- Prolonged sedentary behavior was linked with significantly increased cancer mortality risk.
- Interrupting sedentary behavior with brief periods of light, moderate, or vigorous physical was associated with reduced cancer risks.
- The findings suggest that not only the amount of sedentary time but how sedentary time is accumulated may influence cancer risk.
Prolonged, uninterrupted sedentary behavior was associated with a significantly increased likelihood of dying of cancer, a study of more than 90,000 people showed.
Every additional hour of prolonged sedentary behavior per day was linked with a 10% higher hazard of cancer mortality. Replacing an hour of sedentary behavior daily with light physical activity or with 30 minutes of moderate physical activity was associated with reduced cancer mortality risk — which was 22% lower with an additional 5 minutes of vigorous physical activity.
Similar associations were observed for several specific cancer sites as well, including lung, breast, and oral cancers, and leukemia or non-Hodgkin’s lymphoma.
Using objective measurements of physical activity, the study adds to previous evidence by more specifically quantifying the risks of sedentary behavior and the benefits of physical activity, reported Frederick Ho, PhD, of the University of Glasgow in Scotland, and colleagues in PLoS Medicine.
“These findings suggest that not only the total amount of sedentary time, but also how sedentary time is accumulated, may be important for cancer risk,” the authors stated in a summary of the findings. “Reducing long, uninterrupted periods of sedentary behavior and replacing them with PA [physical activity], even those with light intensity, may be a practical target for future interventions.”
Strengths, Limitations
The study’s methodology lends credence to the findings regarding relationships between sedentary behavior, physical activity, and cancer risk, according to Rachel Richardson, of Cochrane in York, England.
“The main strength is the use of a reliable instrument [accelerometer] to measure people’s sedentary and active behavior,” Richardson said in a statement on the Science Media Centre. “Studies such as these often use self-reported activity, which is an unreliable way of measuring behavior. Another strength is the use of electronic medical records to obtain information about cancer incidence and mortality.”
Nonetheless, the study has limitations that should be kept in mind when evaluating the results, beginning with the patient population.
“The people in this study are not representative of the general U.K. population,” said Richardson. “They are older and likely to be more health conscious than the majority. For example, they have lower rates of obesity and lower rates of smoking.”
Inherent to studies relying on registry data, investigators could not control for all relevant variables. Additionally, study participants wore the accelerometers for only a week, which might not be representative of a person’s general activity levels, Richardson continued. Finally, the study had a brief follow-up period after accelerometer measurements.
“A final key limitation is that this is an observational study,” said Richardson. “This means that we can’t be sure that the outcomes were a result of the exposure [amount and type of physical activity]. Although the authors have accounted for several factors which may influence the outcomes [such as diet] there may be other factors involved.”
Sedentary behavior accounts for a majority of waking time for children and adults. Multiple studies have shown that excessive sedentary behavior, such as daily prolonged sitting, increases the risk of cardiovascular disease, type 2 diabetes, obesity, certain cancers, and all-cause mortality, Ho and colleagues noted in their introduction. The adverse effects of sedentary behavior are most prominent in people with low levels of physical activity. The association may result from sedentary behavior replacing physical activity or as an independent effect, the authors added.
More recently, evidence has suggested that how people accumulate sedentary behavior is an important contributor to risk beyond the total amount of sedentary behavior. Prolonged uninterrupted periods of sedentary behavior may pose the greatest hazard, and several health agencies have recommended activity breaks as a potential strategy to reduce health risks associated with sedentary behavior.
Study Details, Key Findings
To address existing gaps in knowledge, Ho and colleagues conducted a prospective cohort study to examine whether prolonged and interrupted sedentary behavior have different associations with cancer risk. The study evaluated associations of prolonged and interrupted sedentary behavior with cancer incidence, cancer mortality, obesity-related cancers, cancers related to type 2 diabetes, and 23 site-specific cancers. They also examined the effect of replacing sedentary behavior with other behaviors, including physical activity.
Data analysis involved 91,292 participants in the U.K. Biobank prospective cohort study. None of the participants had a history of cancer at baseline, and all of them had a week of physical activity data from an accelerometer, which measured duration and intensity of physical activity. The investigators classified physical activity as low (walking at a slow speed), moderate (walking at a moderate speed), and vigorous (fast walking and high-energy activities). They defined prolonged sedentary behavior as an episode of at least 30 minutes when at least 90% of the time was sedentary.
Study participants had a mean age of 56, and women accounted for 56% of the total. Increased levels of sedentary behavior were associated with increased consumption of red and processed meats, whereas increasing levels of interrupted sedentary behavior were associated with lower consumption of red meat and increased consumption of fruits and vegetables.
The data showed that 12,392 participants developed cancer, and 1,726 died of cancer. Both cancer incidence and mortality increased with the daily amount of sedentary behavior. The incidence of obesity-related and diabetes-related cancers also increased with duration of daily sedentary behavior.
In a statistical model adjusted for age, sex, and ethnicity, every additional hour of prolonged sedentary behavior was associated with a cancer mortality hazard of 1.10 (95% CI 1.07-1.12), whereas interrupted sedentary behavior was protective (HR 0.81, 95% CI 0.77-0.85). A significant association also was seen with cancer incidence (HR 1.03, 95% CI 1.02-1.04 and HR 0.93, 95% CI 0.92-0.95). A model adjusted for socioeconomic and multiple lifestyle factors yielded similar results, as did both models for obesity-related and diabetes-related cancers.
Ho and colleagues examined the impact of replacing prolonged sedentary behavior with various levels of physical activity. One hour of low-level physical activity daily was associated with a 12% lower hazard (95% CI 0.79-0.99). A half hour of moderate physical activity was associated with an 8% lower hazard (95% CI 0.86-0.99), and 5 minutes of vigorous physical activity was associated with 22% lower hazard (95% CI 0.46-1.30).
The association between physical activity and cancer incidence was more modest, ranging from 1-6%, but it achieved statistical significance for low and vigorous activity (P=0.003, P<0.001, respectively). The results were similar for obesity- and diabetes-related cancers.
“Our findings suggest that the health effects of sedentary behavior may depend not only on total sedentary time but also on whether that time is accumulated in prolonged bouts or interrupted by activity,” the authors stated. “This pattern is biologically plausible. Experimental studies have shown that interrupting prolonged sitting with short bouts of activity can improve metabolic responses compared with uninterrupted sitting. Prolonged sedentary behaviors were found to promote chronic low-grade inflammation as well as suppressing efficient immune function. In particular, higher device-measured total sedentary behavior has been associated with endothelial dysfunction and low-grade inflammation independent of physical activity levels.”
