Friday January 17 2014
Sitting for more than 11 hours a day is unhealthy
“Why sitting for too long can be deadly for older women... even if they go to the gym,” reports the Mail Online.
The study this news is based on found an association between sedentary behaviour (sitting or lying down for much of the day) and an increased risk of potentially fatal lifestyle-associated diseases such as heart disease and cancer.
This study followed a sample of almost 100,000 postmenopausal women from across the US. It assessed how long they spent being sedentary and followed them up over an average of 12 years to look at their risk of death. It took into account how physically active the women were, to see if being sedentary had an effect by itself.
The study found a general trend that women who were sedentary for longer tended to have a greater risk of death during follow-up.
The main limitation of the study is that many of the measures were collected through self-report, which increases the possibility of inaccuracies.
Still, there is an increasing body of evidence linking sedentary behaviour with increased risk of chronic diseases. Some fitness gurus have even gone so far as to say that “sitting down is the new smoking”.
While this may be a little over the top, the earlier you adopt healthy habits, the more likely they are to persist throughout your life, and the more benefit you will get from them.
Read more advice about how you can start becoming fitter and healthier through exercise and diet.
Where did the story come from?
The study was carried out by researchers from Cornell University, Ithaca, Stony Brook University School of Medicine, New York, and other US institutions. Funding was provided by the National Heart, Lung, and Blood Institute, National Institutes of Health, and US Department of Health and Human Services.
The study was published in the peer-reviewed American Journal of Prevalence Medicine.
The Mail Online’s report of the study is generally accurate, though the claim that “middle-aged women who spend too long sitting down are at greater risk of health problems – no matter how much exercise they do” is inherently illogical. If women are doing lots of exercise then they are not sitting down.
What kind of research was this?
This was a cohort study of postmenopausal women aiming to investigate the association between the time they spent in sedentary behaviour, and the risk of death overall, and from specific disease causes of cardiovascular disease and cancer.
The researchers say that few previous studies have specifically looked at older women; and also other studies have not taken into account time spent being physically active.
What did the research involve?
The study included women from the Women’s Health Initiative (WHI) Observational Study (OS) and Extension Study (ES). It assessed the women’s sedentary behaviour, physical activity, and other characteristics at the start of the study. Researchers then followed them for up to 17 years to see who died in this period, and their cause of death. The researchers then looked at whether women who were sedentary for longer were more likely to die during follow-up, even after taking into account how active they were.
A total 93,676 women aged 50 to 79 were initially recruited between 1993 and 1998 from 40 clinical centres across the US, and had data collected through interviews, questionnaires and clinical assessments. The main WHI ended in 2005, and the ES included annual mail follow-up from 2005 to 2010.
At recruitment, questionnaires asked about the total hours spent in sedentary behaviour with the questions:
- During a usual day and night, about how many hours do you spend sitting? Including time spent sitting at work, sitting eating, driving or riding in a car or bus, and sitting up watching TV or talking.
- During a usual day and night, about how many hours do you spend sleeping or lying down with your feet up? Including time spent sleeping or trying to sleep at night, resting or napping, and lying down watching TV.
- A third question asked participants to estimate the number of hours typically spent sleeping each night.
Total sedentary time was sitting time plus lying time with sleeping time subtracted. Women were divided into three categories of daily sedentary time:
- less than four hours
- four to eight hours
- eight to 11 hours
- more than 11 hours
Deaths were identified up to 2010 using hospital records, autopsy reports, death certificates, and records from the National Center for Health Statistic’s National Death Index.
They took into account various confounding factors that may influence the relationship between sedentary time and mortality, including:
- marital status
- body mass index (BMI)
- smoking and alcohol use
- history of chronic diseases (coronary heart disease, heart failure, stroke, diabetes, cancer, arthritis, hypertensive, number of falls in the past year, chronic obstructive pulmonary disease [COPD], and hip fractures before age of 55)
- hormone use
- depressed mood
- living alone
- problems performing activities of daily living
- self-reported health
- moderate to vigorous physical activity (MVPA), measured using a validated questionnaire
The analyses included all 92,234 women (average age 63.6) who had data on sedentary time available.
What were the basic results?
Average sedentary time of women in the sample was 8.5 hours a day. Generally women with higher sedentary time were:
- less likely to be of white ethnicity
- less likely to have received higher education,
- more likely to have higher BMI
- more likely to have lower physical activity levels
- more likely to smoke
- more likely to rate their health as fair or poor
During the average 12 years of follow-up, 13,316 women (14.4%) died. Overall there was a general trend for increasing sedentary time to be associated with increasing all-cause mortality, and mortality from cardiovascular disease (such as stroke and heart disease), heart disease specifically, and cancer.
Compared to women with the lowest sedentary time (less than four hours), those with the highest (more than 11 hours) had:
- a 12% increased risk of all-cause mortality (hazard ratio [HR] 1.12, 95% confidence interval [CI] 1.05 to 1.21)
- 27% increased risk of heart disease death (HR 1.27, 95% CI 1.04 to 1.55)
- 21% increased risk of cancer death (HR 1.21, 95% CI 1.07 to 1.37)
Women who were sedentary for between four and 11 hours a day were at no higher risk of death overall compared with women sedentary for less than four hours a day.
In general these women were also at no greater risk of the specific causes of death with the exception of cancer. A 21% higher risk of cancer death was found compared with those women who were sedentary for less than four hours.
How did the researchers interpret the results?
The researchers conclude that there was a relationship between greater amounts of sedentary time and mortality risk after controlling for multiple potential confounders.
This cohort study looking at the link between sedentary time in postmenopausal women and their risk of death, benefits from its large sample size of almost 100,000 women, and 12 year follow-up.
It finds, as previous research has found, that increased sedentary time is generally associated with increased risk of death.
The main risk was for women with the highest sedentary time (greater than 11 hours sitting per day) who were at increased risk of death from any cause and deaths from heart disease or cancer compared with women sitting for less than four hours a day.
The links were less clear for women sedentary for between four and 11 hours a day.
The study also benefits from adjusting for many confounding factors that may influence the relationship between sedentary time and mortality – including sedentary activity. The main limitation is that many of the measures taken in the study – for example sedentary time, physical activity, and medical history – were collected through self-reported mailed questionnaires.
This may reduce the reliability of some of these measures. Self-reports would not be as accurate as looking at medical records or objectively measuring activity using monitors, for example.
Though it is unclear whether self-reporting would mean that women underestimate or overestimate the time they spent sitting down during the day (but if you forced us to guess, we would go for the former).
Also, the study findings were only obtained from postmenopausal women and may not apply to men or younger groups of women.
Overall the findings support current lifestyle advice that less sedentary time and more physical activity can improve health.
Read more about increasing your activity levels and why not try our tool that could help you find a type of exercise you enjoy – if you do enjoy a type of exercise you are more likely to stick to it.
Analysis by Bazian. Edited by NHS Choices. Follow Behind the Headlines on Twitter.