Sedentary living is not just a lack of activity

The terms sedentary and physical inactivity are often used interchangeably. Assuming the correct definitions are being used then they are separate and different concepts. This means that someone can be classed as physically active yet still have a sedentary lifestyle. To explain, I will use two examples of different lifestyles to illustrate how both constructs differ.

Nigel*, one of our regular Lifestyle members, tells me that he is very active when I ask him a few general questions about his lifestyle. “I lift weights twice per week and run for 45 mins three times a week”. This is excellent. Nigel is right, he is active; he achieves at least 150-minutes of moderate physical activity (PA) per week, guidelines set by the department for health (DOH) to improve health and reduce the risk of chronic disease. Nigel tells me he likes to exercise to keep him happy, reduce stress and to “keep him young”. By maintaining his good physical health and remaining active, his chance of developing the many of today’s age-related health conditions is massively reduced.

However, Nigel is not active the rest of the day; he drives to work, earlier than he has to so that he can get a car parking space close to the entrance. He works hard and is focused, so much so in that he does not gets up from his desk much. He tries to walk a bit at lunch but some of the days, lunch is eaten alongside catching up with emails so that he can leave promptly at 4:45pm. From 5–6pm is gym time, Nigel’s favourite part of the day and the part that he associates his lifestyle with. Nigel is physically active for part of his day but spends about 90% of the time being sedentary.

Natasha* is really not a fan of the gym; the concept of paying money to run on the spot on a motorised treadmill seems pointless and extremely dull to her. Natasha tells me she does not like to get out of breath and is ashamed to admit that she is totally sedentary and doesn’t do any exercise. Her lifestyle however would be considered healthy; he diet is good, she has never smoked and only drinks alcohol occasionally. She is falling short on her moderate activity which could be increased in order to improve the health of her cardiovascular and musculoskeletal system. However, despite Natasha not engaging in any structured physical exercise, she is certainly not sedentary; she does not own a car so 5 days a week she enjoys the 20- minute walk to the Primary school she works in. She spends the majority of her time on her feet, walking around and engaging with the children both in and outside the classroom before making the commute home, stopping to get her food shopping in on the way. Aside from sitting down during meal times or doing paper work, Natasha spends most of her day in light physical activity with much less sedentary time (about 15%).

Nigel and Natasha’s lifestyle’s and daily routines are fairly typical but very different from one another, but which is more active? which one would be considered the healthier lifestyle? To compare Nigel and Natasha’s activity and determine how they differ, we must define what exactly we mean when referring to behaviours which are sedentary or physically inactive. Physical activity is any movement of our body that uses energy. If we think of levels or types of activity as a continuum of energy use which is measured using the Metabolic Equivalent of Task scale (MET). 1 MET is considered our Resting Metabolic Rate (RMR) which is just the energy required to maintain all our physiological functions i.e sitting still and reading this article. The amount of oxygen consumed at rest is estimated at 3.5 ml per kilogram of bodyweight per minute (mL/kg/min). If we compare the energy and oxygen used during a moderate bout of walking at a good constant pace we reach the range of 3–6 MET’s so that’s 3–6 times more energy and oxygen than sitting still. Sedentary behaviour is categorised as equal to or less than 1.5MET’s and typically involves watching TV, sitting at a computer, driving, reading etc. Returning to the DOH guidelines outlined at the beginning of the article, less than 150-mins of moderate activity (3–6 MET’s) per week is considered being relatively physically inactive. However, like Natasha, those people may spend a significant portion of their day being lightly active (1.5- 3 MET’s).

The number of MET’s cumulated within their typical 12- hour day is anything between 14–22.5 MET for Nigel and 15- 30.6 MET for Natasha, so would you consider Natasha to be more active than Nigel?

Both physical inactivity and sedentary behaviour are major determinants for poor mental and physical health. It has been repeatedly shown that those least active are far more likely to suffer from obesity, heart disease, cancer, depression, diabetes, osteoporosis, back pain, to name a just a few. However, sedentary behaviour is emerging as a separate independent risk factor for ill health, aside from PA levels. This has been highlighted in a number of research studies associating mortality and disease with total sitting time (marker for sedentary time) 2,3. Although, before definite conclusions can be made, more controlled research trials are required exploring the detrimental impact of sedentary behaviour.

The impact of spending a large proportion of the day being sedentary has received a lot of research in recent years, in fact it has become a hot topic. The value of this research is becoming increasingly important as technology advances and our lives become more and more convenient. After all, who really has time now days to go to Sainsbury’s and do their weekly shop (despite being able to get everything in one store now). Order on line, I can fit in the delivery at 7am Monday morning. Whether it is while we work, socialise or relax by watching box set marathons. There is even a programme where we watch other people watching TV.

At the risk of anyone spending too long sitting reading this article, I have not gone into the physiological consequences of sedentary behaviour; these are numerous and so will save that for another time.

Reference

1. Matthews CE, George SM, Moore SC, Bowles HR, Blair A, Park Y, Troiano RP, Hollenbeck A, Schatzkin A. Amount of time spent in sedentary behaviors and cause-specific mortality in US adults. Am J Clin Nutr. 2012;95:437–45.

2. van der Ploeg HP, Chey T, Korda RJ, Banks E, Bauman AE. Sitting time and all-cause mortality risk in 222,497 Australian adults. Arch Intern Med 2012;172(6):494–500.

3. Chau JY, Grunseit A, Chey T, Stamatakis E, Matthews C, Brown W, Bauman A, van der Ploeg HP. Daily sitting time and all-cause mortality: a meta-analysis. PLoS One. 2013

4. Ekelund U, Steene-Johannessen J, Brown WJ, Fagerland MW, Owen N, Powell KE, Bauman A, Lee IM. For the lancet physical activity series 2 executive committee and the lancet sedentary behaviour working group. Does physical activity attenuate, or even eliminate, the detrimental association of sitting time with mortality? A harmonised meta-analysis of data from more than 1 million men and women. Lancet. 2016;388(10051):1302–10.