July 2, 2026

expert reaction to study looking at prolonged sedentary behaviour and cancer risk

A study published in PLOS Medicine looks at sedentary behaviour and cancer incidence and mortality. 

Prof Kevin McConway, Emeritus Professor of Applied Statistics, Open University, said:

“This is an interesting study, but I think its findings are complicated to interpret.  The researchers themselves point out difficulties of interpretation, and of knowing what causes what, in their research paper, and these issues are also mentioned in the press release.  The researchers suggest that further research, possibly including clinical trials, is needed to go forward on these, points, and I agree.

“But I don’t agree with some of the researchers’ conclusions from their study, or at any rate I think that they just aren’t as clear as the researchers’ paper might indicate.

“Previous work had indicated that people who spent more time being sedentary, such as sitting watching TV, had a higher cancer risk than those who spent less time sedentary.

“This new study concludes that the extent of this increase in risk depends on whether the sedentary time occurs in relatively prolonged sessions, or is split up more, and indeed that it is the amount of sedentary time in long sessions that is correlated with cancer risk, rather than the sedentary time that is more broken up by physical activity.

“I agree that that is one possible explanation of the findings.  But I think there are other explanations too, that don’t necessarily support the idea of overwhelming importance of how much sedentary sessions were broken up by other activity.

“My doubts on this arise because, in my view, the first set of statistical models that the researchers used don’t have a clear enough interpretation, particularly in how they modelled sedentary behaviour that was interrupted by other kinds of activity.

“The researchers also carried out a second set of statistical analyses, so-called isotemporal substitution models, where they used a theoretical way of estimating the effect on cancer risks if people replaced one daily hour of one type of sedentary behaviour (prolonged or interrupted) either by the other type of sedentary behaviour, or by physical activity.

“They can only do this using statistical modelling, because they have no actual data on people actually changing their physical activity patterns.  That’s because each participant only had their activity measured once (over a seven-day period).

“I think that the results from this second set of analyses lead to different conclusions on the relative importance of interruptions to sedentary activity to cancer risk, than do the researchers’ interpretations of their first set of analyses.

“And that’s why I say that the researchers’ conclusions just aren’t clear enough to be of any use without a lot of further research.”

Prof Stephen Burgess, Statistician at the MRC Biostatistics Unit, University of Cambridge, said:

“This is a statistical analysis of an observational study, so the results represent the output from a modelling exercise rather than an experiment that changes people’s behaviour.  It tells us that people who spend more time sitting have higher cancer risk, rather than what would happen if someone spent less time sitting.  In other words, it can tell us about correlation rather than causation.  A correlation like this is only indicative of causation if all other differences between those who sit for longer and shorter have been accounted for.  While the authors have made a commendable effort to do this, it is impossible to be certain that all relevant factors have been addressed.  It is interesting that correlations with cancer risk are in different directions for prolonged sedentary behaviour versus interrupted sedentary behaviour.  However, these correlations may reflect differences in social factors such as job type and income rather than being attributable to sedentary behaviour in a causal sense.”

Rachel Richardson, Methods Support Unit Manager, The Cochrane Collaboration, said:

“This study of sedentary behaviour and the incidence of cancer and cancer mortality has several strengths.  The main strength is the use of a reliable instrument (an accelerometer) to measure people’s sedentary and active behaviour.  Studies such as these often use self-reported activity, which is an unreliable way of measuring behaviour.  Another strength is the use of electronic medical records to obtain information about cancer incidence and mortality.

“However there are a number of key limitations, several of which are acknowledged by the authors.

Representativeness of people in the study

“The people in this study are not representative of the general UK population.  They are older (average age 56 years) and likely to be more health-conscious than the majority, for example, they have lower rates of obesity and lower rates of smoking.

Timing of measurements

“As is always the case when using registry data, the authors do not have control over when all the relevant variables are measured.  It appears that baseline measurements were taken when participants were recruited into the UK Biobank study (2006 to 2010): this includes information on BMI, diet and smoking.  However, activity data from the accelerometers were measured between 2013 and 2015, and so it is likely that the baseline data were no longer representative.  This matters because the authors adjusted their results to account for baseline factors.  Another concern is that the accelerometers were only worn for seven days: such a short period may or may not be representative of someone’s general activity levels.  A final concern is that people have not been followed up for a long time after the accelerometer measurements: data on mortality and cancer incidence come from 2022.

Statistics

“Although the effect estimates seem to be striking (9% increase in cancer mortality for an additional hour of prolonged sedentary behaviour and a reduction of 12% if an hour of prolonged sedentary behaviour is replaced by an hour of light physical activity), it is important to also consider the margins of error around these estimates (the confidence intervals).  The effect estimate for the increase in cancer mortality is HR 1.09 (95% confidence interval 1.06 to 1.11).  This means that the increase with prolonged sedentary behaviour could be as low as 6%.  The effect estimate for the reduction when substituting prolonged SB with light physical activity is HR 0.88 (95% confidence interval 0.79 to 0.99).  This means that the reduction could be as low as 1%.

Observational data

“A final key limitation is that this is an observational study (as opposed to a randomised controlled trial).  This means that we can’t be sure that the outcomes (cancer incidence and mortality) were a result of the exposure (amount and type of physical activity).  Although the authors have accounted for several factors which may influence the outcome (e.g. diet), there may be other factors involved (residual confounding).”

‘Accelerometry-measured prolonged and interrupted sedentary behavior and cancer incidence and mortality: A cohort study of 91,292 UK Biobank participants’ by Ziyi Zhou et al. was published in PLOS Medicine at 19:00 UK time on Thursday 2 July 2026. 

DOI: 10.1371/journal.pmed.1004767

Declared interests

Prof Kevin McConway: “I have no conflicts of interest to declare.”

Prof Stephen Burgess: “I have no relevant conflict of interest to declare.”

Rachel Richardson: “No interests to declare.”