We read with great interest the meta-analysis by Cong et al (2014) recently published in British Journal of Cancer. As the authors acknowledge, sedentary behaviour is distinct from the lack of moderate- to vigorous-intensity physical activity. As the first quantitative review of the studies examining associations of sedentary behaviour on colon and rectal cancer risk, this article makes a timely and novel contribution to the literature. However, we are concerned that the combined risk estimates generated by this meta-analysis may not accurately reflect the effect that can be attributed to sedentary behaviour.
Many of the risk estimates included in the meta-analysis are from studies that investigated the association between occupational physical activity and the risk of colon and/or rectal cancers. As noted by Yates et al (2011), the ordinal scales commonly used to assess occupational physical activity (e.g., ‘sedentary’, ‘moderate’, ‘high’) are not necessarily ordinal scales of sedentary behaviour. As high levels of sedentary behaviour can co-exist with high levels of physical activity, even within specific occupations, using these estimates of occupational physical activity to infer sedentary behaviour is likely to introduce substantial misclassification bias.
A related issue is the inclusion of studies that have classified sedentary behaviour based on job title. While we do not believe it is necessarily wrong to include estimates of sedentary behaviour that are job title based, it is important to note that this method does not take into account within-job variation, seasonal changes or changes in job requirements over time (LaPorte et al, 1985), and may not reflect the actual activities performed on the job (Ainsworth et al, 1999). We would recommend that in future meta-analyses and reviews, these studies be given a lower exposure assessment quality rating than studies using self-reported or objectively assessed measures of sedentary behaviour. In addition, we suggest that subgroup analyses are conducted to investigate whether the results of studies relying on job title-based measures of sedentary behaviour differ from the results of studies with self-reported or objectively assessed measures of sedentary behaviour.
Another issue that arises when using ordinal scales of occupational physical activity (job title-based or self-reported) in a sedentary behaviour context is the selection of the appropriate referent category. The most suitable referent group to compare jobs with high amounts of sedentary behaviour with are jobs that involve ‘mostly standing’ or ‘light’ activity. Within the meta-analysis performed by Cong et al (2014), there are several instances where the authors selected the most physically active category as the referent group (Garabrant et al, 1984; Fraser and Pearce, 1993; Weiderpass et al, 2003; Moradi et al, 2008). The relative risks generated by comparing the sedentary category with the most physically active will not solely reflect the effect of sedentary behaviour on colorectal cancer risk; part of the risk estimate will be attributed to the (inverse) of the risk reduction associated with physical activity. A similar error was made with the inclusion of data from two studies that compared recreational sedentary behaviour with recreational physical activity (Thune and Lund, 1996; Colbert et al, 2001).
There are two final points that we would like to raise. First, the risk estimates included in the meta-analysis from the Campbell et al (2013) study pertain to colorectal cancer-specific survival rather than colorectal cancer risk. Second, there are three studies for which the authors have included risk estimates for two different measures of sedentary behaviour (e.g., recreational and occupational sedentary behaviour) in the primary meta-analysis (Thune and Lund, 1996; Colbert et al, 2001; Howard et al, 2008). This is effectively including the same study population twice, so these studies are arguably contributing more weight to the overall effect size than appropriate.
Despite the concerns raised, we acknowledge that the meta-analysis presented by Cong et al (2014) has drawn attention to the potential role of sedentary behaviour in colon and rectal cancer aetiology. Clearly further studies, using well-designed and tested measures of sedentary behaviour, are required in this field.
References
Ainsworth BE, Richardson MT, Jacobs DR Jr, Leon AS, Sternfield B (1999) Accuracy of recall of occupational physical activity by questionnaire. J Clin Epidemiol 52: 219–227.
Campbell PT, Patel AV, Newton CC, Jacobs EJ, Gapstur SM (2013) Associations of recreational physical activity and leisure time spent sitting with colorectal cancer survival. J Clin Oncol 31: 876–885.
Colbert LH, Hartman TJ, Malila N, Limburg PJ, Pietinen P, Virtamo J, Taylor PR, Albanes D (2001) Physical activity in relation to cancer of the colon and rectum in a cohort of male smokers. Cancer Epidemiol Biomarkers Prev 10: 265–268.
Cong YJ, Gan Y, Sun HL, Deng J, Cao SY, Xu X, Lu ZX (2014) Association of sedentary behaviour with colon and rectal cancer: a meta-analysis of observational studies. Br J Cancer 110: 817–826.
Fraser G, Pearce N (1993) Occupational physical activity and risk of cancer of the colon and rectum in New Zealand males. Cancer Causes Control 4: 45–50.
Garabrant DH, Peters JM, Mack TM, Bernstein L (1984) Job activity and colon cancer risk. Am J Epidemiol 119: 1005–1014.
Howard RA, Freedman DM, Park Y, Hollenbeck A, Schatzkin A, Leitzmann MF (2008) Physical activity, sedentary behaviour, and the risk of colon and rectal cancer in the NIH-AARP Diet and Health Study. Cancer Causes Control 19: 939–953.
LaPorte RE, Montoye HJ, Caspersen CJ (1985) Assessment of physical activity in epidemiologic research: problems and prospects. Public Health Rep 100: 131–146.
Moradi T, Gridley G, Bjork J, Dosemeci M, Ji BT, Berkel HJ, Lemeshow S (2008) Occupational physical activity and risk for cancer of the colon and rectum in Sweden among men and women by anatomic subsite. Eur J Cancer Prev 17: 201–208.
Thune I, Lund E (1996) Physical activity and risk of colorectal cancer in men and women. Br J Cancer 73: 1134–1140.
Weiderpass E, Vainio H, Kauppinen T, Vasama-Neuvonen K, Partanen T, Pukkala E (2003) Occupational exposures and gastrointestinal cancers among Finnish women. J Occup Environ Med 45: 305–315.
Yates T, Wilmot EG, Davies MJ, Gorely T, Edwardson C, Biddle S, Khunti K (2011) Sedentary behaviour: what’s in a definition? Am J Prev Med 40: e33–e34.
Author information
Authors and Affiliations
Corresponding author
Ethics declarations
Competing interests
The authors declare no conflict of interest.
Rights and permissions
This work is licensed under the Creative Commons Attribution-NonCommercial-Share Alike 3.0 Unported License. To view a copy of this license, visit http://creativecommons.org/licenses/by-nc-sa/3.0/
About this article
Cite this article
Lynch, B., Boyle, T. Distinguishing sedentary from inactive: implications for meta-analyses. Br J Cancer 111, 2202–2203 (2014). https://doi.org/10.1038/bjc.2014.106
Published:
Issue Date:
DOI: https://doi.org/10.1038/bjc.2014.106
This article is cited by
-
Reply: Comment on ‘Association of sedentary behaviour with colon and rectal cancer: a meta-analysis of observational studies’
British Journal of Cancer (2014)