with frequency matching on ethnicity, the three age groups, and the seven health-planning districts. We used a modified approach to the Waksberg (20Go) random digit dialing method that has been described previously (8Go). There were a total of 1,039 eligible controls ascertained from approximately 3,400 respondents who completed the telephone-screening interview including 511 Hispanic and 528 non-Hispanic White women. Of those eligible, 844 (81.2 percent) were successfully interviewed. Participation rates were 75.9 percent for Hispanics and 86.4 percent for non-Hispanic Whites.


    Data collection
 TOP
 ABSTRACT
 INTRODUCTION
 MATERIALS AND METHODS
 Identification of case subjects
 Identification of controls
 Data collection
 Physical activity, body mass...
 Covariates
 Statistical analysis
 RESULTS
 DISCUSSION
 REFERENCES
 
In-person interviews were conducted at a location of the participant's choice. Written informed consent was obtained at the onset of the interview. The median time between diagnosis and interview was 193 days. To aid in recall, interviewers used a calendar that recorded major life events. Only events that occurred before each woman's reference date were recorded (date of diagnosis for cases and date of interview for controls). All questionnaires were translated into Spanish, and interviews were conducted in Spanish or English according to the participant's preference.


    Physical activity, body mass index, and energy intake
 TOP
 ABSTRACT
 INTRODUCTION
 MATERIALS AND METHODS
 Identification of case subjects
 Identification of controls
 Data collection
 Physical activity, body mass...
 Covariates
 Statistical analysis
 RESULTS
 DISCUSSION
 REFERENCES
 
Self-reported categories of nonoccupational physical activity were assessed during the in-person interview. Participants were asked to indicate activities that they had done on a regular basis (for at least 6 months) during the year prior to the reference date and the amount of time per week (<1, 1.5, 2–3, 4–6, 7–10, >=11 hours) spent doing each activity. Activities included walking/hiking, running/jogging, exercise class, biking, dancing, lap swimming, tennis, squash/racquetball, calisthenics/rowing, bowling, golf, softball/baseball, basketball, volleyball, housework, and heavy outside work.

For each participant, we computed weekly average energy expenditure by assigning metabolic equivalents (METs) in kcal/kg/hour to each activity and multiplying by the midpoint of the hours per week categories to obtain MET-hours/week (21Go). MET-hours were used to rank women's total energy expenditure for all nonoccupational physical activity and for energy expended during vigorous physical activities. Vigorous activities were defined as activities associated with >=5 METs. Usual adult body mass index was calculated using self-reported usual adult weight and height (weight (kg)/height (m)2).

Dietary intake was collected using a modified version of a quantitative food frequency questionnaire that was validated in our study population and has been previously used in other Hispanic populations (22Go, 23Go). The food frequency questionnaire included unique food items that were important sources of nutrients among New Mexico women. Participants were asked to recall usual food intake during a 4-week period approximately 6 months prior to the reference date. Frequency of consumption and portion size were used to calculate total energy and nutrient intake per day using a database developed by the Human Nutrition Center at the University of Texas-Houston (24Go).


    Covariates
 TOP
 ABSTRACT
 INTRODUCTION
 MATERIALS AND METHODS
 Identification of case subjects
 Identification of controls
 Data collection
 Physical activity, body mass...
 Covariates
 Statistical analysis
 RESULTS
 DISCUSSION
 REFERENCES
 
Parity was defined as the number of pregnancies lasting 6 months or longer with outcomes of either a single birth, multiple births, or a stillbirth. Age at first full-term birth was defined as the age of the woman at the end of her first pregnancy lasting 6 months or longer, regardless of the outcome of the pregnancy. Duration of lactation was the cumulative number of months of breastfeeding for all children. Menstrual history, history of hysterectomy with or without oophorectomy, and use of estrogen replacement therapy were used to determine menopausal status as previously described (8Go). The categories of menopausal status used in this analysis include premenopausal, postmenopausal, and unknown. The cumulative number of years of oral contraceptive use and hormone replacement therapy was collected using a life events calendar. Hormone replacement therapy included cumulative number of years of estrogen or estrogen plus progesterone use. Family history of breast cancer was coded as positive if the respondent reported a first-degree relative, mother, sister, or daughter, with breast cancer. History of fibrocystic disease (yes/no) was defined as having been diagnosed with fibrocystic breast disease.


    Statistical analysis
 TOP
 ABSTRACT
 INTRODUCTION
 MATERIALS AND METHODS
 Identification of case subjects
 Identification of controls
 Data collection
 Physical activity, body mass...
 Covariates
 Statistical analysis
 RESULTS
 DISCUSSION
 REFERENCES
 
Conditional logistic regression, which conditioned on the frequency-matched variables (three age groups, geographic district, and ethnicity), was used to compute odds ratios and 95 percent confidence intervals. Multivariate odds ratios from conditional logistic regression were used to estimate the joint effects of physical activity, energy intake, and body mass index adjusted for reproductive and nonreproductive factors (25Go). We selected potential confounding variables based on review of the literature, univariate analyses, and change of the effect estimates by 10 percent or more in multivariate analyses. To assess differences in risk for Hispanics and non-Hispanic Whites, analyses were stratified by ethnicity with models conditioned on age groups and geographic districts. An overall test of equality of effects for Hispanics and non-Hispanic Whites was conducted by comparing models using the likelihood ratio test. Tests of trend were computed using the median values within each category of activity level. Modification of the effects of physical activity, body mass index, and energy intake by ethnicity or the other factors was investigated using stratified models and tested by comparing appropriate likelihood ratio statistics in nested models that included interaction terms. Stratified models were fitted to assess differences in effects between pre- and postmenopausal women. Women with unknown menopausal status were excluded from the stratified analyses (n = 4). All analyses were performed using Statistical Analysis System software (26Go).


    RESULTS
 TOP
 ABSTRACT
 INTRODUCTION
 MATERIALS AND METHODS
 Identification of case subjects
 Identification of controls
 Data collection
 Physical activity, body mass...
 Covariates
 Statistical analysis
 RESULTS
 DISCUSSION
 REFERENCES
 
Distributions of the selected demographic characteristics of cases and controls are presented in table 1. Hispanic women with breast cancer were less educated, used hormone replacement therapy less often, and had lower parity than did Hispanic controls (table 1). Non-Hispanic White cases had a shorter duration of oral contraceptive use, a stronger family history of breast cancer, lower fat intake, and more fibrocystic breast disease than did non-Hispanic White controls. In general, Hispanic women had less education, had an earlier age at first full-term birth, had a higher parity, used less hormone replacement therapy, had less fibrocystic breast disease, and had fewer first-degree relatives with breast cancer compared with non-Hispanic White women. Fat intake was higher among Hispanic than non-Hispanic White women (96 vs. 90 g/day, p = 0.01). Hispanic participants were estimated to have a higher daily energy intake than non-Hispanic White participants (2,484 vs. 2,281 kcal/day, p < 0.001), respectively. Energy intake varied between cases and controls (table 1). Among non-Hispanic White women, cases reported lower energy intake than did controls. Among Hispanic cases and controls, energy intake showed no consistent differences. Hispanic participants had a higher body mass index than did non-Hispanic White participants (table 1). At the reference date, the mean age of cases was 54 years (age at diagnosis) and was 53 years for controls. As expected from the frequency-matched design, the age distribution for cases and controls was not statistically different between Hispanic or non-Hispanic White participants. Approximately one third of the women were premenopausal.


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TABLE 1. Distribution for selected demographic characteristics and breast cancer risk factors among cases and controls, New Mexico Women's Health Study, 1992–1994*

 
Among both Hispanic and non-Hispanic White participants, the five most commonly reported nonoccupational activities were housework, walking, heavy outside work, biking, and dancing. Hispanic women reported a greater number of activities (mean number of activities = 8.6) and total weekly hours of activity (mean = 13.4 hours) than did non-Hispanic White women (mean number of activities = 7.1, mean = 12.4 hours).

Total and vigorous MET-hours for nonoccupational physical activity were lower in both Hispanic and non-Hispanic White cases than in their respective controls (table 2). Cases were more likely than controls to report no vigorous activities and less likely to expend more than 25 MET-hours/week in vigorous activity. Differences in total physical activity levels followed the same pattern as vigorous physical activity levels, but they were less pronounced among Hispanic women.


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TABLE 2. Distribution of total and vigorous physical activity (MET*-hours/week) for Hispanic and non-Hispanic White participants, New Mexico Women's Health Study, 1992–1994

 
High levels of vigorous physical activity were associated with substantial reductions in breast cancer risk in Hispanic and non-Hispanic White women (table 3). Among women who expended 25 or more MET-hours/week performing vigorous activity, Hispanic women had a 66 percent decrease in risk compared with a 40 percent decrease for non-Hispanic White women who performed no vigorous activity. Furthermore, increasing amounts of vigorous activity were associated with greater reductions in risk. Total physical activity levels were also associated with reduced risk for breast cancer, except among non-Hispanic White women who showed no clear evidence of a reduction in risk.


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TABLE 3. Effects of physical activity on breast cancer risk in premenopausal and postmenopausal Hispanic and non-Hispanic White women, adjusted odds ratios* and 95% confidence intervals, New Mexico Women's Health Study, 1992–1994

 
The larger protective effect of physical activity among Hispanic than non-Hispanic White women resulted primarily from ethnic differences in the protective effect among premenopausal women (table 3). Hispanic women who were premenopausal had substantial reductions in the risk of breast cancer associated with increasing MET-hours/week of both vigorous activities and total activity, and the effects were larger than among non-Hispanic White women. Risk among premenopausal non-Hispanic White women was not associated with activity levels. Among postmenopausal women, high levels of total and vigorous activity were associated with an approximate 50 percent reduction in risk in both ethnic groups. Further adjustments for family history of breast cancer, history of fibrocystic disease, education, body mass index at age 18, weight change, and dietary fat intake did not result in substantial changes in any of the effect estimates for physical activity. The effects of physical activity did not vary significantly across levels of usual body mass index, body mass index at age 18, weight gain, energy intake, or parity (data not shown).


    DISCUSSION
 TOP
 ABSTRACT
 INTRODUCTION
 MATERIALS AND METHODS
 Identification of case subjects
 Identification of controls
 Data collection
 Physical activity, body mass...
 Covariates
 Statistical analysis
 RESULTS
 DISCUSSION
 REFERENCES
 
The relation between a woman's risk of breast cancer and physical activity has been the subject of a growing number of investigations (15Go, 17Go). Our results support the hypothesis that high levels of physical activity reduce breast cancer risk in both Hispanic and non-Hispanic White women. The protective effects of physical activity in both ethnic groups are consistent with the evolving consensus that high energy expenditure during leisure time physical activity is associated with an approximate 30 percent reduction in breast cancer risk (15Go, 17Go).

In our study population, Hispanic women showed a stronger protective effect of nonoccupational physical activity than did non-Hispanic White women. More than 30 studies of physical activity and breast cancer risk have been reported, but few studies have investigated the effects of physical activity on breast cancer risk among women from diverse racial and ethnic groups, and, to our knowledge, none has investigated the effects of physical activity among Hispanic women (15Go, 18Go). Our findings suggest that physical activity may be an important protective behavior among Hispanic women and that more targeted research is warranted to investigate the effects of physical activity in other Hispanic populations.

The findings of studies that examined physical activity among postmenopausal women have shown heterogeneous results that ranged from a significant decrease in risk to no association and an increase in risk (15Go, 18Go). In the present study, the substantial protective effects of high levels of energy expenditure were apparent in both Hispanic and non-Hispanic White postmenopausal women and indicate that recent activity levels may contribute to reduced risk in postmenopausal women. Because recent activity is likely to be correlated with past activity levels, it is difficult to clearly determine if this protective effect is due to recent activity or to a cumulative lifetime effect.

The effects of physical activity in younger and premenopausal women, in general, have been found to be somewhat more consistent and to have a larger protective effect of physical activity than in postmenopausal women (14Go, 15Go, 18Go). We found that the effects of physical activity were larger among premenopausal Hispanic than non-Hispanic White women. The heterogeneity among premenopausal women suggests that it may be important to examine the effects of physical activity within ethnic groups and menopausal status.

The interpretation of the evidence for a protective effect of physical activity is complex because levels of physical activity are interrelated with other physiologic risk factors including energy intake and obesity (15Go, 18Go). Substantial variation in the effects of physical activity has been reported for lean and nonlean women, suggesting that energy intake and body mass may modify or confound the effects of physical activity (15Go, 18Go, 27Go, 28Go). If energy balance is important in breast carcinogenesis, then consideration of the simultaneous effects of physical activity, body mass index, and energy intake may be important for clarifying the relation of physical activity with breast cancer, as well as the underlying mechanism for its inverse association with breast cancer risk. In our study, variation in body mass index at age 18, usual adult body mass index, or current body mass index, as well as adult weight gain, and energy intake did not account for the strong protective effects of physical activity, suggesting that high levels of physical activity do not primarily act through effects on obesity or energy intake. Furthermore, the effects of physical activity did not vary substantially when stratified by body mass index or adult weight gain, indicating that the protective effects of recent activity occur for lean and obese women and are unaffected by weight gain. Based on these results, we conclude that the association of physical activity with breast cancer risk is independent of body mass index and energy intake.

The mechanisms for the protective effects of physical activity have yet to be clarified. Potential etiologic mechanisms include changes in fat distribution, alterations in reproductive function and sex hormone levels, metabolic and growth hormones, and modulation of immune function (15Go, 17Go, 18Go). In addition, the protective effects of physical activity may be mediated directly through effects on ovarian and peptide hormone production in premenopausal women (29GoGo–31Go). A leading candidate mechanism is that high levels of physical activity result in a preferential reduction of intraabdominal fat stores that leads to lower exposure to estrogens and other hormonal breast epithelial cell mitogens, such as insulin-like growth factor 1 and insulin (15Go, 17Go, 18Go). Because Hispanic women have a higher prevalence of obesity and relatively greater intraabdominal fat than do non-Hispanic White women, the effects of physical activity may be larger among Hispanics. We were unable to investigate this hypothesis because we lack data on the relevant component of body fat.

A number of limitations affect interpretation of our results. Response rates were reasonably high for both Hispanic and non-Hispanic subjects, indicating that a large selection bias is unlikely, but a potential bias may have arisen from our sampling scheme for control subjects. Control subjects were selected by random digit dialing methods that may introduce differences between cases and controls because telephone coverage varies by socioeconomic variables and other health-related factors in New Mexico (32Go). To reduce this possibility, we selected controls from strata defined by age, ethnicity, and geographic region to ensure a distribution similar to that of the cases. Assessment of energy intake using self-report has recognized limitations (33Go). The accuracy of self-report varies by respondent body mass index, with greater error at the extremes of the distribution. To assess the effects of this misreporting, we restricted our analyses to women with a body mass index between 20 and 25 and found that the estimates for physical activity showed the same patterns in the restricted sample as for the overall study.

Assessment of physical activity is difficult, and a standardized approach has yet to be developed for use in epidemiologic studies. Accordingly, studies of the relation of physical activity and breast cancer have used a variety of methods to assess usual patterns of physical activity. These methods differ in the type of physical activity (leisure, nonoccupational, occupational), period of assessment (lifetime, age periods, recent), and the specificity of parameters needed to estimate energy expenditure (activity, frequency, duration, and intensity) (14Go, 16Go, 28Go, 34Go, 35Go). All the methods are likely to result in some degree of misclassification of participants' activity levels (36Go). We assessed the type and duration of usual nonoccupational time physical activity performed regularly on a weekly basis during a 1-year time period, the year prior to the reference date. We chose to include both leisure time activities and nonoccupational activities (housework and heavy outside), because this provides a more complete assessment of energy expenditure outside of paid employment in our study population. We lack data to directly assess the validity or reliability of our assessment of physical activity in our study population, but the use of a 1-year period is likely to misclassify some participants, especially if lifetime levels of activity or activity during specific life stages is the biologically relevant exposure period. Furthermore, we did not include occupational activities that may be more important in rural and minority populations (37GoGo–39Go). Selecting cases and controls within geographic strata with relatively homogeneous population densities and industries minimized the potential for differential bias. Such misclassification or misspecification of the relevant time period likely would be the same for cases and controls, thus reducing the magnitude of protective effects we observed. The use of recent activity levels is supported by findings in some, but not all, studies that show recent physical activity is associated with decreased breast cancer risk (14Go, 28Go). Although differential misclassification among cases and controls is possible, recall of physical activity was unlikely to be substantially different in cases and controls in this study because participants were interviewed in the early 1990s before physical activity received a great deal of media attention as a protective factor for breast cancer (15Go, 40Go). It is also possible that reporting of prediagnosis physical activity may be affected by treatment among cases. Women who are less active because of poor well-being from active breast cancer treatment might underreport physical activity in the year prior to diagnosis. This would tend to overestimate the effect of physical activity in a case-control design. However, we are not aware of studies that show how current levels of activity after the diagnosis of breast cancer affect the validity of recall about prediagnosis activity levels. We also examined the sensitivity of our findings to changes in our definitions of physical activity metrics. We redefined vigorous activity as activities with greater than or equal to 6 METs and found that the effects in Hispanic women were robust but were no longer significant among non-Hispanic White women. We also conducted analyses using physical activity metrics that excluded housework and outside work and found that the effect estimates were reduced in magnitude and no longer significant for non-Hispanic White women for both total and vigorous activity.

Our findings have public health significance. The proportion of Hispanic women who live sedentary lifestyles is high and rising (39Go, 41Go). Because Hispanic women appear to be at greater risk for breast cancer from low levels of physical activity than are non-Hispanic Whites, the increasing prevalence of sedentary lifestyles may presage a disproportionate increase in breast cancer occurrence among this growing ethnic population. Interventions are needed to reverse the trends toward increasingly sedentary lifestyles not only to prevent breast cancer but also to reduce the other increasing burdens of diabetes and cardiovascular disease.

In conclusion, Hispanic and non-Hispanic women with high levels of physical activity during nonoccupational physical activity were at substantially reduced risk of breast cancer. The overall protective effects of physical activity were larger in Hispanic than non-Hispanic White women as a result of a protective effect among premenopausal Hispanic women that was not apparent among premenopausal non-Hispanic White women.


    ACKNOWLEDGMENTS
 
This research was supported by grant R01-CA55730 from the National Cancer Institute and by the Norris Comprehensive Cancer Center, University of Southern California Health Sciences Center.


    NOTES
 
Reprint requests to Dr. Frank Gilliland, USC Keck School of Medicine, 540 Alcazar, #236, Los Angeles, CA 90033 (e-mail: gillilan{at}usc.edu).


    REFERENCES
 TOP
 ABSTRACT
 INTRODUCTION
 MATERIALS AND METHODS
 Identification of case subjects
 Identification of controls
 Data collection
 Physical activity, body mass...
 Covariates
 Statistical analysis
 RESULTS
 DISCUSSION
 REFERENCES
 

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Received for publication August 30, 2000. Accepted for publication February 14, 2001.