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

Sedentary work and breast cancer risk: A systematic review and meta-analysis

OBJECTIVES

This systematic review and meta-analysis aimed to assess sedentary work's contribution to breast cancer risk quantitatively using thorough research articles.

METHODS

We performed a meta-analysis using a registered protocol in PROSPERO (registration number: CRD42020204629). Literature from PubMed, Embase, and Cochrane involving sedentary work and breast cancer risk was reviewed. We calculated the overall pooled risk ratios (RRs) and 95% CI with a random-effect model from the included studies. Furthermore, we performed stratified analyses by characteristics of studies.

RESULTS

Thirty-one studies (13 cohort studies and 18 case-control studies) were included in the analysis. The overall effect of the pooled analysis was an RR of 1.16 (95% CI 1.08-1.23). The results were 1.20 (95% CI 1.10-1.30) and 1.12 (95% CI 1.02-1.23) for cohort and case-control studies. The effect of sedentary work did not seem to be consistently attenuated by controlling body mass index, menopausal status, or experience of hormone replacement therapy.

CONCLUSION

The results from this meta-analysis suggest that sedentary behavior within the occupational domain was associated with a 15.5% increased risk of breast cancer. It is essential to reduce the sedentary time spent at work and to secure time for leisure-time physical activity among sedentary workers as a primary preventive measure.

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.

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.

Physical activity and breast cancer prevention

Breast cancer is the most commonly diagnosed invasive malignancy and the second leading cause of cancer death in women. This chapter considers epidemiologic evidence regarding the association between physical activity and breast cancer risk from 73 studies conducted around the world. Across these studies there was a 25% average risk reduction amongst physically active women as compared to the least active women. The associations were strongest for recreational activity, for activity sustained over the lifetime or done after menopause, and for activity that is of moderate to vigorous intensity and performed regularly. There is also some evidence for a stronger effect of physical activity amongst postmenopausal women, women who are normal weight, have no family history of breast cancer, and are parous. It is likely that physical activity is associated with decreased breast cancer risk via multiple interrelated biologic pathways that may involve adiposity, sex hormones, insulin resistance, adipokines, and chronic inflammation. Future research should include prospective observational epidemiologic studies relating proposed biomarkers to breast cancer risk and also randomized controlled trials to examine how physical activity influences the proposed biomarkers. Exercise trials will provide more clarity regarding the appropriate type, dose, and timing of activity that relate to breast cancer risk reduction.

Leisure time and occupational physical activity among racial or ethnic minorities

PURPOSE

One of the factors distinguishing non-Latino blacks (NLB) and Latinos from non-Latino whites (NLW) is higher rates of occupational physical activity (OPA) and less participation in leisure time physical activity (LTPA). We examined participation in OPA and LTPA among employed individuals and the relationship between OPA and LTPA across select racial or ethnic groups and Latino subgroups.

METHODS

We pooled data from 2000 to 2003 of the National Health Interview Survey. We divided the survey participants into three groups: 1) those with no LTPA, 2) those who reported some LTPA but not for sufficient time and intensity to meet recommended guidelines, and 3) those who reported LTPA at levels that met or exceeded recommendations. We used ordinal logistic regression to examine whether NLB and Latinos or Latino subgroups were less likely to report LTPA than NLW while controlling for social, economic, and demographic factors that may have accounted for group differences. We further examined the prevalence of OPA and the relationship between LTPA and OPA.

RESULTS

Among employed individuals, NLB and Latinos had significantly more individuals reporting no LTPA compared with NLW. Latinos had the greatest proportion of individuals reporting no LTPA. Furthermore, it was found that significantly more Latinos had physically active occupations compared with NLB and NLB compared with NLW, respectively. Among employed Latinos, Cubans and Dominicans were most likely to report no LTPA, and Mexicans had the greatest percentage of workers with a physically active occupation. LTPA was not significantly associated with having a physically active occupation across races and Latino subgroups.

CONCLUSIONS

Participation in LTPA among ethnic or racial minorities is lower than that of NLW, and the OPA rates are higher. OPA does not significantly impact participation in LTPA in employed adults.

A systematic review of the evidence for Canada's Physical Activity Guidelines for Adults

This systematic review examines critically the scientific basis for Canada's Physical Activity Guide for Healthy Active Living for adults. Particular reference is given to the dose-response relationship between physical activity and premature all-cause mortality and seven chronic diseases (cardiovascular disease, stroke, hypertension, colon cancer, breast cancer, type 2 diabetes (diabetes mellitus) and osteoporosis). The strength of the relationship between physical activity and specific health outcomes is evaluated critically. Literature was obtained through searching electronic databases (e.g., MEDLINE, EMBASE), cross-referencing, and through the authors' knowledge of the area. For inclusion in our systematic review articles must have at least 3 levels of physical activity and the concomitant risk for each chronic disease. The quality of included studies was appraised using a modified Downs and Black tool. Through this search we identified a total of 254 articles that met the eligibility criteria related to premature all-cause mortality (N = 70), cardiovascular disease (N = 49), stroke (N = 25), hypertension (N = 12), colon cancer (N = 33), breast cancer (N = 43), type 2 diabetes (N = 20), and osteoporosis (N = 2). Overall, the current literature supports clearly the dose-response relationship between physical activity and the seven chronic conditions identified. Moreover, higher levels of physical activity reduce the risk for premature all-cause mortality. The current Canadian guidelines appear to be appropriate to reduce the risk for the seven chronic conditions identified above and all-cause mortality.

Risk factors for surgically treated benign prostatic hyperplasia in Western Australia

OBJECTIVE

To determine the relationship between personal, hormonal and lifestyle risk factors and surgically treated benign prostatic hyperplasia (BPH).

MATERIALS AND METHODS

A population-based case-control study was conducted in Western Australia (WA) on men aged 40-75 years who were surgically treated at public and private hospitals for BPH during 2001-2002. Controls were recruited from the WA electoral roll. Cases and controls were compared with regard to demographic and lifestyle factors and proxy measures of hormonal status using logistic regression. Data were available for 398 cases and 471 controls.

RESULTS

No associations with BPH were found for family history of prostate cancer in father or brother, serving in the military in a combat area, pattern of baldness, smoking status, obesity, alcohol intake and occupational physical activity. The only inverse relationship was observed with heavy alcohol drinking (>30g/day), however, this was not statistically significant. An increased risk of BPH, not statistically significant, was observed for British-born men compared to Australian born and for history of vasectomy. The analysis was repeated after excluding 28% of controls with moderate and severe symptoms of BPH and 7% of cases with mild symptoms prior to surgery, and our results remained essentially unchanged.

CONCLUSIONS

The results suggest that there are few risk factors for BPH although perhaps country of birth, vasectomy and heavy alcohol consumption may be considered further.

Lifetime physical activity and the risk of breast cancer: a case-control study

BACKGROUND

The association between breast cancer and lifetime histories of physical activity was studied to determine whether exercise may reduce development of this cancer.

METHODS

The case-control study was based on 250 women living in the Region of Western Pomerania with histological confirmed breast cancer operated during 1999-2003 in the Szczecin hospitals, and 301 controls, free of any cancer diagnosis, aged 35-75 years. Physical activity was assessed using a self-administered questionnaire with questions on type of activity, duration, frequency, and intensity for each type of activity. Data on physical activity from exercise/sports, household and outdoor chores, and occupational activity separately throughout a woman's lifetime were collected. Recreational physical activity was examined for the four age periods (14-20, 21-34, 35-50, and after the age of 50 years). Logistic regression was used to compute odds ratios (ORs) and 95% confidence intervals (CI) and a full assessment of confounding was included in analysis.

RESULTS

Lifetime total physical activity among all women was associated with a reduced breast cancer risk; after controlling for potential confounders the multivariate ORs associated with <110, 110-150, and >150 metabolic equivalent (MET)-hours/week/year were 1.00 (referent), 0.60 (95% CI: 0.60-1.06), and 0.43 (95% CI: 0.25-0.75), respectively (P(trend)=0.004). Analyses by type of lifetime activity for household and recreational activities revealed significant risk reductions: 1.00 (referent); 0.51, 95% CI: 0.29-0.87; 0.54, 95% CI: 0.31-0.94 (P(trend)=0.002), and 1.00 (referent); 0.54, 95% CI: 0.31-0.94; 0.40, 95% CI: 0.22-0.70 (P(trend)<0.001), respectively. For lifetime occupational activity a modest association could not be ruled out when measured the activity by average annual hours/week; the ORs were 0.51, 95% CI: 0.29-0.91, and 0.58, 95% CI: 0.33-1.00, the inverse trend in risk was not significant (P(trend)=0.51). Women who started recreational activity after age of 20 years had much more higher breast cancer risk than either those who were active between ages 14 and 20 years and were inactive after age 20 years or continued their activity throughout adult life.

CONCLUSIONS

These data are in concordance with the hypothesis that lifetime total physical activity is associated with decreased breast cancer risk. They also suggest that recreational physical activity at ages 14-20 years is the most beneficial. In addition, these findings confirm the majority of previous reports which implicated physical inactivity as important risk factor for breast cancer.

Physical activity and stomach cancer risk: The influence of intensity and timing during the lifetime

Physical inactivity is linked to risk for cancers of the colon, breast, lung and endometrium, but few data exist on this association with stomach cancer. We evaluated the association between recreational physical activity and incident stomach cancer in a case-control study. The data yielded odds ratios suggestive of approximately 20-40% reduced risk of stomach cancer when comparing more frequent (3+ times/week) to less frequent (<1/month) strenuous activities. The magnitudes of the associations for strenuous physical activities were consistent across the teens, early 30s, and early 50s. Compared to the least active quartile, the second, third, and fourth quartiles of average lifetime strenuous physical activity yielded odds ratios of 0.82, 0.69, and 0.58 (95% CI, 0.41-0.83), respectively. Odds ratios for moderate activity generally suggested a modest, but not statistically significant, inverse association. Increased strenuous physical activity, throughout the lifespan, was associated with decreased risk of incident stomach cancer.

The effect of lifestyle factors on gynaecological cancer

Several lifestyle factors affect a woman's risk of gynaecological cancer and-potentially-can be modified to reduce risk. This chapter summarises the evidence for the effect of lifestyle factors on the incidence of gynaecological malignancy. The incidence of obesity is increasing in the developed world such that it now contributes as much as smoking to overall cancer deaths. Women with a body mass index (BMI)>40 have a 60% higher risk of dying from all cancers than women of normal weight. They are also at increased risk from gynaecological cancer. Dietary factors significantly influence the risk of gynaecological cancer: fruit, vegetables and antioxidants reduce risk whereas high animal fat and energy intakes increase risk. Alcohol intake adversely affects breast cancer risk, possibly accounting for 4% of all breast cancers. Physical activity protects against ovarian, endometrial and postmenopausal breast cancer, independently of BMI. The oral contraceptive pill has a substantial and long-lasting effect on the prevention of ovarian and endometrial cancer and is one of the best examples of large-scale chemoprevention in the developed world. Childbearing is protective against ovarian, endometrial and breast cancer but increases the risk of cervical cancer. Smoking acts as a cofactor in cervical carcinogenesis and increases the risk of ovarian cancer, particularly mucinous tumours.

Science and society: the Long Island Breast Cancer Study Project

In the early 1990s, breast cancer advocates petitioned the United States Congress to investigate the high rates of breast cancer on Long Island in the state of New York. The resulting law led to the Long Island Breast Cancer Study Project (LIBCSP)--more than ten research projects designed to study the possible causes of this increased incidence of cancer. This project reported that there was no evidence that environmental exposures were responsible. Controversial from its start, the LIBCSP has had an important role in efforts to understand the reasons for the high rates of breast cancer in some regions of the United States.

When worlds collide: observations on the integration of epidemiology and transportation behavioral analysis in the study of walking

Since obesity has emerged as a public health crisis in the United States, the factors that influence physical activity are of interest to both epidemiologists and transportation researchers. This article describes different approaches taken by the two disciplines to this issue. "Utilitarian" walking to accomplish a task, as opposed to structured exercise, could be a highly sustainable way for people to achieve recommended levels of physical activity. Transportation planners have begun to investigate factors of urban form and transportation services that influence the choice to walk. Epidemiologists have become more aware of the importance of factors in the built and social environment that could influence health behaviors like walking. Few transportation studies focus on the generation of the utilitarian walk trip as the key variable; rather, they include it in more general discussions of urban form. Likewise, most epidemiologic studies have not focused on utilitarian walking, but have folded it into an overall measure of physical activity that emphasizes structured exercise. Further transportation research should examine the effects of improved mobility services in addition to alterations of the built environment. Integration of epidemiologic and transportation behavioral research could enhance our understanding of the role of urban and transportation factors on physical activity.

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.

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.

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.

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.