Am. J. Epidemiol.-2010-Patel-419-29

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American Journal of Epidemiology

The Author 2010. Published by Oxford University Press on behalf of the Johns Hopkins Bloomberg School of

Public Health. All rights reserved. For permissions, please e-mail: [email protected].

Vol. 172, No. 4

DOI: 10.1093/aje/kwq155

Advance Access publication:

July 22, 2010

Original Contribution

Leisure Time Spent Sitting in Relation to Total Mortality in a Prospective Cohort of

US Adults

Alpa V. Patel*, Leslie Bernstein, Anusila Deka, Heather Spencer Feigelson, Peter T. Campbell,Susan M. Gapstur, Graham A. Colditz, and Michael J. Thun

* Correspondence to Dr. Alpa V. Patel, Epidemiology Research Program, American Cancer Society, 250 Williams Street NW,

Atlanta, GA 30303 (e-mail: [email protected]).

Initially submitted January 7, 2010; accepted for publication April 29, 2010.

The obesity epidemic is attributed in part to reduced physical activity. Evidence supports that reducing time spent

sitting, regardless of activity, may improve the metabolic consequences of obesity. Analyses were conducted in

a large prospective study of US adults enrolled by the American Cancer Society to examine leisure time spent

sitting and physical activity in relation to mortality. Time spent sitting and physical activity were queried by ques-

tionnaire on 53,440 men and 69,776 women who were disease free at enrollment. The authors identified 11,307

deaths in men and 7,923 deaths in women during the 14-year follow-up. After adjustment for smoking, body mass

index, and other factors, time spent sitting (!6 vs.


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(CPS-II) Nutrition Cohort. This cohort has the advantage ofbeing very large with approximately 184,000 US adults andover 19,000 deaths for whom detailed information on timespent sitting and physical activity was collected at baseline.

MATERIALS AND METHODS

Study population

Men and women in this analysis were drawn from the184,190 participants in the CPS-II Nutrition Cohort (here-after referred to as the Nutrition Cohort), a prospectivestudy of cancer incidence and mortality begun by the Amer-ican Cancer Society in 1992 (24). The Nutrition Cohort isa subgroup of approximately 1.2 million participants in thebaseline CPS-II cohort, a prospective mortality study estab-lished by the American Cancer Society in 1982 (25). Mem-bers of the CPS-II cohort who resided in 21 states withpopulation-based state cancer registries and were 5074years of age in 1992 were invited to participate by complet-ing a mailed questionnaire. The 10-page mailed question-

naire included questions on demographic, reproductive,medical, behavioral, and lifestyle factors. The recruitmentand characteristics of the Nutrition Cohort are described indetail elsewhere (24).

We excluded sequentially from this analysis men andwomen who reported a personal history of cancer (n 21,785), heart attack (n 11,560), stroke (n 2,513), oremphysema/other lung disease (n 9,321) at the time ofenrollment. We also excluded individuals with missing dataon physical activity (n 4,240), missing sitting time (n 2,954), missing or extreme (top and bottom 0.1%) values ofbody mass index (n 2,121), or missing smoking status (n 1,347) at baseline. Finally, to reduce the possibility of un-diagnosed serious illness at baseline that would preclude orinterfere with physical activity, we excluded individuals whoreported both no daily life activities and no light housekeep-ing (n 4,730), as well as those who died from any causewithin the first year of follow-up (n 403). After exclusions,the analytical cohort consisted of 123,216 individuals (53,440men and 69,776 women) with a mean age of 63.6 (standarddeviation, 6.0) years in men and 61.9 (standard deviation, 6.5)years in women when enrolled in the study in 1992.

Mortality endpoints

The primary endpoint was death from any cause occur-ring between 1 year after the time of enrollment and De-cember 31, 2006. Deaths were identified through biennialautomated linkage of the entire cohort with the NationalDeath Index (26). Death certificates or codes for cause ofdeath have been obtained for 98.7% of all known deaths.Causes of death were classified by using the InternationalClassification of Diseases (ICD), Ninth Revision (27), fordeaths occurring from 1992 to 1998 and the Tenth Revision(28) for deaths from 1999 to 2006. Specific causes of deathwere grouped into 3 broad categories: cardiovascular dis-ease (ICD, Ninth Revision, codes 390459 and ICD, TenthRevision, codes I00I99); cancer (ICD, Ninth Revision,codes 140195 and 199208 and ICD, Tenth Revision,codes C00C76 and C80C97); and all other causes.

Measures of time spent sitting and physical activity

Time spent sitting was assessed by using the question,During the past year, on an average day (not counting timespent at your job), how many hours per day did you spendsitting (watching television, reading, etc.)? Responses in-cluded none,8 hours per day. Time spent

sitting was categorized as 0


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other potential confounding factors, and 3) mutually adjust-ing for both physical activity and time spent sitting in addi-tion to all potential confounders. The potential confoundersincluded were race (white, black, other), smoking status(never, current, former), duration (35, >35 years) andfrequency (25 years), body massindex (weight (kg)/height (m)2) (


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Table 1. Age-adjusted Percentages and Means of Selected Baseline Characteristics in 1992, by Hours of Leisure Time Spent Sitting for Wome

Nutrition Cohort

Sitting in 1992

Women


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activity at baseline with those from our first follow-up sur-vey in 1997 to examine sustained (5-year) measures. Resultsfrom these analyses did not differ from those presented forbaseline alone (data not shown).

Associations between time spent sitting and physical ac-tivity were stronger for cardiovascular disease mortalitythan for cancer (Table 4). Time spent sitting was associatedwith an increased risk of cardiovascular disease mortality inboth men and women, whereas it was associated with in-

creased cancer mortality only among women. There wasa statistically significant inverse relation between physical

activity and cardiovascular disease mortality beginning atrelatively low levels of activity in both men (Ptrend 0.0001) and women (Ptrend < 0.0001). In contrast, totalphysical activity was not significantly associated with lowercancer mortality among men and only modestly associatedwith lower cancer mortality in women. Longer time spentsitting was associated with higher death rates from all othercauses, and physical activity was inversely associated withdeath rates from other causes. The most common conditions

in this category were respiratory diseases (22.7% in men,20.4% in women), central nervous system diseases (20.3%

Figure 1. Combined multivariate-adjusted rate ratios (P< 0.05) for leisure time spent sitting and physical activity in relation to all-cause mortality,women only, in the Cancer Prevention Study II Nutrition Cohort, 19932006. MET, metabolic equivalent.

Figure 2. Combined multivariate-adjusted rate ratios (P< 0.05) for leisure time spent sitting and physical activity in relation to all-cause mortality,men only, in the Cancer Prevention Study II Nutrition Cohort, 19932006. MET, metabolic equivalent.

424 Patel et al.

Am J Epidemiol 2010;172:419429


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in men, 19.8% in women), digestive diseases (9.6% in men,10.4% in women), and diabetes (7.8% in men, 6.3% inwomen).

DISCUSSION

In this large prospective cohort, women who reportedsitting for more than 6 hours during their leisure time versusless than 3 hours a day had an approximately 40% higherall-cause death rate, and men had an approximately 20%higher death rate. This association was independent of theamount of physical activity. The combination of both sittingmore and being less physically active (>6 hours/day sittingand


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First, time spent sitting might be more easily measured thanphysical activity and/or may reflect a different aspect ofinactivity than other indices usually used in epidemiologicstudies. However, this potential misclassification of expo-sure is unlikely to fully explain our findings, because timespent sitting was significantly associated with mortality

even among men and women with the highest levels ofphysical activity.

Second, time spent sitting might be associated with otherunhealthy behaviors that are either not captured or incom-pletely captured through questionnaires. Total energy ex-penditure is reduced among individuals who are sedentary.

Table 4. Relative Risk of Death From CardiovascularDisease, Cancer, and All Other Causes According to Leisure

Time Spent Sitting and Physical Activity Among Women and Men, Cancer Prevention Study II Nutrition Cohort,

19932006

Cardiovascular Disease Cancer Other Causes

No. ofDeaths

RelativeRiska

95% CINo. of

DeathsRelative

Riska95% CI

No. ofDeaths

RelativeRiska

95% CI

Women

Sitting in 1992,hours/day

0


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However, consistent with previous studies, the present studyfound no correlation between physical activity and timespent sitting (r 0.03). Time spent sitting is also associ-ated with greater food consumption and subsequent weightgain, especially when watching television (16, 34, 35). Timespent sitting was previously shown to be associated withincreased weight gain in this cohort (18). While residual

confounding by obesity could contribute to the associationbetween sitting time and mortality, this association was at-tenuated but not eliminated by controlling for or stratifyingon body mass index.

Third, prolonged time spent sitting, independent of phys-ical activity, has important metabolic consequences thatmay influence specific biomarkers (such as triglycerides,high density lipoprotein cholesterol, fasting plasma glucose,resting blood pressure, and leptin) of obesity and cardiovas-cular and other chronic diseases (811). Animal studieshave also shown that sedentary time substantially sup-presses enzymes centrally involved in lipid metabolismwithin skeletal muscle, and low levels of daily life activityare sufficient to improve enzyme activity (3638). Further-more, substantial evidence in both adults and children fromobservational studies and randomized clinical trials showsthat reducing time spent sitting lowers the risk of obesityand type II diabetes (19, 3942).

Over the past century, a number of technologic changeshave contributed to a decrease in total daily energy expen-diture. For example, during the 20062007 broadcast year,the average US household reported 8 hours of televisionwatching per day, which is an increase of 1 hour per dayof television watching from only a decade ago (43). Al-though leisure-time physical activity levels have remainedrelatively constant over the past few decades (44, 45), it iswell recognized that technologic advances in the workplace

have also greatly reduced occupational physical activity.This reduction in overall physical activity, in conjunctionwith increased time spent sitting and higher caloric intake,has contributed in large part to the rise in obesity and likelyinfluenced temporal trends in cardiovascular disease, type 2diabetes, and some cancers.

The strengths of our study include the large sample size,prospective design, and ability to control for many potentialconfounding factors. The lack of occupational physical ac-tivity data is a potential limitation; however, we believe thisto have minimal impact on daily physical activity levelsbecause the majority of study participants were retired/homemakers (57% of men and 80% of women) and, amongthose that were not retired, few worked in jobs that involvedany activity (21% of men and 7% of women). Because wemeasured only leisure time spent sitting, the lack of occu-pational sitting time may have underestimated sitting timeamong working individuals, since much of their sitting timemay have occurred at work. However, adjusting for employ-ment status (employed, retired, or homemaker) did notchange risk estimates for time spent sitting or physicalactivity. Furthermore, we conducted a sensitivity analysisamong only men and women who were retired or home-makers, and results were virtually identical to those in theoverall cohort. Another limitation is the use of self-reportedmeasures of time spent sitting, physical activity, and all

other covariates including height and weight. Although thephysical activity and sitting time questions we used are sub-

ject to misreporting, they are very similar to those used andvalidated in the Nurses Health Study II, a prospective studywith similar participant characteristics, which found a corre-lation of 0.79 between activity reported on recalls and ques-tionnaire (46). These measures have also been associated

with various cancers in this cohort (4750). Finally, we werenot able to differentiate between types of sitting (i.e., whilewatching television, reading, driving), and the energy ex-penditure and other behaviors may vary with different typesof sitting.

In conclusion, we found that both leisure time spent sit-ting and physical activity are independently associated withtotal mortality. Associations were stronger for cardiovascu-lar disease mortality than for cancer mortality. Public healthmessages and guidelines should be refined to include reduc-ing time spent sitting in addition to promoting physicalactivity. Because a sizeable fraction of the populationspends much of their time sitting, it is beneficial to encour-age sedentary individuals to stand up and walk around aswell as to reach optimal levels of physical activity.

ACKNOWLEDGMENTS

Author affiliations: Epidemiology Research Program,American Cancer Society, Atlanta, Georgia (Alpa V. Patel,Ausila Deka, Peter T. Campbell, Susan M. Gapstur, MichaelJ. Thun); City of Hope, Duarte, California (LeslieBernstein); Kaiser Permanente, Denver, Colorado (HeatherSpencer Feigelson); and Washington University SitemanCancer Center, St. Louis, Missouri (Graham A. Colditz).

The authors would like to acknowledge the late Drs.Eugenia E. Calle and Carmen Rodriguez who were instru-mental in conducting and guiding this research. The prepa-ration of this manuscript would not have been possiblewithout them.

Conflict of interest: none declared.

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Am. J. Epidemiol.-2010-Patel-419-29