A major new study suggests that one of the most routine parts of modern life may be linked to a higher risk of cancer.
Researchers at the University of Glasgow tracked the health of more than 91,000 adults in the UK Biobank, using wrist-worn activity monitors to measure how active or inactive they were across a seven-day period. The participants were then followed for an average of about 12 years, making this one of the largest studies to examine how sedentary behaviour may affect long-term cancer risk.
For many people, sitting takes up a large part of the day, often somewhere between six and ten hours. That can include time spent at a computer, commuting, watching television, or other seated daily activities.
Previous research has already connected too much inactivity with conditions such as obesity, type 2 diabetes, heart disease and osteoporosis. However, the latest findings suggest the risks may extend further than previously understood, particularly when sitting happens in long, uninterrupted stretches.
The study found that prolonged sedentary behaviour was associated with a greater likelihood of both developing cancer and dying from it. In this research, prolonged sedentary time meant bouts of at least 30 minutes in which people were sedentary for at least 90 percent of the time.
According to the researchers, each additional hour of prolonged sitting was associated with roughly a 9 percent increase in the risk of dying from cancer and a 3 percent increase in the risk of developing it. For cancers linked to obesity and metabolic health, such as colon and pancreatic cancer, the increase in risk was even higher at around 5 percent per additional hour.
The University of Glasgow team based its analysis on 91,292 people from the UK Biobank, all aged between 37 and 73 and all with no previous cancer diagnosis at the time monitoring began.
Each person wore a wrist-based activity tracker continuously for one week, giving scientists a detailed record of their movement patterns throughout the day and night.
A machine-learning system was then used to classify the data into different levels of movement, including sedentary time, light activity and moderate-to-vigorous exercise. The researchers also separated sitting time into two forms: prolonged sedentary behaviour, defined as periods of at least 30 minutes in which 90 percent of the time was spent sitting, and interrupted sedentary behaviour, where movement broke up those periods.
Participants were followed for around 12 years, during which researchers recorded diagnoses of a range of cancers, including breast, colorectal, pancreatic, kidney, liver and thyroid cancers.
The team believes one possible explanation is that extended periods of sitting may contribute to inflammation in the body, which can create an environment where DNA damage is more likely and abnormal cells have a better chance of developing into tumours.
Another possible factor is insulin resistance. Sedentary lifestyles are known to contribute to this process, which is strongly associated with type 2 diabetes and has also been linked in other research to tumour development.
There was also a more encouraging side to the findings. Even relatively small changes appeared to be linked to better outcomes. Replacing 30 minutes of daily sitting with light movement, such as walking or doing housework, was associated with a lower risk of dying from cancer.
Exchanging that same half-hour for moderate exercise, including brisk walking or cycling, was linked to a further reduction in cancer death risk. Meanwhile, just five minutes a day of vigorous activity, such as swimming or running, was associated with a meaningful drop in overall cancer risk.
“Our findings suggest that the health effects of sedentary behaviour may depend not only on total sedentary time, but also on whether that time is accumulated in prolonged bouts or interrupted by activity,” the study authors said.
The researchers emphasised that the study shows a link rather than proving that sitting directly causes cancer. Even so, they said the results strengthen the case for breaking up long periods of inactivity with regular movement throughout the day.
The findings fit with wider public health advice that encourages adults to reduce sitting time and interrupt long periods of inactivity. Experts say the most practical change for many people may be simple: stand up, stretch, or take a short walk at least every 30 minutes.