Why is sitting for long periods of time bad for the body?
In short, the detrimental effects of sitting for prolonged hours can be divided into acute, or short-term, and long-term effects.
The acute effects that may be measurable within hours and include changes in the body’s ability to metabolize fat and carbohydrates.
If sitting time is even longer and in combination with physical inactivity these changes may manifest in hyperlipidemia (elevated blood lipid levels) and insulin resistance. High blood lipids and insulin resistance are precursors of cardio-vascular disease and type 2 diabetes.
Additional long-term effects may include a positive energy balance and therefore gain in body weight, high blood pressure, osteoporosis only to name a few.
By how much does sitting for 8 hours a day increase your risk of death?
This risk is NOT possible to estimate as it depends on a number of factors including physical activity levels.
Can you please outline your recent study analyzing the impact of exercise on the risk of death for people who sit for prolonged periods of time?
Compelling evidence from many observational studies shows that lack of physical activity increases the risks of many non-communicable diseases such as type 2 diabetes, cardiovascular disease, stroke, some cancers, and premature mortality.
Sedentary behavior, on the other hand, has emerged as a potential risk factor for many chronic conditions and mortality during the last decade. However, it was unknown whether physical activity may attenuate, or even eliminate, the detrimental association between sitting time with death.
Our study was a systematic review including a meta-analysis, which is considered the strongest proof of evidence. We systematically searched the literature for studies including data on sitting time, physical activity and mortality from cardio-vascular diseases, cancers, and all-causes.
We identified 16 studies and contacted all principal investigators of these studies and asked whether they were willing to reanalyze their data according to a harmonized protocol. All but one, agreed to participate in the project. We then also included two studies where pertinent data were available but not published.
In a second step, we performed a meta-analysis taking into account all individual study results. We examined the joint association between sitting time, in four groups, and physical activity, also stratified in four groups (i.e. in total 16 groups) where the most active group in combination with the lowest amount of sitting was the reference group. Thus, the risk of death in all other groups were compared with this reference group.
This primary analysis included more than 1 million healthy men and women who were followed between 2 and 18 years. The reference group reported between 60 and 75 minutes of moderate intensity activity (e.g. brisk walking or cycling) every day in combination with less than four hours of sitting every day.
We thereafter repeated all analyses where we substituted sitting time with time spent viewing TV in those study where this variable was available (465,000 men and women). We also estimated the risk of death from cancer and cardio-vascular diseases in those studies where these data were available.
What were your main findings? Was the increased risk of death in people who sit for 8 hours a day eliminated by exercise?
Our results clearly suggested that 60 to 75 minutes of physical activity such as brisk walking or cycling eliminated the association between sitting time and death. In other words, there was no increased risk in those who were active at this level regardless if they sat for more than 8 hours per day.
This high activity level was reported by about 25% of the participants. We then performed additional analyses in a sub-sample where we also estimated whether the risk was increased in those who reported more than 10 hours of sitting per day and the results were unchanged.
The results for TV viewing were similar, however, watching TV for more than three hours every day was associated with increased risk of mortality regardless of the amount of physical activity except in the most active 25% of the participants where mortality risk was increased only in those who watched TV for more than five hours per day.
Your research looked at “moderate intensity” exercise – how is this defined? So do we need to go to the gym in order to eliminate the risks or are other forms of physical activity such as walking sufficient?
We defined moderate intensity physical activity as activities that increase energy expenditure about three to four times resting energy expenditure. Common examples of such activities are walking at 5.5 km/h or cycling at 16 km/h.
This means that you do not have to go to the gym or participate in vigorous sport activities rather everyday activities than most people can undertake, as part of their everyday life is sufficient to reduce the risk.
Is it necessary to get at least 1 hour of exercise a day to have an impact or would a shorter amount of exercise still have an effect?
Every minute counts. Our data showed that the risk was reduced also in those who participated in less amounts of activity every day compared with those who were almost completely physically inactive.
Many studies have shown that also lower amounts of physical activity of about 15 minutes per day reduces the risk for mortality and chronic diseases compared with those who are completely inactive.
How much evidence is there for high intensity exercise?
We did not specifically examine high intensity activity but it is suffice to assume that high intensity activity provides even further benefits provided the amount of time spent in high intensity activity is similar.
This is also in line with the current physical activity recommendations for public health suggesting that all adults should be active for at least 150 minutes of moderate intensity per week, or 75 minutes of vigorous intensity, or a combination of both. Importantly, some recommendations also suggest that the optimal dose is one hour of moderate intensity activity every day.
What advice would you give to people that have desk-based jobs and can’t escape sitting for long periods?
Take every opportunity to stand up and walk for a few minutes. Try to find opportunities to be physically active as part of your journey to work, during breaks at work and during lunch time.
Why not introduce walking meetings at your job and be as active as you can during leisure time. Take a brisk walk in the evening rather than watching TV and walk the dog even if you don’t have a dog.
Where can readers find more information?
About Prof. Ulf Ekelund
Ulf Ekelund obtained his PhD from the Karolinska Institutet, Stockholm Sweden in 2002 and is currently full professor in physical activity and health at the Department of Sport Medicine, Norwegian School of Sport Sciences, Oslo, Norway.
Before joining the Norwegian School of Sport Sciences he lead a research program in physical activity epidemiology at the Medical Research Council (MRC) Epidemiology Unit, University of Cambridge, UK where he currently holds a part time senior investigator scientist position.
He has published more than 250 peer-review articles including papers in The Lancet, JAMA, and PLOS Medicine and his current H-index is 60. Ekelund is on the executive board for the International Society for Physical Activity and Health, Fellow and international treasurer on the Board of Trustees for the American College of Sport Medicine, associate editor of Medicine and Science in Sport and Exercise and the Journal of Physical Activity and Health. He was awarded the new investigator award from the American College of Sports Medicine in 2007 and the UK Nutrition Society Silver Medal in 2013.
The paper on physical activity, sitting time and mortality published in the Lancet is currently ranked #67 according to its media attention (Altmetrics) among more than 6 million research papers recorded in the database.
Department of Sport medicine
Norwegian School of Sport Sciences
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