Strenuous physical activity in young adulthood and risk of breast cancer (United States)

Influence of physical activity at a young age and lifetime physical activity on the risks of 3 obesity-related cancers: systematic review and meta-analysis of observational studies

CONTEXT

Excess weight has been linked to increased risks of 13 types of cancers. Physical activity is a non-nutritional modifiable lifestyle factor that is not only crucial for weight control but is also known to regulate hormones and metabolic pathways that may contribute to carcinogenesis. There is solid evidence that being physically active during middle and late adulthood lowers the risks of 3 obesity-related cancers, namely breast cancer, colon cancer, and endometrial cancer. However, the associations between physical activity at a young age (childhood, adolescence, and young adulthood; age 5 to ≤30 yr) and lifetime physical activity and the risks of breast cancer, colon cancer, and endometrial cancer are less defined.

OBJECTIVE

The present systematic review and meta-analysis of observational studies was performed in accordance with the MOOSE guidelines to determine whether physical activity at a young age and lifetime physical activity may lower the risks of breast cancer, colon cancer, and endometrial cancer.

DATA SOURCES

The PubMed and Web of Science databases were searched for relevant observational studies published from inception to July 2018.

STUDY SELECTION

Observational studies (prospective cohort, case-cohort, nested case-control, historical cohort, and case-control) were considered relevant if they investigated the association between physical activity at a young age or lifetime physical activity and the risks of developing selected cancers.

DATA EXTRACTION

A random-effects meta-analysis was performed to generate the summary relative risk (RR) with 95%CI for the highest vs the lowest category of physical activity of any type.

RESULTS

Eighty publications were included in the present meta-analysis. Higher physical activity at a young age was associated with lower risks of breast cancer (RR 0.81, 95%CI 0.76, 0.87) and colon cancer (RR 0.67, 95%CI 0.50, 0.88). Similarly, lifetime physical activity was inversely associated with the risks of breast cancer (RR 0.79, 95%CI 0.72, 0.86) and colon cancer (RR 0.75, 95%CI 0.69, 0.82). For breast cancer, menopausal status did not appear to modify the observed inverse association. The benefit with respect to endometrial cancer risk reduction was only observed with higher lifetime physical activity (RR 0.77, 95%CI 0.67, 0.88), not with higher physical activity at a young age (RR 0.89, 95%CI 0.73, 1.07).

CONCLUSIONS

Being physically active over a lifetime, starting from early childhood, may lower the risks of developing breast cancer, colon cancer, and endometrial cancer.

Social and Dietary Changes Associated with Obesity and Breast Cancer Risk

Women’s dietary behaviour and physical activity have changed as they have entered the workplace in developed and developing countries around the world. Women now manage dual roles, one at work and one at home, resulting in less time for traditional meal preparation and housekeeping, and hence less physical activity. Technological innovations such as kitchen gadgets and other labour-saving devices have also eliminated much of the strenuous physical activity associated with performing household chores. As a result of these social changes, there has been a transition in dietary behaviour and physical activity. Specifically, many women now lead more sedentary lifestyles and eat higher calorie diets than ever before, which in turn is fuelling the obesity epidemic—one of the risk factors for many diseases, such as diabetes and probably breast cancer, now one of the most prevalent cancers among women. This paper describes the epidemiology of breast cancer and the probable role of physical inactivity, diet and obesity in its development.

Physical activity and the risk of postmenopausal breast cancer - the Norwegian Women and Cancer Study

Background

The relationship between physical activity (PA) throughout life and the risk of postmenopausal breast cancer overall and by estrogen receptor (ER) and progesterone receptor (PR) status, has been reported, but without consistent results. The present study aimed to investigate PA from young age to adulthood in participants of the Norwegian Women and Cancer (NOWAC) Study, in order to determine whether changes in PA level affect the risk of postmenopausal breast cancer.

Methods

1767 invasive breast cancer cases were identified among 80,202 postmenopausal participants of the NOWAC Study during 8.2 years of median follow-up. PA levels at age 14 years, 30 years and at cohort enrollment were obtained via a self-administered questionnaire. Multivariate Cox proportional hazard regression models were used to estimate relative risks and 95% confidence intervals of the risk of postmenopausal breast cancer overall and by ER/PR status.

Results

Risk of postmenopausal breast cancer overall and by ER/PR status was not associated with physical activity level at enrollment. Women with a low PA level at age 30 had an increased risk of ER+/PR + breast tumors (P for trend = 0.04) compared to women with a moderate physical activity level at age 30. Women with a low physical activity level at all three periods of life had a 20% significantly reduced risk of postmenopausal breast cancer, as well as a reduced risk of ER+/PR + and ER+/PR- breast tumors, compared with women who maintained a moderate physical activity level. However, when analyses were corrected for multiple tests, the result was no longer statistically significant. The findings were consistent over strata of age, body mass index and use of hormone replacement therapy.

Conclusions

The study results from this large Norwegian cohort do not support an association between physical activity at different periods of life and the risk of postmenopausal breast cancer.

Fat or fit: the joint effects of physical activity, weight gain, and body size on breast cancer risk

BACKGROUND

Although physical activity reduces breast cancer risk, issues critical to providing clear public health messages remain to be elucidated. These include the minimum duration and intensity necessary for risk reduction and the optimal time period for occurrence, as well as subgroup effects, particularly with regard to tumor heterogeneity and body size.

METHODS

This study investigated the relationship between recreational physical activity (RPA) and breast cancer risk, in addition to characterizing the joint effects of activity level, weight gain, and body size, through use of a population-based sample of 1504 cases (N = 233 in situ, N = 1271 invasive) and 1555 controls (aged 20-98 years) from the Long Island Breast Cancer Study Project, in Long Island, New York.

RESULTS

A nonlinear dose-response association was observed between breast cancer risk and RPA during the reproductive period and after menopause. Women in the third quartile of activity experienced the greatest benefit with an approximate 30% risk reduction for reproductive (odds ratio = 0.67; 95% confidence interval = 0.48-0.94) and postmenopausal activity (odds ratio = 0.70; 95% confidence interval = 0.52-0.95). Little to no difference was observed regarding intensity of activity or hormone receptor status. Joint assessment of RPA, weight gain, and body size revealed that women with unfavorable energy balance profiles were at increased breast cancer risk. A significant multiplicative interaction was observed between RPA and adult weight gain (P = .033).

CONCLUSIONS

RPA at any intensity level during the reproductive and postmenopausal years have the greatest benefit for reducing breast cancer risk. Substantial postmenopausal weight gain may eliminate the benefits of regular activity.

Weight, dietary behavior, and physical activity in childhood and adolescence: implications for adult cancer risk

Lifestyle factors related to energy balance, including weight, dietary behavior and physical activity, are associated with cancer risk. The period of childhood and growth into adolescence and early adulthood may re-present a 'cumulative risk' for later adult-onset cancers. We review a number of epidemiologic studies that have examined associations among childhood and adolescent body size, diet, and physical activity with adult cancer risk. These studies suggest that unhealthy behaviors that develop early in life and persist over time may increase the risk of some cancer types, such as premenopausal breast, ovarian, endometrial, colon and renal cancer, adversely affect cancer-related morbidities, and increase mortality. Continued research is needed to further determine and refine how timing and degree of such exposures in early childhood and adolescence relate to adult cancer risk. Presently, sufficient evidence suggests a continued need for stronger primary prevention in cancer and obesity research via modified lifestyle behaviors earlier in the developmental spectrum, i.e. during childhood and adolescence.

Physical activity and breast cancer risk among pre- and postmenopausal women in the U.S. Radiologic Technologists cohort

To clarify aspects of the association between physical activity and breast cancer, such as activity intensity required, and possible effect modification by factors such as menopausal hormone therapy (MHT) use. We prospectively examined physical activity in relation to breast cancer risk among 45,631 women participating in the U.S. Radiologic Technologists cohort. Participants provided information at baseline regarding hours spent per week engaging in strenuous activity, walking/hiking for exercise, and walking at home or work. We estimated multivariable relative risks (RR) and 95% confidence intervals (CI) of breast cancer using Cox regression. We identified 864 incident-invasive breast cancers. Greatest risk reduction was observed among women who reported walking/hiking for exercise 10 or more hours per week (RR, 0.57; 95% CI, 0.34-0.95) compared with those reporting no walking/hiking. The association between walking/hiking for exercise and breast cancer was modified by MHT use (p for interaction = 0.039). Postmenopausal women who never used MHT had reduced risks of breast cancer associated with physical activity whereas no relation was observed among ever users of MHT. Our study suggests moderate intensity physical activity, such as walking, may protect against breast cancer. Further, the relation between physical activity and breast cancer may be modified by MHT use.

Physical activity and survival after diagnosis of invasive breast cancer

Previous studies suggest that increased physical activity may lower the risk of breast cancer incidence, but less is known about whether levels of physical activity after breast cancer diagnosis can influence survival. We prospectively examined the relation between postdiagnosis recreational physical activity and risk of breast cancer death in women who had a previous invasive breast cancer diagnosed between 1988 and 2001 (at ages 20-79 years). All women completed a questionnaire on recent postdiagnosis physical activity and other lifestyle factors. Among 4,482 women without history of recurrence at the time of completing the questionnaire, 109 died from breast cancer within 6 years of enrollment. Physical activity was expressed as metabolic equivalent task-hours per week (MET-h/wk); hazard ratios (HR) and 95% confidence intervals (95% CI) were estimated using Cox proportional hazards regression. After adjusting for age at diagnosis, stage of disease, state of residence, interval between diagnosis and physical activity assessment, body mass index, menopausal status, hormone therapy use, energy intake, education, family history of breast cancer, and treatment modality compared with women expending <2.8 MET-h/wk in physical activity, women who engaged in greater levels of activity had a significantly lower risk of dying from breast cancer (HR, 0.65; 95% CI, 0.39-1.08 for 2.8-7.9 MET-h/wk; HR, 0.59; 95% CI, 0.35-1.01 for 8.0-20.9 MET-h/wk; and HR, 0.51; 95% CI, 0.29-0.89 for > or =21.0 MET-h/wk; P for trend = 0.05). Results were similar for overall survival (HR, 0.44; 95% CI, 0.32-0.60 for > or =21.0 versus <2.8 MET-h/wk; P for trend <0.001) and were similar regardless of a woman's age, stage of disease, and body mass index. This study provides support for reduced overall mortality and mortality from breast cancer among women who engage in physical activity after breast cancer diagnosis.

Lifetime recreational and occupational physical activity and risk of in situ and invasive breast cancer

Numerous studies have observed reduced breast cancer risk with increasing levels of physical activity, yet these findings have been inconsistent about optimal times of activity and effect modification by other factors. We investigated the association between recreational and occupational physical activity and breast cancer risk in a population-based case-control study in Massachusetts, New Hampshire, and Wisconsin. During structured telephone interviews, 7,630 controls, 1,689 in situ, and 6,391 invasive breast cancer cases, ages 20 to 69 years, reported lifetime history of recreational physical activity and occupation. Neither lifetime recreational nor strenuous occupational physical activity appeared to be associated with risk of breast carcinoma in situ. In contrast, recreational physical activity was associated with a reduced risk of invasive breast cancer. After adjustment for potentially confounding factors, women averaging >6 h per week of strenuous recreational activity over their lifetime had a 23% reduction in the odds ratio of invasive breast cancer when compared with women reporting no recreational activity (95% confidence interval, 0.65-0.92; P(trend) = 0.05). However, this reduction in risk was limited to women without a first-degree family history of breast cancer (P(interaction) = 0.02). Inverse associations were observed for physical activity early in life, in the postmenopausal years, and in the recent past, but these findings were confined to women without a family history of breast cancer. Lifetime strenuous occupational activity was not associated with invasive breast cancer risk. These results provide further evidence that, for most women, physical activity may reduce the risk of invasive breast cancer.

Physical Activity and Breast Cancer

BACKGROUND

Many epidemiologic studies have found an association between physical activity and breast cancer risk, although this has not been a consistent finding.

METHODS

Studies were identified through a systematic review of literature available on PubMed through February 2006. We included all cohort and case-control studies that assessed total or leisure time activities in relation to occurrence or mortality of breast cancer. The fully adjusted risk estimates and 95% confidence intervals for the highest versus lowest level of activity were documented for each study as well as evidence for a dose-response relationship. Methodologic quality was also assessed. Due to statistical and methodologic heterogeneity among studies, we did not carry out statistical pooling. To draw conclusions, we performed a best-evidence synthesis taking study quality into account.

RESULTS

Nineteen cohort studies and 29 case-control studies were evaluated. There was strong evidence for an inverse association between physical activity and postmenopausal breast cancer with risk reductions ranging from 20% to 80%. For premenopausal breast cancer, however, the evidence was much weaker. For pre- and postmenopausal breast cancer combined, physical activity was associated with a modest (15-20%) decreased risk. Evidence for a dose-response relationship was observed in approximately half of the higher-quality studies that reported a decreased risk. A trend analysis indicated a 6% (95% confidence interval = 3% to 8%) decrease in breast cancer risk for each additional hour of physical activity per week assuming that the level of activity would be sustained.

CONCLUSIONS

There is evidence for an inverse association between physical activity and breast cancer risk. The evidence is stronger for postmenopausal breast cancer than for premenopausal breast cancer.

Early life factors and incidence of proliferative benign breast disease

Proliferative benign breast disease is a marker of increased breast cancer risk, yet little is known about its etiology. Most studies of benign breast disease have focused only on risk factors in adulthood, despite growing evidence that factors in early life influence breast cancer risk. We explored the relations of several early life factors with incidence of proliferative benign breast disease among 71,896 premenopausal women in the Nurses' Health Study II who recalled their body fatness at young ages, physical activity in adolescence, birthweight, and history of being breastfed. Between 1991 and 1997, 901 of these women were identified as having proliferative benign breast disease from a centralized pathology review. Relative risks (RR) and 95% confidence intervals (95% CI) were estimated from Cox proportional hazards models. Greater childhood body fatness (ages 5-10) was associated with decreased risk of proliferative benign breast disease; the multivariate RR (95% CI) for the most overweight compared with the most lean was 0.61 (0.44-0.86; P(trend) < 0.0001) and remained significant after adjustment for current body mass index. Body mass index at age 18 was also inversely associated with incidence of proliferative benign breast disease, with a multivariate RR (95% CI) of 0.67 (0.52-0.88) for those who were > or =25 kg/m(2) compared with those who were <19 kg/m(2) (P(trend) = 0.001). There were no clear associations for physical activity in adolescence, birthweight, or being breastfed. These results indicate that premenopausal women who were heavier at young ages have lower incidence of proliferative benign breast disease, consistent with previous findings for breast cancer.

What causes cancer? Reports from sixth-grade girls

BACKGROUND

We evaluated students' perception of cancer causation among sixth-grade girls living in Wisconsin.

METHOD

We asked female students to list up to 3 causes of cancer in a cross-sectional health survey.

RESULTS

A total of 141 answers were given by 53 students. The most frequently listed causes of cancer were cigarette smoking (91%), drugs (32%), an unhealthy diet (25%), and heredity (25%). Less than 10% of students listed other potentially modifiable risk factors such as exercise, sun exposure, or alcohol consumption.

CONCLUSION

Students in this study appeared to link a range of high-risk behaviors with health consequences, although the specific health risks may not be understood.

Effect of Physical Activity on Women at Increased Risk of Breast Cancer: Results from the E3N Cohort Study

PURPOSE

There is a need to investigate the type, duration, frequency, and intensity of physical activity that are critical to reduce the risk of breast cancer, and if this relation differs among subgroups of women.

METHODS

We analyzed the relation between physical activity and breast cancer incidence between 1990 and 2002 (n=3,424 cases), among 90,509 women of the French E3N cohort, ages between 40 and 65 years in 1990. We gave special attention to effect modification by body mass index (BMI), family history of breast cancer, parity, and hormone replacement therapy (HRT).

RESULTS

A linear decrease in risk of breast cancer was observed with increasing amounts of moderate (P(trend)<0.01) and vigorous (P(trend)<0.0001) recreational activities. Compared with women who reported no recreational activities, those with more than five weekly hours of vigorous recreational activity had a relative risk of 0.62 (0.49-0.78). This decrease was still observed among women who were overweight, nulliparous, had a family history of breast cancer, or used HRT. Compared with the whole cohort, among nulliparous women, the reduction of risk observed was of a higher magnitude, although the test for heterogeneity did not reach statistical significance.

CONCLUSION

A risk reduction of breast cancer was particularly observed with vigorous recreational activity. Further investigations are needed to confirm that intensity is an important variable to consider in risk reduction and to identify the precise biological mechanisms involved in such a risk reduction.

Effects of a School-based Obesity-prevention Intervention on Menarche (United States)

OBJECTIVE

Early menarche is a risk factor for breast cancer. Since body composition influences age at menarche we decided to estimate the effects of a school-based intervention for the prevention of obesity on the initiation of menses in young girls.

METHODS

Ten schools were randomized to a modified curriculum or no curricular changes for 2 school-years. Data of 508 pre-menarcheal girls at baseline (age range: 10-13 years) were analyzed.

RESULTS

Girls attending an intervention school experienced menarche less frequently than girls attending control schools during the intervention period (intervention schools = 54%, control schools = 59%; RR = 0.76; 95% CI [0.66, 0.87]). Attending an intervention school was also associated with lower increase in BMI (-0.3 kg/m2; p = 0.003), lower gains in triceps skinfold thickness (-1.5 mm; p = 0.007), decreased television viewing (-0.6 h/day; p<0.0001) and increased physical activity (3.1 MET-h/week; p = 0.032). Including these changes as predictors of menarche incidence attenuated the intervention effect (RR = 0.94; 95% CI [0.80, 1.10]).

CONCLUSIONS

The intervention delayed menarche in this group of girls. The delay was produced by increased physical activity, reduced television viewing and changes in BMI and fat distribution. These findings may have implications for the primary prevention of breast cancer.

Physical Activity in Different Periods of Life and the Risk of Breast Cancer: The Norwegian-Swedish Women's Lifestyle and Health Cohort Study

Background: Physical activity has been found to be associated with decreased risk of breast cancer in postmenopausal women in the majority of epidemiologic studies, but the association is inconsistent in premenopausal women.

Methods: We studied the effect of physical activity at various ages on the incidence of breast cancer in 99,504 women from 30 to 49 years of age at enrollment in the Women's Lifestyle and Health Study, a prospective population-based cohort study in Norway and Sweden. Physical activity level on an ordinal scale at age 14, age 30, and age at enrollment, participation in competitive sports, as well as information on other covariates was obtained using a self-administered questionnaire. Complete follow-up with data on incident invasive breast cancer and mortality was collected by linkage to national registries. The relation between physical activity and time to breast cancer development was assessed using Cox proportional hazard models, controlling for potential confounders.

Results: During an average 9.1 years of follow-up, there were 1,166 incident breast cancer cases. The mean age of the women was 41 years at enrollment, and the mean age at breast cancer diagnosis was 48 years. Compared to inactive women, women with higher levels of physical activity at enrollment had a similar risk of incident breast cancer (adjusted relative risk, 1.24 for vigorous activity compared with no activity; 95% CI, 0.85-1.82). Physical activity at age 30 or at age 14 also did not afford any significant protection from breast cancer, nor did a consistently high level of activity from younger ages to enrollment.

Conclusions: We did not find evidence of a protective effect of physical activity on breast cancer risk in this group of primarily premenopausal women.

Effects of a Moderate Intensity Exercise Intervention on Estrogen Metabolism in Postmenopausal Women

Physical activity has been associated with reduced breast cancer risk, potentially via hormonal pathways, and high urinary excretion of 2-hydroxyestrone (2-OH E1) relative to 16α-hydroxyestrone (16α-OH E1) also has been associated with reduced breast cancer risk. Studies suggest that body composition and exercise can influence estrogen metabolism. We determined the effects of a 12-month moderate intensity aerobic exercise intervention on urinary 2-OH E1, 16α-OH E1, and their ratio in overweight and obese, previously sedentary, postmenopausal women, ages 50–75 years. Women were randomized to a 12-month exercise intervention (n = 87) or stretching control group (n = 86); 170 completed the study. Urinary 2- and 16α-OH E1 were measured in spot urines collected at baseline, 3, and 12 months. Body composition was measured at baseline and 12 months. Differences between exercisers and controls for excretion of estrogen metabolites were determined using general estimating equations. Further analyses assessed change in estrogen metabolites and their ratio by subgroups of change in body composition. Overall, there were no significant effects of the exercise intervention on 2-OH E1, 16α-OH E1, or their ratio (P &gt; 0.05). There appeared to be an effect of change in intra-abdominal fat and adherence to the exercise intervention on change in the estrogen metabolites or their ratio. However, this did not reflect a potentially desirable change in estrogen metabolites associated with the exercise intervention. Thus, this 12-month moderate intensity exercise intervention did not significantly alter urinary excretion of 2-OH E1, 16α-OH E1, or their ratio in this population of women.

Physical activity in adolescence and young adulthood and breast cancer risk: a quantitative review

Physical activity habits are potentially modifiable and could therefore be targeted in a primary prevention strategy against breast cancer, provided there is causality and a sufficiently strong relationship. Our objective of this quantitative review was to provide a summary estimate of the association of moderate/vigorous recreational physical activity during adolescence/young adulthood with breast cancer risk, and to determine whether a dose-response relationship exists. Data sources included studies in humans relating physical activity to breast cancer risk, published between January 1966 and October 2002, identified on Medline, the Web of Science, from reference lists and related reviews. The main characteristics of each study, the point estimates of relative risk (RR) and confidence intervals (CI) were extracted from 19 case-control and four cohort studies. Comparing the highest to the lowest category of physical activity, the summary RR from the random effects model was 0.81 (95% CI 0.73-0.89). This almost 20% risk reduction proved to be fairly consistent, despite variation in populations and methods. Each one-hour increase of recreational physical activity/week during adolescence was associated with a 3% (95% CI 0-6%) risk reduction. Physical activity in 12-24-year-old females significantly reduces risk of breast cancer. Heterogeneity may be explained by different methods to measure activity.

Moderate exercise training slows mammary tumour growth in adolescent rats

Adolescence and young adulthood may be critical windows in establishing risk for breast cancer development in humans. Epidemiological data suggest that exercise during this life stage is associated with decreased breast cancer risk yet few experimental studies to elucidate the mechanism have been performed. The purpose of these studies was to evaluate the effects of moderate exercise training on mammary tumour development in adolescent rats using the 1-methyl 1-nitrosourea (MNU) chemical carcinogen model. Exercise (EX) consisted of moderate-intensity treadmill running 30 min/day, 5 days a week. A total of 274 animals were used: 94 in study 1 and 180 in study 2. Animals were injected with MNU (50 and 25 mg/kg body weight in studies 1 and 2, respectively) at 21 days of age and began training at 28 days of age. Groups of animals (n=10-30 depending on the study and time point) were sacrificed every 2 weeks for 8 weeks to evaluate tumour development. No difference in median tumour-free survival time was observed in the EX versus sham-exercise (SHAM), nor were there any differences in multiplicity at either a high or moderate dose of MNU. Latency to first tumour palpated was increased in both studies by 3-4 days. Consistent across both studies, tumour weights were less and the growth rates of the tumours, defined as tumour weight divided by the number of days elapsed since the tumour was first palpated, were reduced in the EX group. The data suggest that latency is increased and tumour growth is retarded in response to moderate exercise training.

Physical activity and breast cancer risk among Asian-American women in Los Angeles: a case-control study

BACKGROUND

To the authors' knowledge, there have been few studies published to date regarding physical activity patterns and breast cancer risk in Asian and Asian-American women.

METHODS

The authors conducted a population-based case-control study of 501 Asian-American women with incident breast cancer and a control group of 594 Asian-American women in Los Angeles County to evaluate the role of lifetime physical activity on breast cancer risk. Information concerning lifetime recreational physical activity (i.e., type of activity, duration [years], and frequency [average hours per week]) and occupational physical activity was obtained using a structured questionnaire that was administered in person.

RESULTS

Increasing years and levels (average metabolic equivalent [MET] hours per week) of lifetime recreational activity were associated with a significantly reduced risk of breast cancer after adjusting for demographic factors, migration history, and menstrual and reproductive factors. Compared with women who had no lifetime recreational physical activity, <or= 3 MET hours per week, > 3-6 MET hours per week, > 6-12 MET hours per week, and > 12 MET hours per week of activity were associated with significantly reduced risk, with odds ratios (and 95% confidence intervals) of 0.91 (0.55-1.49), 0.65 (0.39-1.10), 0.53 (0.31-0.90), and 0.47 (0.28-0.80), respectively (P value for trend < 0.001). The risk of breast cancer was associated inversely with occupational physical activity, although the result was not statistically significant.

CONCLUSIONS

The findings of the current study provide further support for the finding that physical activity has a protective role in breast cancer.

Exposures in childhood, adolescence and early adulthood and breast cancer risk: a systematic review of the literature

A growing body of work indicates that exposures over the life course have important roles to play in the aetiology of breast cancer. This review synthesises the literature that has been published in the area of early life events and female breast cancer risk. The review finds some evidence, primarily from cohort studies on the relationship between birthweight and breast cancer, to suggest that in utero events are related to breast cancer risk in adulthood. Strong evidence to support a positive association between height and breast cancer exists. Postulated mechanisms for this relationship include the role of early diet in subsequent disease risk, and the influence of endogenous growth factors mediating the relationship. There is some evidence to suggest that leg length is the component of height which is generating the observed associations between height and breast cancer. There is no consistent pattern of association between relative weight in childhood or adolescence and risk of breast cancer. The evidence to suggest an association between physical activity in early life and breast cancer risk is convincing from case-control studies, but is not fully substantiated by the results of three cohort studies. There are inconsistent results regarding the association between smoking at a young age and breast cancer risk. There is little evidence for an association between passive smoking in early life and breast cancer risk. No clear association between early drinking and breast cancer risk exists. These results are discussed in relation to possible underlying mechanisms and health promotion strategies which could reduce breast cancer risk.

Physical exercise reduces risk of breast cancer in Japanese women

To evaluate the effects of physical exercise on breast cancer risk, a large-scale case-referent study of 2376 incident breast cancer cases and 18,977 non-cancer referents was conducted using data from the hospital-based epidemiologic research program at Aichi Cancer Center (HERPACC). To adjust appropriately for possible confounders, we examined the effects within subgroups of the study population. The multivariable-adjusted odds ratio (OR) was 0.81 (95% confidence interval (CI): 0.69-0.94) for twice a week or more regular exercise. We observed a decreased risk of breast cancer for women who regularly exercised for health twice a week or more, irrespective of menopausal status, and were able to detect greater risk reductions within particular subgroups, including women who were parous, without a family history or non-drinkers. Among premenopausal women, a particularly strong protective effect of physical exercise was observed (OR=0.57, 95%CI: 0.28-1.15) for those women whose body mass index (BMI) was high (BM > or = 25). In contrast, risk reduction was found (OR=0.71, 95%CI: 0.50-1.01) among postmenopausal women whose BMI was medium (BMI: 22-25). Stratification of history of stomach cancer screening to adjust modifying effects of healthy consciousness allows a more precise assessment of the protective effect of exercise twice a week or more, independent of stomach cancer screening history. This study provides evidence that physical exercise, especially exercise twice a week or more, reduces the risk of breast cancer among Japanese women.

Physical activity and cancer prevention: etiologic evidence and biological mechanisms

Scientific evidence is accumulating on physical activity as a means for the primary prevention of cancer. Nearly 170 observational epidemiologic studies of physical activity and cancer risk at a number of specific cancer sites have been conducted. The evidence for decreased risk with increased physical activity is classified as convincing for breast and colon cancers, probable for prostate cancer, possible for lung and endometrial cancers and insufficient for cancers at all other sites. Despite the large number of studies conducted on physical activity and cancer, most have been hampered by incomplete assessment of physical activity and a lack of full examination of effect modification and confounding. Several plausible hypothesized biological mechanisms exist for the association between physical activity and cancer, including changes in endogenous sexual and metabolic hormone levels and growth factors, decreased obesity and central adiposity and possibly changes in immune function. Weight control may play a particularly important role because links between excess weight and increased cancer risk have been established for several sites, and central adiposity has been particularly implicated in promoting metabolic conditions amenable to carcinogenesis. Based on existing evidence, some public health organizations have issued physical activity guidelines for cancer prevention, generally recommending at least 30 min of moderate-to-vigorous intensity physical activity on > or =5 d/wk. Although most research has focused on the efficacy of physical activity in cancer prevention, evidence is increasing that exercise also influences other aspects of the cancer experience, including cancer detection, coping, rehabilitation and survival after diagnosis.

Effect of exercise on the rat mammary gland: implications for carcinogenesis

Physical activity has been associated with decreased risk for developing breast cancer yet to date, the mechanism remains unknown. The purpose of this investigation was to evaluate the effects of moderate exercise training on the normal mammary gland in an attempt to identify alterations or differences that might be associated with tumour inhibition. A total of 170 female Sprague-Dawley rats were randomized to baseline (n=10), exercise (EX; n=80), or sham-exercise groups (SHAM; n=80). Treadmill training (20-25 m min-1, 15% grade, 30 min day-1, 5 days week-1) was started at 28 days of age (DOA). Animals were killed at 28, 42, 56, 70 and 84 DOA. Mammary glands were evaluated by histology and immunohistochemistry. Terminal end buds (TEB), structures susceptible to carcinogenesis, were counted. Sexual maturation, estradiol and progesterone, and organ and muscle weights were also evaluated. No differences in growth, sexual maturation, or steroid hormones were observed in response to training. No difference in the number of TEBs was observed at any timepoint between EX and SHAM. Proliferation was significantly increased at 56 DOA and tended to be increased at 42 and 70 DOA in the EX animals whereas cell death was significantly increased at 70 DOA and tended to be increased at 84 DOA in the EX animals. These data suggest no difference in the number of carcinogen-susceptible structures as a result of moderate exercise. The changes in cell proliferation and apoptosis with exercise training suggest altered cell turnover that will necessitate future study particularly with relevance to carcinogenesis.

Influence of physical activity in different age and life periods on the risk of breast cancer

We conducted a population-based case-control study of 1,237 incident breast cancer cases and 1,241 controls in Alberta between 1995 and 1997 to examine the effect of physical activity performed at different ages and life periods on breast cancer risk. In this study, we measured all types of physical activity done throughout life with a questionnaire developed and tested specifically for this study. We found that breast cancer risk was most associated with a risk reduction for activity done later in life, particularly between menopause and the reference year, for which we observed an odds ratio of 0.70 (95% confidence interval = 0.52-0.95). We also stratified the study participants into four categories according to their patterns of physical activity performed before and after menopause. For the women who sustained physical activity throughout life vs those who were never active, we found an odds ratio of 0.58 (95% confidence interval = 0.41-0.83). This study suggests that sustained activity throughout life and particularly activity done later in life may have the most benefit in reducing breast cancer risk.

The effect of physical activity on breast cancer risk: a cohort study of 30,548 women

BACKGROUND

In epidemiological studies abundant physical activity has been related to decreased breast cancer risk, though the results have been inconsistent. The purpose of this paper was to study the association of physical activity at leisure and commuting to work and incidence of breast cancer.

METHODS

The study cohort consisted of 30,548 female participants of the Finnish adult health behaviour survey, based on annual random samples of Finns aged 15-64, collected in 1978-1993. By the end of 1995, 332 breast cancer cases had been diagnosed in the cohort. Relative risks of breast cancer were adjusted for age at survey, body mass index (BMI), education, length of follow-up, parity and age at first birth using Poisson regression models.

RESULTS

Compared to women exercising less than once a week, the adjusted relative risk of breast cancer for women exercising once a week was 0.80 (95% confidence interval (CI): 0.58-1.10), for women exercising 2-3 times per week 0.92 (95% CI: 0.78-1.22) and for women exercising daily 1.01 (95% CI: 0.72-1.42). Women who reported commuting, walking or bicycling to work 30 min or more daily had slightly lower adjusted risk of breast cancer (RR: 0.87, 95% CI: 0.62-1.24) than women working at home, being unemployed or driving a car to working place.

CONCLUSION

Although a small protective effect of regular physical activity for breast cancer incidence was found in physical activity when commuting to work, the role of the physical activity in breast cancer prevention is still an open question.

Baseline recreational physical activity, history of sports participation, and postmenopausal breast carcinoma risk in the Netherlands Cohort Study

BACKGROUND

The aim of the current study was to evaluate the relation between physical activity and breast carcinoma risk with specific emphasis on interaction with other aspects of energy balance.

METHODS

The Netherlands Cohort Study on diet and cancer was conducted among 62,537 women ages 55-69 years at baseline. Information regarding baseline recreational physical activity, history of sports participation, and occupational physical activity was collected with a questionnaire in 1986. After 7.3 years of follow-up, 1208 incident breast carcinoma cases were available for case-cohort analyses.

RESULTS

A summed total of baseline recreational physical activity (including walking, cycling, gardening) showed an inverse association with breast carcinoma risk. Women who were active in the above-mentioned activities for > 90 minutes a day had a rate ratio (RR) of 0.76 (95% confidence interval [95% CI], 0.58-0.99) compared with women who were active < 30 minutes a day. Women who ever participated into sports before baseline had a RR of 1.13 (95% CI, 0.94-1.37) compared with women who never participated in sports. The relation between sports participation and breast carcinoma risk did not appear to be dependent on the time window of participation (before/after menarche, before/after birth of the first child, before/after age 20 years). No interaction was found between baseline recreational physical activity, body mass index (BMI) (kg/m(2)), energy intake, and weight gain/loss during adult life in relation to breast carcinoma, although in the subgroup of women with a high BMI we found a stronger inverse relation between recreational physical activity and breast carcinoma risk independent of energy intake. Occupational physical activity was not found to be related to breast carcinoma risk.

CONCLUSIONS

The current study findings support the hypothesis that recreational physical activity is associated inversely with breast carcinoma risk.

Relation between intensity of physical activity and breast cancer risk reduction

PURPOSE

To examine the influence of frequency, duration, and intensity of physical activity on risk of breast cancer and to compare breast cancer risks associated with self-reported versus assigned intensity levels of activity.

METHODS

A population-based case-control study of 1233 incident breast cancer cases and 1241 controls was conducted in Alberta between 1995 and 1997. The frequency, duration and intensity of occupational, household, and recreational activities were measured throughout lifetime using the Lifetime Total Physical Activity Questionnaire and cognitive interviewing methods. Unconditional logistic regression analyses were used to estimate odds ratios and a full assessment of confounding and effect modification was undertaken. Odds ratios for self-reported and compendium-based assigned levels of activity were compared for lifetime total activity and by type of activity.

RESULTS

Breast cancer risk reductions were comparable when self-reported and assigned intensity values were used, although the results and trends were more evident with the assigned intensity data. Moderate-intensity occupational and household activities decreased breast cancer risk, whereas recreational activity, at any intensity level, did not contribute to a breast cancer risk reduction.

CONCLUSION

This study found that moderate-intensity activities were the major contributors to the decrease in breast cancer risk found in this study and that risk reductions were more evident when the frequency and duration of activity alone were modeled. Of the three types of activity considered, the greatest risk reductions observed were for occupational and household activities.

Physical activity and cancer risk: dose-response and cancer, all sites and site-specific

PURPOSE

The association between physical activity and overall and site-specific cancer risk is elaborated in relation to whether any observed dose-response association between physical activity and cancer can be interpreted in terms of how much physical activity (type, intensity, duration, frequency) is needed to influence site- and gender-specific cancer risk.

METHODS

Observational studies were reviewed that have examined the independent effect of the volume of occupational physical activity (OPA) and/or leisure time physical activity (LPA) on overall and site-specific cancer risk.

RESULTS

The evidence of cohort and case-control studies suggests that both leisure time and occupational physical activity protect against overall cancer risk, with a graded dose-response association suggested in both sexes. Confounding effects such as diet, body weight, and parity are often included as a covariate in the analyses, with little influence on the observed associations. A crude graded inverse dose-response association was observed between physical activity and colon cancer in 48 studies including 40,674 colon/colorectal cancer cases for both sexes. A dose-response effect of physical activity on colon cancer risk was especially observed, when participation in activities of at least moderate activity (>4.5 MET) and demonstrated by activities expressed as MET-hours per week. An observed inverse association with a dose-response relationship between physical activity and breast cancer was also identified in the majority of the 41 studies including 108,031 breast cancer cases. The dose-response relationship was in particular observed in case-control studies and supported by observations in cohort studies when participation in activities of at least moderate activity (>4.5 MET) and demonstrated by activities expressed by MET-hours per week. This association between physical activity and breast cancer risk is possibly dependent on age at exposure, age at diagnosis, menopausal status and other effect modifiers, e.g., body mass index. Furthermore, data concerning carcinoma of other cancers (prostate, lung, endometrium, ovary, and testicular cancers) are required.

CONCLUSION

A protective effect of physical activity on site-specific cancer risk with a dose-response association between physical activity and colon and pre- and postmenopausal breast cancer supported by identified biological mechanisms has been observed. The optimal permutation of type, intensity, duration, and frequency of physical activity across the lifespan is unclear, but it is gender, age, and site specific and supports moderate activity (>4.5 MET) more than light activities (<4.5 MET). The complicated nature of the physical activity variable, combined with lack of knowledge regarding possible biological mechanisms operating between physical activity and cancer, warrants further studies including controlled clinical randomized trials.

Energetic factors, ovarian steroids and the risk of breast cancer

In industrial countries, women often have excess metabolic energy due to high food consumption and low physical activity. High lifetime energy availability results in high lifetime levels of ovarian steroid hormones. Oestrogens and progesterone are hypothesized to play a crucial role in the development and prognosis of breast cancer. Epidemiological studies document the importance of physical activity and caloric limitations in reducing breast cancer risk. The risk of breast cancer is much higher in industrial countries than in developing countries, where women are characterized by lower energy intake and higher energy expenditure. It is likely, that the beneficial effects of physical activity and of negative energy balance are mediated by the reduced levels of ovarian steroids. While both weight loss and physical activity may have similar efficacy in suppressing ovarian function and, therefore, in reducing the risk of breast cancer, we suggest that it may be more advantageous for premenstrual women to achieve lifetime reduction in steroid levels by increasing their physical activity, rather than by weight loss due to caloric restriction alone.

Physical activity and incidence of postmenopausal breast cancer

Whether physical activity reduces the risk of postmenopausal breast cancer is uncertain; few studies have addressed this issue. We examined the association of leisure physical activity with breast cancer incidence among 37,105 postmenopausal participants in the Iowa Women's Health Study. Women reporting the highest level of physical activity at baseline compared with women with the lowest level of activity had an age-adjusted relative risk of breast cancer of 0.92 (95% confidence interval = 0.80-1.05). Women reporting any regular leisure-time physical activity had a relative risk of 0.97 (95% confidence interval = 0.87-1.08) compared with those reporting no such regular physical activity. Adjustment for potential confounders did not appreciably alter the findings. There is little evidence from this study that physical activity later in life is associated to any appreciable extent with breast cancer incidence.

Physical activity and breast cancer risk in women aged 20-54 years

BACKGROUND/METHODS

Although several studies have suggested that physical activity is associated with a decreased risk of breast cancer, such a decrease has not been found consistently, perhaps because physical activity was assessed in different ways and for restricted periods. Few studies have assessed the risk of breast cancer in relation to lifetime physical activity. We used data from a population-based, case-control study, including 918 case subjects (aged 20-54 years) and 918 age-matched population control subjects, to examine associations between breast cancer risk and physical activity at ages 10-12 years and 13-15 years, lifetime recreational activity, and title of longest held job.

RESULTS

Women who were more active than their peers at ages 10-12 years had a lower risk of breast cancer (odds ratio [OR] = 0.68; 95% confidence interval [CI] = 0.49-0.94). Women who had ever engaged in recreational physical activity had a reduced risk of breast cancer compared with inactive women (OR = 0.70; 95% CI = 0.56-0.88). Neither very early recreational activity (before age 20 years) nor recent activity (last 5 years) was associated with a greater reduction in risk than recreational activity in the intermediate period. Furthermore, women who started recreational activities after age 20 years and women who started earlier and continued their activities throughout adult life experienced a similar reduction in risk. Lean women, i.e., women with a body mass index (weight in kg/[height in m](2)) less than 21. 8 kg/m(2), appeared to have a lower risk associated with recreational physical activity than women with a body mass index greater than 24.5 kg/m(2) (OR = 0.57 [95% CI = 0.40-0.82] and OR = 0. 92 [95% CI = 0.65-1.29], respectively).

CONCLUSIONS

Our findings support the hypothesis that recreational physical activity is associated with a decreased risk of breast cancer. Physical activity in early or recent life does not appear to be associated with additional beneficial effects.

Occupational physical activity and breast cancer risk in the upper Cape Cod cancer incidence study

BACKGROUND

In several epidemiological studies, breast cancer risk has been reduced among women who reported high levels of occupational or leisure-time physical activity. We used data from a population-based case control study to evaluate the effect of occupational physical activity on breast cancer risk.

METHODS

Two hundred-thirty three incident cases of breast cancer and 670 controls or their next of kin were interviewed for information on breast cancer risk factors and a complete job history. Physical activity level of jobs were classified using a Department of Labor rating scheme. We calculated adjusted odds ratios for light and medium/heavy activity jobs compared to sedentary jobs.

RESULTS

Odds ratios for women who held medium/heavy jobs for less than 10 years and more than ten years were, respectively, 0.7 (95% CI = 0.4,1.3) and 1.7 (95% CI = 0.9,3.3).

CONCLUSIONS

In these data there was no evidence that holding a job of medium/heavy activity reduced breast cancer risk. The study was limited by misclassification inherent in the occupational exposure scheme and by the lack of information on leisure time physical activity. The modest risk increase for subjects holding a medium/heavy job for at least 10 years probably represents either confounding or random variation.

Lifetime exercise activity and breast cancer risk among post-menopausal women

Lifetime exercise activity has been linked to breast cancer risk among young women. However, no study has specifically evaluated whether lifetime exercise activity is related to the breast cancer risk of post-menopausal women. We conducted a population-based case-control study of post-menopausal white women (1123 newly diagnosed cases and 904 healthy controls) aged 55-64 who lived in Los Angeles County, California, USA to evaluate this relationship. Although neither exercise activity from menarche to age 40 years, nor exercise after age 40 separately predicted breast cancer risk, risk was lower among women who had exercised each week for at least 17.6 MET-hours (metabolic equivalent of energy expenditure multiplied by hours of activity) since menarche than among inactive women (odds ratio (OR) = 0.55; 95% confidence interval (CI) 0.37-0.83). Exercise activity was not protective for women who gained considerable (> 17%) weight during adulthood. However, among women with more stable weight, breast cancer risk was substantially reduced for those who consistently exercised at high levels throughout their lifetime (OR = 0.42; 95% CI 0.24-0.75), those who exercised more than 4 h per week for at least 12 years (OR = 0.59; 95% CI 0.40-0.88), and those who exercised vigorously (24.5 MET-hours per week) during the most recent 10 years (OR = 0.52; 95% CI 0.32-0.85). Strenuous exercise appears to reduce breast cancer risk among post-menopausal women who do not gain sizable amounts of weight during adulthood.

Diet and mammary gland carcinogenesis

The variation in human breast cancer incidence rates worldwide suggests that lifestyle factors, especially diet, influence breast cancer risk. There is convincing evidence that diets associated with rapid growth and greater adult height increase breast cancer risk. In addition, diet and other lifestyle factors which lead to high body mass, especially during postmenopausal years, also appear to increase risk. Several dietary components have been evaluated in epidemiological and animal studies for their role in breast cancer. Dietary fat was once implicated in the high incidence of breast cancer in the Western world, but its role in breast cancer is now controversial. In contrast, alcohol consumption is currently recognized as the best-established dietary risk factor in this disease. Carcinogens that cause mammary gland cancer in rats such as heterocyclic amines and polycyclic aromatic hydrocarbons are found in cooked meat, but it is not yet known if these carcinogens are etiological factors in human breast cancer. Fruits and vegetables are rich in potential chemopreventive factors that may lower breast cancer risk. Practical approaches to dietary modification that include increasing fruit and vegetable consumption, eating a low fat diet, reducing cooked meat consumption, and avoiding alcohol are likely to be of potential overall benefit in lowering the risk of human breast cancer.

Relationship between the risk of breast cancer and physical activity. An epidemiological perspective

The possible protective effect of physical exercise on the risk of breast cancer has gained attention during the last few years. According to the current concepts of biological mechanisms underlying the possible protection, physical activity may alter menstrual function by reducing the number of ovulatory menstrual cycles via a hormone-related pathway and, thus, reducing the cumulative exposure to progesterone and estrogen. Experimental studies on the effects of exercise suggest changes in menstrual characteristics and in ovarian hormone serum levels. The majority of epidemiological studies, in spite of limitations in estimating physical activity and partly incomplete control of confounding factors, suggest a protective effect of physical activity. Whether physical activity offers a means of primary prevention of breast cancer is still an open issue.

Physical activity and the primary prevention of cancer

Evidence has been accumulating that suggests that physical activity may help reduce the risk of cancer. Physically active people have been shown to have a decreased rate of all-cancer mortality. The incidence of colon, breast, and perhaps prostate cancer are decreased in more active people when compared with their sedentary peers. Chronic physical activity may decrease tumor risk by its effect on natural immunity, antioxidant defenses, improved energy balance, hormonal changes, or by other unknown mechanisms.

Physical activity and women in the United States: an overview of health benefits, prevalence, and intervention opportunities

Despite decades of physical activity research and interventions conducted on men, very little is known about the patterns of physical activity among US women. Rates from several national surveys show much lower rates of physical activity for women than for men. Among women, rates may vary by socioeconomic status. Studies relating physical activity and experience with heart disease, cancer, osteoporosis, and mental health are discussed. Interventions in the workplace and the community may increase the level of physical activity among US women. A history of not participating in exercise and lack of time for this activity appear to be important constraints for many women. The Surgeon General's Report on physical activity sanctioned future research on specific groups, such as women. Applied research coupled with community and workplace policies that support women's efforts to be more physically active may decrease the rates of some chronic diseases in this population.

Recreational and occupational physical activities and risk of breast cancer

Physical activity has been hypothesized to reduce breast cancer risk, but an inverse association has not been consistently reported. In this review, we critically evaluate for coherence, validity, and bias the epidemiologic studies on recreational or occupational physical activity, discuss the biologic plausibility of the association, and identify areas for future research. Results from seven of nine studies suggest that higher levels of occupational physical activity may be associated with a reduction in risk, at least among a subgroup of women. Eleven of 16 investigations on recreational exercise reported a 12%-60% decrease in risk among premenopausal and postmenopausal women, although a dose-response trend was not evident in most of the studies. The reduction in risk associated with exercise was more likely to be observed in case-control studies than in cohort studies. Most investigations incompletely assessed physical activity, which contributed to conflicting findings on the optimal time period, duration, frequency, or intensity of activity to minimize risk. Physical activity may exert its effects through changes in menstrual characteristics, reduced body size, or alterations in immune function. In summary, most epidemiologic studies of physical activity reported a reduction in the risk of breast cancer among physically active women. Future research studies should focus on using a cohort design to rule out recall bias as a possible explanation for the decrease in risk associated with exercise, on improving assessment of lifetime physical activity from all sources to clarify whether there is a dose-response relation or an optimal time period, duration, frequency, or intensity of activity, and on elucidating the underlying mechanisms for the inverse association.

The epidemiology of physical activity and cancer

Experimental studies in animals and epidemiological studies in human populations support an inverse association between exercise and the development of cancer. Physical activity has been shown to be protective against the development of breast and colon cancer and may also be important for other kinds of cancer such as that of the prostate. The proposed biological mechanisms for the physical activity--cancer association include exercise's effect on immune function, transit time of digestion, hormones, and body fat. There has been little research on physical activity and the effect on progression of cancer, although there are studies to suggest that it may slow the clinical course of the disease. Furthermore, exercise may be beneficial in the treatment of cancer through mood elevation, decreased loss of lean tissue, and increased quality of life. Much is still to be learned about the effect of exercise on cancer. The intensity, duration, frequency, and type of exercise that is relevant need to be clarified. As well, the time period during life when exercise is important has not been determined. It seems reasonable to conclude that exercise, a modifiable risk factor, is beneficial in preventing certain forms of cancer. Public health interventions may hold promise for cancer prevention.

Physical activity and the risk of breast cancer

BACKGROUND

Because physical activity may affect hormonal concentrations and energy balance, we decided to investigate whether everyday exercise is related to the risk of breast cancer.

METHODS

During 1974 to 1978 and 1977 to 1983, a total of 25,624 women, 20 to 54 years of age at entry, enrolled in health surveys and answered questionnaires about leisure-time and work activity.

RESULTS

During a median follow-up of 13.7 years, we identified 351 cases of invasive breast cancer among the 25,624 women in the cohort. Greater leisure-time activity was associated with a reduced risk of breast cancer, after adjustments for age, body-mass index (the weight in kilograms divided by the square of the height in meters), height, parity, and county of residence (relative risk, 0.63; 95 percent confidence interval, 0.42 to 0.95), among women who exercised regularly, as compared with sedentary women (P for trend=0.04). In regularly exercising women, the reduction in risk was greater in premenopausal women than in postmenopausal women, and greater in younger women (<45 years at study entry) than in older women (> or =45 years) (relative risk, 0.38; 95 percent confidence interval, 0.19 to 0.79). In stratified analyses the risk of breast cancer was lowest in lean women (body-mass index, <22.8) who exercised at least four hours per week (relative risk, 0.28; 95 percent confidence interval, 0.11 to 0.70). The risk was also reduced with higher levels of activity at work, and again there was a more pronounced effect among premenopausal than postmenopausal women.

CONCLUSIONS

Physical activity during leisure time and at work is associated with a reduced risk of breast cancer.

Cancer risk after a hospital discharge diagnosis of endometriosis

OBJECTIVES

Our goal was to determine the risk of cancer after hospitalization for endometriosis.

STUDY DESIGN

Records of 20,686 women hospitalized with endometriosis during the period 1969 to 1983, as identified through the nationwide Swedish Inpatient Register, were linked against the National Swedish Cancer Registry through 1989 to identify all subsequent diagnoses of cancer. The study subjects were followed up for a mean of 11.4 years, with the cohort contributing 216,851 woman years of follow-up. Standardized incidence ratios were computed by the use of age- and period-specific incidence rates derived from the Swedish population. Because of the high proportion of subjects with gynecologic operations (55.6%), evaluation of the risk of gynecologic cancers involved truncation of person years at the time of any such operation.

RESULTS

The overall cancer risk was 1.2 (95% confidence interval 1.1 to 1.3). Significant excesses were observed for breast cancer (standardized incidence ratio = 1.3, 95% confidence interval 1.1 to 1.4), ovarian cancer (1.9, 1.3 to 2.8), and hematopoietic malignancies (1.4, 1.0 to 1.8); this latter excess was largely driven by an excess risk of non-Hodgkin's lymphoma (1.8, 1.2 to 2.6). The risk of ovarian cancer was particularly elevated among subjects with a long-standing history of ovarian endometriosis (4.2, 2.0 to 7.7). Cervical cancer risk was slightly reduced (0.7, 0.4 to 1.3) whereas no association was observed for cancer of the endometrium (1.1, 0.6 to 1.9).

CONCLUSIONS

These findings suggest that further attention be given to the risk of breast, ovarian and hematopoietic cancers among women with endometriosis and to exploring possible hormonal and immunologic reasons for the excess risks.