"Women who lead a sedentary lifestyle have faster-ageing cells than those who exercise every day," BBC News reports.
This research looked at telomeres – often likened to the caps at the end of shoelaces, they are made up of molecules that protect strands of chromosomes from "fraying".
Telomeres shorten every time the genetic information in cells is duplicated. It's believed that this leads to cell ageing and death.
In a sample of older women, the researchers looked at whether there was an association between time spent sitting down and telomere length.
Telomeres are measured in the small sections of nucleic acids that make up DNA, known as base pairs.
Among women in the study who did less than about 40 minutes of physical activity a day, those who sat longest had shorter telomeres by an average 170 base pairs.
The researchers say telomeres shorten at a rate of 21 base pairs a year – using a rough "back of a fag packet" calculation, 170 equals about eight years.
Sitting time did not seem linked to telomere length for women who did at least 30 minutes of physical activity a day.
We don't know whether the results apply to men or younger people.
And, importantly, as the study only looked at the women's activity levels and telomeres at one point in time, we don't know whether activity levels or sitting causes telomeres to shorten.
Still, arguably, most of us would benefit from spending less time sitting down.
The study was carried out by researchers from the University of California, San Diego State University, the State University of New York at Buffalo, the University of Washington, the Fred Hutchinson Cancer Research Centre, George Washington University, the University of Florida and Northwestern University, all in the US.
It was funded by the US National Heart, Lung and Blood Institute.
All of the UK media outlets that covered the study implied that a direct cause and effect relationship between sitting down and cell ageing had been proven.
For example, the Mail's headline stated that, "Women who spend at least 10 hours on their backsides each day speed up their aging process."
This is untrue. While there certainly seems to be an association worthy of further research, no causal link has been established.
This cross-sectional study used data from women taking part in a much bigger study of health called the Women's Health Initiative.
Cross-sectional studies can find correlations between different factors – in this case, sitting time and telomere length.
But because this type of study only looks at one point in time, researchers can't say which factor happened first, so it's not very useful for telling us whether one causes the other.
Researchers used information about 1,481 women aged over 65 who'd taken part in various sub-studies of the Women's Health Initiative.
They used information from women who'd had their physical activity measured using accelerometers (devices that measure movement) and had also given DNA samples that had been tested for telomere length.
After accounting for other factors, they looked at whether telomere length was linked to the amount of time spent sitting.
The information about physical activity was measured over one week, during which time women wore their accelerometer all the time, except when bathing or swimming.
Women taking part also completed a questionnaire about their physical activity and kept a record of their sleep. Telomere length was measured from DNA in blood cells.
The researchers took account of the following possible confounding factors:
They also redid their calculations to divide the women into those who did more or less than the average amount of physical activity (about 40 minutes).
They then looked at the link between time spent sitting and telomere length for women who did more or less than 40 minutes physical activity a day.
They also looked at the link between sitting and telomere length for women who did 30 minutes or more a day, the recommended activity level for all adults.
It's unclear whether these additional calculations were planned from the start of the study, or whether the researchers decided to do them because the initial findings did not show a link between time spent sitting and telomere length.
The length of time spent sitting was not linked to telomere length for women who did 30 minutes or more of moderate physical exercise a day.
For women who did less than the average amount of moderate physical activity each day, time spent sitting did show a link to telomere length.
Among these women, those who spent more than about 10 hours a day sitting had shorter telomeres than those who spent less than about eight hours a day sitting. The average difference was 170 base pairs (95% confidence interval [CI] 4 to 340).
Women who spent the most time sitting were more likely to be older, white, obese and have long-term illnesses.
The researchers said their results suggest that, "Prolonged sedentary time and limited engagement in moderate to vigorous physical activity may act synergistically to shorten leukocyte telomere length among older women."
In other words, being both sedentary for long periods and not getting much physical activity may act together to shorten telomeres in blood cells.
They speculated that causes of the link might include insulin resistance, lack of the anti-inflammatory responses the body has to exercise, or obesity.
They also acknowledged women who have long-term illnesses are more likely to have a sedentary lifestyle, and the illness rather than the lack of exercise may cause shortened telomeres.
It's not news to anyone that being more physically active and spending less time sitting around is likely to keep people in better health.
But this study has many limitations that make it difficult for us to rely on its results.
While they are used as a marker for ageing cells, telomeres are not a direct measure of ageing. Although shortened telomeres have been linked to certain diseases, everyone's telomeres shorten over time.
Saying shorter telomeres make someone "biologically older" doesn't mean much. This hasn't stopped the emergence of private companies offering to measure your telomeres – but it's unclear what exactly you could usefully do with that information.
And the only cells studied in this research were blood cells, so we don't know whether the results would have held for brain cells, muscle cells or any other cells in the body.
Doctors have tried to disentangle the effects of physical activity from the effects of being sedentary before without much success.
Generally, as in this study, research seems to show that if you get plenty of moderate to vigorous physical exercise, the amount of time you spend sitting or lying down doesn't make much difference.
The researchers carried out a lot of comparisons and used multiple models to try to show sedentary time was linked to telomere length.
In most of these models, once you take account of women's age, ethnicity, body mass index and long-term illnesses, there was no link.
Only when the researchers stratified the results by how much physical activity women did could they show a link in one category: those who did the least physical activity.
That suggests sedentary behaviour is not the strongest factor to affect telomere length.
Another problem with the study is it only looked at telomere length and physical activity at one point in the women's lives.
We don't know how much physical activity they'd done throughout their lives, or whether their telomeres had shortened faster than other women recently or at an earlier stage in life.
The study doesn't add much to what we already know: physical activity is likely to be beneficial for people at all stages of life, and everyone should aim to get at least the recommended level of 30 minutes of moderate to vigorous physical activity a day.