Every hour spent sitting down increases the risk of cancer by nearly a tenth

Each extra hour sitting down may raise your risk of deadly cancer, a study suggests.

The average American spends between six and 10 hours a day sitting and engaging in sedentary behaviors like watching TV or working at a desk. 

Science has long shown that sedentary behavior raises the risk of obesity, diabetes, heart disease and osteoporosis.

Now, researchers in Scotland have found sitting may lead to a higher chance of developing and dying of multiple forms of cancer. 

The team combed through health data from nearly 100,000 adults who wore activity monitors for a week and were then followed for more than a decade after. 

They found that prolonged sedentary behavior – spending at least 90 percent of a 30-minute interval sitting – was linked to a three percent higher risk of developing cancer and a nine percent greater chance of dying from the disease.

And the risk of cancers linked to obesity, such as pancreatic and colon cancer – increased by five percent. 

However, the team also found replacing just 30 minutes of sitting a day with light exercise like walking was associated with an 18 percent lower chance of dying from cancer.

‘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 study authors, from the University of Glasgow, said. 

‘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.’

The study, published in the journal PLOS Medicine, looked at data from 91,292 adults included in the UK Biobank database.

Participants were between 37 and 73 years old and had no history of cancer. They were asked to wear a wrist monitor for 24 hours per day over the course of a week.

Researchers used a machine-learning program to classify each 10-second period into one of the following categories: sedentary behavior, light physical activity, moderate physical activity and vigorous physical activity. 

They then separated sedentary behavior into two subcategories. The first was prolonged sedentary behavior, defined as at least 30 minutes where 90 percent of time was spent sitting continuously, such as watching TV or working at a desk without getting up. 

The second subcategory, interrupted sedentary behavior, was defined as sitting less than 30 minutes or interrupting sitting with movement like getting up and walking briefly. 

Over a 12-year follow-up period, researchers monitored patients to see if they developed any of the following forms of cancer: breast, colorectal, pancreatic, kidney, liver, thyroid, ovarian, gallbladder, esophageal, bladder, leukemia or non-Hodgkin lymphoma. 

The team found every additional hour of prolonged sedentary behavior was associated with a three percent higher risk of any form of cancer. There was also a five percent greater risk of obesity-related cancers – such as colon, kidney and pancreatic cancer – and a five percent higher chance of diabetes-related cancers – which can include breast, liver and thyroid cancer. 

Additionally, participants faced a nine percent higher risk of dying from any form of cancer with each additional hour of prolonged sedentary behavior. 

Sitting for long periods of time increases inflammation throughout the body, which creates a prime environment for cell DNA to become damaged and mutate, causing cancerous tumors. 

Inactivity also promotes insulin resistance, the main driver of type 2 diabetes, which has been shown to promote tumor growth and stop cancer cells from dying. 

On the flip side, researchers wanted to understand how becoming more physically active may ward off cancer. They found each additional hour of interrupted sedentary behavior was associated with a six percent lower chance of developing any cancer.

Additionally, the risk of obesity-related and diabetes-related cancers decreased by nine and 10 percent, respectively. 

And each extra hour of interrupted sedentary behavior was associated with an 18 percent lower risk of death from any cancer. 

The researchers also noted replacing 30 minutes a day with moderate activity – such as brisk walking or cycling – was tied to an eight percent lower chance of cancer death. 

And replacing just five minutes a day with vigorous activity – running, swimming laps or hiking uphill – was associated with a four percent lower cancer risk. Diabetes-related cancers decreased 11 percent, while obesity-related disease was tied to a nine percent decline. 

The researchers caution that the findings show associations rather than directly proving sedentary behavior increases cancer risk. However, they note that the study adds to a growing body of evidence showcasing the benefits of light exercise like walking.

‘Current health guidelines focus heavily on moderate or vigorous exercise, but our findings show that light movement shouldn’t be ignored,’ the study authors said. 

‘Moving forward, clinical trials will help us move beyond blanket advice and develop personalized strategies for breaking up time.’