Abstract

Research suggests that sedentary behavior may increase the risk of some chronic diseases. The aims of the study were to examine whether sedentary work is associated with colorectal cancer and to determine whether the association differs by subsite. A total of 918 cases and 1,021 controls participated in a population-based case-control study of colorectal cancer in Western Australia in 2005–2007. Data were collected on lifestyle, physical activity, and lifetime job history. The estimated effects of sedentary work on the risk of cancers of the proximal colon, distal colon, and rectum were analyzed by using multinomial logistic regression. Compared with participants who did not spend any time in sedentary work, participants who spent 10 or more years in sedentary work had almost twice the risk of distal colon cancer (adjusted odds ratio = 1.94, 95% confidence interval: 1.28, 2.93) and a 44% increased risk of rectal cancer (adjusted odds ratio = 1.44, 95% confidence interval: 0.96, 2.18). This association was independent of recreational physical activity and was seen even among the most recreationally active participants. Sedentary work was not associated with the risk of proximal colon cancer. These results suggest that long-term sedentary work may increase the risk of distal colon cancer and rectal cancer.

Research suggests that sedentary behavior may increase the risk of some chronic diseases (1). Sedentary behavior is characterized by prolonged sitting and other activities requiring very low energy expenditure (<1.5 metabolic equivalents (METs)) (1–3) and is considered to be an independent exposure, distinct from low levels of physical activity (1). Research on the more well-recognized risk factor of physical activity has shown that a low level of work-related activity is associated with an increased risk of colorectal cancer (4). However, these studies have mostly compared light-intensity occupations (or combined sedentary and light-intensity occupations) with physically active work. Physical activity and sedentary behavior (often called “too much sitting”) may act through different mechanisms (1) and, as such, it is not clear if a low level of physical activity, a high level of sedentary behavior, or a combination of both is behind the associations seen. Previous studies have generally not adjusted for recreational physical activity and also have not investigated whether recreational physical activity modified the effect of sedentary work on colorectal cancer risk. Additionally, few of these studies have investigated the effect of long-term sedentary work on the risk of colorectal cancer; most have examined only occupational activity at 1 point in time.

Another unresolved issue is whether sedentary work has a different effect on distal and proximal colon cancer risks. It has been suggested that distal and proximal colon cancers may in fact be 2 different diseases: There are morphological, molecular, and epidemiologic differences between distal and proximal colon cancers, and research suggests that they may have different environmental and genetic risk factors (5). Different effects by tumor subsite may have screening implications (6) and may provide further clues into the etiology of colon cancer.

We conducted a case-control study to investigate these issues. The aims of the study were as follows: to examine whether sedentary work was associated with colorectal cancer; to determine whether the association differed by subsite; and to investigate whether recreational physical activity modified the effect of sedentary work on colorectal cancer risk.

MATERIALS AND METHODS

The Western Australian Bowel Health Study

A case-control study called the Western Australian Bowel Health Study (WABOHS) took place in Western Australia between 2005 and 2007. Details of this study have been described previously (7). Briefly, all participants were aged between 40 and 79 years and resided in Western Australia. Incident cases of colorectal cancer occurring between June 2005 and August 2007 were recruited through the Western Australian Cancer Registry. With the exception of nonmelanoma skin cancer, it is mandatory to register all cases of cancer in Australia. Carcinomas occurring in the hepatic flexure, cecum, ascending colon, and transverse colon were classified as proximal colon cancers; carcinomas occurring in the splenic flexure, descending colon, and sigmoid colon were classified as distal colon cancers; and carcinomas occurring in the rectosigmoid junction and rectum were classified as rectal cancers. Controls were randomly selected from the Western Australian electoral roll, which is considered to be virtually complete as registration on the electoral roll is compulsory in Australia. Information on lifestyle, diet, occupation, and medication use was collected by using self-administered questionnaires from 918 histopathologically confirmed cases (59.5% response fraction) and 1,021 age- and sex-matched controls (46.5% response fraction). Forty-eight cases and 25 controls were excluded because of missing data on 1 or more covariates, leaving 996 controls and 870 cases in this analysis. Ethics approval for the Western Australian Bowel Health Study was obtained from the University of Western Australia Human Research Ethics Committee and the Confidentiality of Health Information Committee within the Western Australian Department of Health, and written informed consent was obtained from all participants.

Exposure measurement

Participants were asked to complete their lifetime occupational history, from their first job to retirement. For each job that a participant held, he/she was required to record the job title and duties, company, location, age started and stopped, and whether the job was full time, part time, casual, or seasonal. Occupational physical activity was calculated by classifying each job (based on job title and duties) that a participant held into one of 5 categories of Physical Demands Strength Rating, according to the US Department of Labor's Dictionary of Occupational Titles (8). The 5 categories are 1) sedentary (e.g., bookkeepers, computing professionals); 2) light (e.g., teachers, hairdressers, housewives); 3) medium (e.g., mechanics, police officers, nurses); 4) heavy (e.g., plumbers, farmers); and 5) very heavy (e.g., miners, fire fighters). The heavy and very heavy categories were combined as there were few participants in the very heavy category. Jobs classified as sedentary by the Dictionary of Occupational Titles involve sitting most of the time, while light jobs involve activity of a light intensity (8).

The number of years that a participant spent in each level of occupational activity was calculated. Part time, casual, and seasonal occupations were considered to be equivalent to 0.5, 0.25, and 0.25 years of full-time work, respectively. A lifetime occupational activity variable was created by assigning participants to the occupational activity category in which they spent the most time. As more than half of the participants were in the “light” category, this was used as the reference level in the model that assessed the effect of lifetime occupational activity on colorectal cancer. To further quantify the effect of sedentary work on colorectal cancer, we categorized the number of years spent in sedentary work as none, more than zero but less than 10 years, and 10 or more years. The latter 2 categories were determined from the distribution among controls that led to 2 groups of roughly equal size.

Statistical analysis

Polytomous logistic regression models were used to estimate the odds ratio for the risk of proximal colon cancer, distal colon cancer, and rectal cancer. Postestimation commands were used to test whether the effect of each exposure differed significantly by subsite (9). As this was an imputed data set (see below), the command “MIM” was used to generate parameter estimates (10). All analyses were performed by using STATA, version 11.1, statistical software (StataCorp LP, College Station, Texas).

Sex and age group (5 year) were included in all models, as the control distribution was matched to the case distribution on these variables. Energy intake 10 years ago, cigarette use, alcohol intake 10 years ago, diabetes, socioeconomic status, body mass index at ages 20 and 40 years, and lifetime recreational physical activity were included in the final analyses as they are established risk factors for colorectal cancer and were considered to be potential confounders (11). The model assessing the effect of years in sedentary work additionally adjusted for combined years in heavy and very heavy occupational activity. Energy intake (kJ/day from food) and alcohol intake (g/day) were categorized on the basis of the control quartiles. Cigarette use was categorized into 3 groups (0, 1–19, ≥20 pack-years). Diabetes was classified as having diabetes, high blood sugar level only, or neither. Socioeconomic status was classified by using the Index of Relative Socio-economic Disadvantage from the Socio-Economic Indexes for Areas (12) and was categorized into 5 groups. Body mass index was classified as under or normal weight (<25 kg/m2), overweight (25–29.9 kg/m2), and obese (≥30 kg/m2). Body mass index at ages 20 years and 40 years was imputed by using the multiple imputation command “ICE” (13), as there were 8% missing data for these variables.

Information on recreational physical activity performed over the adult lifetime was collected by using a questionnaire based upon others that have been shown to be reliable (14–16). Participants were asked to record the recreational physical activity that they performed regularly (>10 times) during 3 periods of their life (up to the date of their participation in the study): 19–34 years; 35–50 years; and 51 years or more. For each age period, a MET-hours per week total was calculated. This total was then categorized as no physical activity (i.e., 0 MET-hours per week), and the remaining active participants were split into 3 groups of roughly equal size (0.1–11.9, 12–29.9, and ≥30 MET-hours per week). A lifetime recreational physical activity score was created by summing the category (0, 1, 2, or 3) from each age period. The total score was then split into 2 groups (0–4, 5–9 for participants aged 51 years or older; 0–2, 3–6 for participants aged <51 years). This was also done for vigorous activities (activities with a MET value of 6 or more) only.

To determine whether recreational physical activity modified the effect of sedentary work on colorectal cancer risk, we included an interaction term in the full model. Recreational physical activity was considered to be a potential effect modifier and is an established risk factor for colorectal cancer. We also examined whether vigorous recreational physical activity or body mass index at age 40 years modified the effect of sedentary work on colorectal cancer risk. Postestimation commands were used to generate odds ratios and confidence intervals for the models that included an interaction term.

Chi-square tests were used to assess the differences between outcomes for the selected characteristics in Table 1. All P values were 2 sided.

Table 1.

Selected Characteristics of the Participants, by Outcome, the Western Australian Bowel Health Study, 2005–2007

 Cases
 
 Proximal ColonCancer
 
Distal ColonCancer
 
RectalCancer
 
All Colorectal Cancer
 
Controls
 
 No. No. No. No. No. 
Sex           
    Male 155 54.6 165 61.6 216 67.9 536 61.6 594 59.6 
    Female 129 45.4 103 38.4 102 32.1 334 38.4 402 40.4 
Age, years           
    40–49 12 4.2 15 5.6 18 5.7 45 5.2 70 7.0 
    50–59 48 16.9 80 29.8 83 26.1 211 24.2 219 22.0 
    60–69 106 37.3 94 35.1 120 37.7 320 36.8 355 35.6 
    70–79 118 41.6 79 29.5 97 30.5 294 33.8 352 35.3 
Socioeconomic status           
    1 (most disadvantaged) 44 15.5 54 20.2 57 17.9 155 17.8 168 16.9 
    2 51 18.0 49 18.3 78 24.5 178 20.5 192 19.3 
    3 63 22.2 41 15.3 70 22.0 174 20.0 231 23.2 
    4 67 23.6 64 23.9 60 18.9 191 22.0 195 19.6 
    5 (least disadvantaged) 59 20.8 60 22.4 53 16.7 172 19.8 210 21.1 
Smoking, pack-years           
    0–0.9 132 46.5 116 43.3 135 42.4 383 44.0 493 49.5 
    1–19.9 62 21.8 66 24.6 79 24.8 207 23.8 236 23.7 
    ≥20 90 31.7 86 32.1 104 32.7 280 32.2 267 26.8 
Body mass index at age 20 years, kg/m2          
    <25 225 84.0 182 73.7 223 76.6 630 78.2 767 83.0 
    25–29.9 39 14.6 60 24.3 56 19.2 155 19.2 136 14.7 
    ≥30 1.5 2.0 12 4.1 21 2.6 21 2.3 
Body mass index at age 40 years, kg/m2          
    <25 143 54.0 123 48.8 158 53.7 425 52.3 538 58.4 
    25–29.9 95 35.8 89 35.3 92 31.3 276 34.0 296 32.1 
    ≥30 27 10.2 40 15.9 44 15.0 111 13.7 87 9.4 
Diabetes and/or high blood sugar           
    Neither 225 79.2 211 78.7 259 81.4 695 79.9 851 85.4 
    High blood sugar 19 6.7 16 6.0 14 4.4 49 5.6 52 5.2 
    Diabetes 40 14.1 41 15.3 45 14.2 126 14.5 93 9.3 
Energy intake from food 10 years ago, kJ/day           
    0–6,301 70 24.6 60 22.4 55 17.3 185 21.3 245 24.6 
    6,302–8,074 61 21.5 64 23.9 77 24.2 202 23.2 250 25.1 
    8,075–10,406 83 29.2 64 23.9 94 29.6 241 27.7 252 25.3 
    ≥10,407 70 24.6 80 29.8 92 28.9 242 27.8 249 25.0 
Alcohol consumption 10 years ago, g/day           
    0–1.1292 71 25.0 56 20.9 72 22.6 199 22.9 248 24.9 
    1.1293–9.514 69 24.3 66 24.6 69 21.7 204 23.4 249 25.0 
    9.515–27.198 69 24.3 59 22.0 76 23.9 204 23.4 248 24.9 
    ≥27.199 75 26.4 87 32.5 101 31.8 263 30.2 251 25.2 
Total lifetime recreational physical activity           
    0 (least active) 112 39.4 112 41.8 137 43.1 361 41.5 387 38.9 
    1 (most active) 172 60.6 156 58.2 181 56.9 509 58.5 609 61.1 
Vigorouslifetime recreational physical activity           
    0 (least active) 197 69.4 209 78.0 245 77.0 651 74.8 708 71.1 
    1 (most active) 87 30.4 59 22.0 73 23.0 219 25.2 288 28.9 
 Cases
 
 Proximal ColonCancer
 
Distal ColonCancer
 
RectalCancer
 
All Colorectal Cancer
 
Controls
 
 No. No. No. No. No. 
Sex           
    Male 155 54.6 165 61.6 216 67.9 536 61.6 594 59.6 
    Female 129 45.4 103 38.4 102 32.1 334 38.4 402 40.4 
Age, years           
    40–49 12 4.2 15 5.6 18 5.7 45 5.2 70 7.0 
    50–59 48 16.9 80 29.8 83 26.1 211 24.2 219 22.0 
    60–69 106 37.3 94 35.1 120 37.7 320 36.8 355 35.6 
    70–79 118 41.6 79 29.5 97 30.5 294 33.8 352 35.3 
Socioeconomic status           
    1 (most disadvantaged) 44 15.5 54 20.2 57 17.9 155 17.8 168 16.9 
    2 51 18.0 49 18.3 78 24.5 178 20.5 192 19.3 
    3 63 22.2 41 15.3 70 22.0 174 20.0 231 23.2 
    4 67 23.6 64 23.9 60 18.9 191 22.0 195 19.6 
    5 (least disadvantaged) 59 20.8 60 22.4 53 16.7 172 19.8 210 21.1 
Smoking, pack-years           
    0–0.9 132 46.5 116 43.3 135 42.4 383 44.0 493 49.5 
    1–19.9 62 21.8 66 24.6 79 24.8 207 23.8 236 23.7 
    ≥20 90 31.7 86 32.1 104 32.7 280 32.2 267 26.8 
Body mass index at age 20 years, kg/m2          
    <25 225 84.0 182 73.7 223 76.6 630 78.2 767 83.0 
    25–29.9 39 14.6 60 24.3 56 19.2 155 19.2 136 14.7 
    ≥30 1.5 2.0 12 4.1 21 2.6 21 2.3 
Body mass index at age 40 years, kg/m2          
    <25 143 54.0 123 48.8 158 53.7 425 52.3 538 58.4 
    25–29.9 95 35.8 89 35.3 92 31.3 276 34.0 296 32.1 
    ≥30 27 10.2 40 15.9 44 15.0 111 13.7 87 9.4 
Diabetes and/or high blood sugar           
    Neither 225 79.2 211 78.7 259 81.4 695 79.9 851 85.4 
    High blood sugar 19 6.7 16 6.0 14 4.4 49 5.6 52 5.2 
    Diabetes 40 14.1 41 15.3 45 14.2 126 14.5 93 9.3 
Energy intake from food 10 years ago, kJ/day           
    0–6,301 70 24.6 60 22.4 55 17.3 185 21.3 245 24.6 
    6,302–8,074 61 21.5 64 23.9 77 24.2 202 23.2 250 25.1 
    8,075–10,406 83 29.2 64 23.9 94 29.6 241 27.7 252 25.3 
    ≥10,407 70 24.6 80 29.8 92 28.9 242 27.8 249 25.0 
Alcohol consumption 10 years ago, g/day           
    0–1.1292 71 25.0 56 20.9 72 22.6 199 22.9 248 24.9 
    1.1293–9.514 69 24.3 66 24.6 69 21.7 204 23.4 249 25.0 
    9.515–27.198 69 24.3 59 22.0 76 23.9 204 23.4 248 24.9 
    ≥27.199 75 26.4 87 32.5 101 31.8 263 30.2 251 25.2 
Total lifetime recreational physical activity           
    0 (least active) 112 39.4 112 41.8 137 43.1 361 41.5 387 38.9 
    1 (most active) 172 60.6 156 58.2 181 56.9 509 58.5 609 61.1 
Vigorouslifetime recreational physical activity           
    0 (least active) 197 69.4 209 78.0 245 77.0 651 74.8 708 71.1 
    1 (most active) 87 30.4 59 22.0 73 23.0 219 25.2 288 28.9 
a

Total case and control numbers do not add up because of missing data.

The controls were selected from the electoral roll, but we did not exclude the cases who were not on the electoral roll. We therefore conducted a sensitivity analysis, in which we repeated all analyses without cases who were not on the electoral roll (n = 54) and compared the effect sizes with those from the original models.

There was no statistically significant interaction by sex in any of the analyses, and the associations observed were similar in both sexes, so all results are presented for males and females combined.

RESULTS

The demographic and various dietary and lifestyle characteristics of the participants by cancer status are shown in Table 1. There was a higher proportion of females among the proximal colon cancer cases than the distal colon or rectal cancers. The age distribution among participants with proximal colon cancer was significantly different from that of participants with distal colon cancer or rectal cancer, with 79% of proximal colon cancer aged over 60 years compared with 65% of distal colon cancer cases and 68% of rectal cancer cases. Participants from more disadvantaged areas were more likely to have rectal cancer than distal colon cancer. Compared with all cases, controls were less likely to be ≥20 pack-year smokers and were also less likely to have diabetes. Proximal colon cancer cases were less likely than distal colon or rectal cancer cases to be overweight or obese at 20 years of age.

Compared with participants who spent the most time in a light job, participants who spent the most time in a sedentary occupation had 2 times the risk of distal colon cancer (adjusted odds ratio (AOR) = 2.07, 95% confidence interval (CI): 1.25, 3.44) (Table 2). Participants who spent 10 or more years in sedentary work had almost twice the risk of distal colon cancer (AOR = 1.94, 95% CI: 1.28, 2.93) and nearly 1.5 times the risk of rectal cancer (AOR = 1.44, 95% CI: 0.96, 2.18) compared with participants who never held a sedentary occupation (Table 2). When the interaction term between total recreational physical activity and sedentary work was included in the model (Table 3), it was not statistically significant (P = 0.29). The risk of distal colon cancer and rectal cancer increased with increasing duration of sedentary work among both the most recreationally active and least recreationally active participants (Table 3). The risk of proximal colon cancer was not linked with sedentary work in the whole population (Table 2), the most recreationally active participants, or the least recreationally active participants (Table 3). Neither vigorous recreational physical activity nor body mass index at age 40 years modified the association between years in sedentary work and colorectal cancer (data not shown).

Table 2.

Association Among Lifetime Occupational Activity, Years in Sedentary Work, and Site-specific Colorectal Cancer Risk, the Western Australian Bowel Health Study, 2005–2007

 Controls, no. Proximal Colon Cancer
 
Distal Colon Cancer
 
Rectal Cancer
 
 No. ORa 95% CI AORb 95% CI No. ORa 95% CI AORb 95% CI No. ORa 95% CI AORb 95% CI 
Lifetime occupational activity                 
    Sedentary 57 18 1.13 0.64, 2.01 1.12 0.63, 2.00 31 2.07 1.26, 3.38 2.07 1.25, 3.44 23 1.27 0.75, 2.16 1.34 0.78, 2.30 
    Light 563 178 1.00 Referent 1.00 Referent 146 1.00 Referent 1.00 Referent 161 1.00 Referent 1.00 Referent 
    Medium 264 68 0.84 0.60, 1.17 0.81 0.58, 1.15 68 0.99 0.70, 1.40 0.92 0.64, 1.31 97 1.12 0.82, 1.54 1.03 0.75, 1.42 
    Heavy/very heavy 112 20 0.56 0.33, 0.96 0.55 0.32, 0.95 23 0.83 0.50, 1.38 0.75 0.44, 1.27 37 0.98 0.63, 1.52 0.88 0.56, 1.38 
Years in sedentary work                 
    0 805 232 1.00 Referent 1.00 Referent 196 1.00 Referent 1.00 Referent 251 1.00 Referent 1.00 Referent 
    >0–<10 95 24 0.93 0.58, 1.49 0.89 0.55, 1.44 27 1.15 0.73, 1.82 1.21 0.76, 1.95 26 0.89 0.56, 1.41 0.93 0.58, 1.49 
    ≥10 years 96 28 1.14 0.72, 1.79 1.11c 0.69, 1.76 45 1.87 1.26, 2.78 1.94c 1.28, 2.93 41 1.29 0.87, 1.93 1.44 0.96, 2.18 
        Ptrend   0.70 0. 84d  0.003 0.002d  0.33 0.14 
 Controls, no. Proximal Colon Cancer
 
Distal Colon Cancer
 
Rectal Cancer
 
 No. ORa 95% CI AORb 95% CI No. ORa 95% CI AORb 95% CI No. ORa 95% CI AORb 95% CI 
Lifetime occupational activity                 
    Sedentary 57 18 1.13 0.64, 2.01 1.12 0.63, 2.00 31 2.07 1.26, 3.38 2.07 1.25, 3.44 23 1.27 0.75, 2.16 1.34 0.78, 2.30 
    Light 563 178 1.00 Referent 1.00 Referent 146 1.00 Referent 1.00 Referent 161 1.00 Referent 1.00 Referent 
    Medium 264 68 0.84 0.60, 1.17 0.81 0.58, 1.15 68 0.99 0.70, 1.40 0.92 0.64, 1.31 97 1.12 0.82, 1.54 1.03 0.75, 1.42 
    Heavy/very heavy 112 20 0.56 0.33, 0.96 0.55 0.32, 0.95 23 0.83 0.50, 1.38 0.75 0.44, 1.27 37 0.98 0.63, 1.52 0.88 0.56, 1.38 
Years in sedentary work                 
    0 805 232 1.00 Referent 1.00 Referent 196 1.00 Referent 1.00 Referent 251 1.00 Referent 1.00 Referent 
    >0–<10 95 24 0.93 0.58, 1.49 0.89 0.55, 1.44 27 1.15 0.73, 1.82 1.21 0.76, 1.95 26 0.89 0.56, 1.41 0.93 0.58, 1.49 
    ≥10 years 96 28 1.14 0.72, 1.79 1.11c 0.69, 1.76 45 1.87 1.26, 2.78 1.94c 1.28, 2.93 41 1.29 0.87, 1.93 1.44 0.96, 2.18 
        Ptrend   0.70 0. 84d  0.003 0.002d  0.33 0.14 

Abbreviations: AOR, adjusted odds ratio; CI, confidence interval; OR, odds ratio.

a

Adjusted for age group and sex.

b

Adjusted for age group, sex, lifetime recreational physical activity level, cigarette smoking (pack-year tertiles), diabetes, educational level, energy intake from food, alcohol intake, body mass index at age 20 years, body mass index at age 40 years, and socioeconomic status. The AOR in the “years in sedentary work” analysis is additionally adjusted for years in a heavy or very heavy occupation.

c

P < 0.05 for the difference between the proximal colon and the distal colon.

d

P < 0.05 for the difference between the proximal colon trend and the distal colon trend.

Table 3.

Effects of Lifetime Recreational Physical Activity and Body Mass Index on the Association Between Years in Sedentary Work and Site-specific Colorectal Cancer Risk, the Western Australian Bowel Health Study, 2005–2007

Years in Sedentary Work Proximal Colon Cancer by Lifetime Recreational Physical Activity
 
Distal Colon Cancer by Lifetime Recreational Physical Activity
 
Rectal Cancer by Lifetime Recreational Physical Activity
 
Groups 0 and 1 (Less Active)
 
Groups 2 and 3 (More Active)
 
Groups 0 and 1 (Less Active)
 
Groups 2 and 3 (More Active)
 
Groups 0 and 1 (Less Active)
 
Groups 2 and 3 (More Active)
 
AORa 95% CI AORa 95% CI AORa 95% CI AORa 95% CI AORa 95% CI AORa 95% CI 
    0 1.13 0.83, 1.53 1.00 Referent 1.25 0.90, 1.73 1.00 Referent 1.20 0.89, 1.62 1.00 Referent 
    >0–<10 0.56 0.21, 1.50 1.13 0.64, 1.97 1.35 0.63, 2.91 1.30 0.72, 2.35 1.22 0.57, 2.59 0.88 0.49, 1.59 
    ≥10 0.68 0.29, 1.58 1.49 0.86, 2.60 1.42 0.71, 2.82 2.60 1.57, 4.31 1.49 0.79, 2.79 1.57 0.93, 2.65 
Years in Sedentary Work Proximal Colon Cancer by Lifetime Recreational Physical Activity
 
Distal Colon Cancer by Lifetime Recreational Physical Activity
 
Rectal Cancer by Lifetime Recreational Physical Activity
 
Groups 0 and 1 (Less Active)
 
Groups 2 and 3 (More Active)
 
Groups 0 and 1 (Less Active)
 
Groups 2 and 3 (More Active)
 
Groups 0 and 1 (Less Active)
 
Groups 2 and 3 (More Active)
 
AORa 95% CI AORa 95% CI AORa 95% CI AORa 95% CI AORa 95% CI AORa 95% CI 
    0 1.13 0.83, 1.53 1.00 Referent 1.25 0.90, 1.73 1.00 Referent 1.20 0.89, 1.62 1.00 Referent 
    >0–<10 0.56 0.21, 1.50 1.13 0.64, 1.97 1.35 0.63, 2.91 1.30 0.72, 2.35 1.22 0.57, 2.59 0.88 0.49, 1.59 
    ≥10 0.68 0.29, 1.58 1.49 0.86, 2.60 1.42 0.71, 2.82 2.60 1.57, 4.31 1.49 0.79, 2.79 1.57 0.93, 2.65 

Abbreviations: AOR, adjusted odds ratio; CI, confidence interval.

a

Adjusted for age group, sex, lifetime recreational physical activity level, cigarette smoking (pack-year tertiles), diabetes, educational level, energy intake from food, alcohol intake, body mass index at age 20 years, body mass index at age 40 years, socioeconomic status, and years in a heavy or very heavy occupation.

Participants who spent the most time in jobs requiring heavy or very heavy activity had a significantly reduced risk of proximal colon cancer (AOR = 0.56, 95% CI: 0.32, 0.96) compared with participants who spent the most time in a light job (Table 2).

There were no meaningful changes to the effect estimates in the above analyses when the cases that were not on the electoral roll were excluded.

DISCUSSION

In this study, we found that participants who spent the most time in sedentary work had a risk of distal colon cancer that was 2 times higher than those who spent the most time in a job requiring light activity. Similarly, participants who spent 10 or more years in sedentary work had almost twice the risk of distal colon cancer and almost 1.5 times the risk of rectal cancer, of those who did not do any sedentary work. Proximal colon cancer risk was not associated with sedentary work, although the risk was significantly reduced among participants who spent the most time in jobs requiring heavy or very heavy activity.

Previous research that has investigated the association between long-term exposure to a low level of work-related activity and colorectal cancer has generally found an increased risk (17–25), although these studies have in the main compared light-intensity work with more physically active work. The 4 studies that have investigated the effect of time spent sitting, or a proxy measure, on colorectal cancer risk have all revealed an increased risk (26–29). Howard et al. (27) found that participants who spent 9 or more hours per day sitting had a 25% greater risk of colon cancer than those who spent less than 3 hours per day sitting. Whittemore et al. (29) found that, compared with Chinese-American men who spent less than 5 hours sitting, those who spent 5–9 hours sitting had 2.4 times the risk of colorectal cancer, and participants who spent 10 or more hours sitting had 3.9 times the risk of colorectal cancer. Colbert et al. (26) found that participants who reported that their occupation in the past year involved mainly sitting had a 67% greater risk of colon cancer and a 41% increased risk of rectal cancer than those who reported “walking quite a lot” in their occupation, although neither of these associations reached statistical significance, and the authors did not find an association between sedentary behavior in leisure time and colon or rectal cancer risk. Finally, Steindorf et al. (28) found that those who spent more than 2 hours a day watching television more than doubled their risk of colorectal cancer compared with those who spent less than 1.14 hours a day watching television.

The positive association found between sedentary work and colorectal cancer in this study was independent of recreational physical activity and was seen among the most recreationally active participants. Indeed, even a high level of vigorous recreational physical activity did not modify the effect of sedentary work. This finding fits with the “Active Couch Potato” phenomenon, in which high amounts of sedentary behavior and physical activity coexist, and lends support to the notion that sedentary behavior affects disease risk independently of physical activity (1).

There are several plausible biologic mechanisms through which sedentary work and sedentary behavior in general may increase the risk of colorectal cancer (refer to reference 30 for a review of the evidence and proposed mechanisms linking sedentary behavior and cancer). Sedentary behavior has been shown to increase blood glucose levels and to decrease insulin sensitivity, independently of physical activity (31). Increased blood glucose and decreased insulin resistance are both thought to promote colorectal cancer carcinogenesis (32, 33). Sedentary behavior has also been linked to an increased risk of diabetes and obesity (again independently of exercise level) (34), both of which are established risk factors for colorectal cancer (11). Bed-rest studies and experiments on mice provide some evidence that sedentary behavior is associated with markers of inflammation and mitochondrial function (30). Other proposed mechanisms through which sedentary behavior may increase the risk of colorectal cancer include increasing levels of proinflammatory factors, decreasing levels of antiinflammatory factors, and decreasing levels of vitamin D (30). Although several of these mechanisms are the same as those proposed for the inverse relation between physical activity and colorectal cancer (35), many of them appear to act independently of physical activity. It has also been proposed that the loss of local contractile activity caused by sitting results in unique gene expressions that may be involved in disease etiology (36). Sitting has been shown to suppress the regulation of skeletal muscle lipoprotein lipase, which plays an important role in lipid metabolism and may have a positive effect on diet-induced adiposity and insulin resistance (36).

Our finding that sedentary work is associated with the risk of distal colon cancer but not proximal colon cancer adds to the evidence suggesting that lifestyle factors may play a larger role in distal colon carcinogenesis than in proximal colon carcinogenesis (37). Fruit and vegetable intake (38), calcium intake (39), and meat consumption (40) all appear to be more strongly associated with distal colon cancer than proximal colon cancer, although the literature concerning physical activity and distal and proximal colon cancer is inconsistent (41). There is no clear mechanistic explanation, however, for why sedentary behavior would increase the risk of distal colon and rectal cancers but not proximal colon cancer. Obesity is one possibility: An increased body mass index may have a greater effect on distal colon cancer (37), and there is evidence that obesity has a greater effect on microsatellite-stable tumors (42), which are more likely to occur in the distal colon than the proximal colon (5). There is some evidence that vitamin D has a stronger protective effect on distal colon and rectal cancers than proximal colon cancer (43); however, there is little evidence that insulin affects proximal and distal colon cancers differently (44–47), and antiinflammatories may have a more pronounced protective effect on proximal tumors than distal tumors (48, 49). Further mechanistic evidence is needed to understand why the association between sedentary behavior (and lifestyle factors in general) and colon cancer may differ by subsite.

The findings of this study have occupational health implications, especially given that advances in technology have led to increasing amounts of sedentary behavior at work and in other settings. Findings from the National Health and Nutrition Examination Survey suggest that the average American adult spends 55% (7.7 hours) of his/her waking time sedentary (50). This percentage increases to more than 60% (≥8 hours) among adults aged 60 years or more. Sedentary work is unlikely to be eliminated from modern life. However, strategies to minimize the amount of time spent sitting, especially long bouts of sitting, have been suggested (1). For example, it has been suggested that new occupational regulations could be introduced to break up prolonged sedentary time by taking regular nonsitting “breaks” (1), which have been shown to have a beneficial effect on metabolic biomarkers (51). A recent systematic review, however, found that there is a lack of evidence concerning the effectiveness of workplace interventions for reducing sitting and called for more research to develop effective programs that specifically target sitting (52). It has also been suggested that future guidelines on physical activity and health should include recommendations regarding sedentary behavior (53); this will become especially important if, as we found in our study, sedentary behavior has a detrimental effect on disease risk even among individuals who are physically active in their leisure time. Some countries, including Australia (54), Canada (55), and the United Kingdom, have or are developing sedentary behavior recommendations for early years (<5 years of age), children, and/or adults.

This study had several limitations. Selection bias is a possible explanation for the results, especially given the low response fraction among the controls (46.5%). Basing occupational activity level on job title is not an ideal assessment and may have led to nondifferential exposure misclassification. Recreational activity was determined by self-report of activities; however, this approach to measuring physical activity has been found to be reliable and valid (56). We had data on only sedentary behavior in 1 domain (occupation) and were not able to take into account sedentary behavior during leisure time, at home, or during transport. This study also had several strengths. It was a population-based study with histopathologically confirmed cases, and we were able to control for many potential confounders including lifetime recreational physical activity. We were also able to investigate whether recreational physical activity modified the effect of sedentary work on colorectal cancer risk.

Aside from the increased risk of distal colon cancer and rectal cancer seen among those who worked in a sedentary occupation for 10 or more years in this study, an increasing level of sedentary behavior has been linked to an increased risk of several other chronic diseases, as well as increased mortality (57, 58). Sedentary behavior appears to be a novel and important risk factor for many chronic diseases (59).

Abbreviations

    Abbreviations
  • AOR

    adjusted odds ratio

  • CI

    confidence interval

  • MET

    metabolic equivalent

Author affiliations: School of Population Health, the University of Western Australia, Perth, Australia (Terry Boyle, Jane Heyworth, Fiona Bull); and Western Australian Institute for Medical Research, the University of Western Australia, Perth, Australia (Terry Boyle, Lin Fritschi).

This work was supported by the Australian National Health and Medical Research Council (grant 353568). T. B. is supported by an Australian Postgraduate Award from the University of Western Australia and a scholarship from the Lions Cancer Institute of Western Australia. L. F. is supported by an Australian National Health and Medical Research Council fellowship.

The authors thank Barry Iacopetta, Cameron Platell, Kieran McCaul, David Crawford, Cassandra Clayforth, Jenny Landrigan, Jen Girschik, Clare Tran, Beatriz Cuesta-Briand, and Anna Timperio for their contributions to this study.

Conflict of interest: none declared.

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