Excessive sedentary time, whether accumulated throughout the day or accrued in prolonged, uninterrupted bouts, is a significant risk factor for all-cause mortality, regardless of exercise habits. Now, a new study says it's not just the total time spent sitting throughout the day, but specifically sitting for long periods of time.
It's possible that prolonged sedentary stretches might hasten death by causing what's known as metabolic toxicity, said Dr. David Alter, head of cardiovascular and metabolic research for the University Health Network-Toronto Rehabilitation Institute in Canada.
Earlier studies that have reported a link between sitting and an early death have relied on people telling researchers how long they sat in a day.
Like many of us, executive assistant Meghan Hampsey spends way too many hours sitting behind a desk.
"That risk is reduced if we exercise at least 150 minutes per week, but not entirely eliminated", he concluded. "One of the reasons for this is related to our skeletal muscles, which require fuel such as glucose to operate and take in glucose from our blood", Diaz said in an email.
"We think a more specific guideline could read something like, 'For every 30 consecutive minutes of sitting, stand up and move/walk for five minutes at brisk pace to reduce the health risks from sitting, ' " he said, adding the study "puts us a step closer to such guidelines", but more research is needed to verify the findings. He said, "We don't yet know what the ideal solutions are to remedy the risks associated with sedentary behavior".
For the current study, researchers examined data on 7,985 adults, age 45 and older, who were asked to wear accelerometers to measure activity levels for one week.
As well as the total amount of sedentary time, the length of sitting bouts was also an important factor. During the study period, the team recorded 340 total deaths considered "all-cause mortality" - any death, regardless of cause.
Overall, sedentary behavior accounted for 77 percent of participants' waking hours, or about 12 hours a day, researchers report in the Annals of Internal Medicine.
Adults are sedentary for nine to 10 hours of their waking day, which can lead to cardiovascular risks and even death. The higher average in his own study is likely "due to the fact we studied a middle- and older-aged population", Diaz wrote.
The greatest risk for mortality was observed in participants with both a high sedentary time (12.5 hours/day) and high bout duration (10 minutes/bout), while the lowest risk for death was seen in those who kept their sitting bouts under 30 minutes.
As total sedentary time increased, so did early death by any cause, the results indicated.
He also explained that sitting for long periods can cause blood to pool in the legs and that the combination of blood pooling, decreased blood flow, and turbulent blood flow is believed to be conducive to the development of plaque formation in the blood vessels.
"We found that there wasn't a threshold or cutoff where one's risk for death dramatically increased", said Diaz, explaining that risk of death increased with more sitting. "Unfortunately the message is more mixed ... it looks like both are bad for you". What's more, studies typically fail to measure the various ways in which people are inactive throughout the day, and how these differences may have influenced risk of death.
"First of all, this is an epidemiological study that shows associations or relationships between sedentary time, sedentary breaks and mortality", she told Gizmodo.
Arguably, he said, the study's most important contribution involved disentangling two sedentary behaviors: total daily sedentary time and uninterrupted sedentary bout duration.
Greater total sedentary time and longer mean sedentary bout duration each had a dose-dependent association with higher risk for all-cause mortality that did not vary by age, sex, race, BMI, or participant exercise habits.
The researchers studied nearly 8000 Americans aged 45 and over to examine the relationship between sedentary behaviour and death for any reason (what researchers call "all-cause mortality").
"The device that has been used in the study [did not allow the researchers] to estimate posture (sitting vs. standing vs. lying), therefore, estimated sedentary time may overestimate sitting time", Zipunnikov told Gizmodo.
People may also be less likely to die young if they break up sedentary time by moving around every half hour than if they remain seated for longer stretches of time without getting up, the study also suggests.
But as other experts we spoke to made clear, these findings may not be generalisable across the entire population, and much more work needs to be done to determine why physical inactivity is so harmful to our health, and the kinds of things we can do to offset these risks.