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Association between sedentary behaviour and mortality : impact of physical activity

by P. Debraux | 15 October 2019

sedentary, behaviour, physical inactivity, activity, exercise, sport, health, mortality, disease, obesity, diabetes, cancer, cardiovascular, problem, sitting time

According to an international consensus, sedentary behaviour corresponds to all awake activities that take place in seated or extended positions. This includes all activities whose energy expenditure is less than or equal to 1.5 METs. MET is a "metabolic equivalent," an arbitrary unit that matches your energy expenditure sitting in front of the television without talking. A sedentary lifestyle should not be confused with physical inactivity. Indeed, physical inactivity implies that a person does not achieve the objectives set by the World Health Organization (WHO) at the level of recommended minimum physical activity. WHO recommends some level of physical activity for health benefits. Thus, it is recommended to spend at least 150 minutes a week doing moderate intensity physical activities (between 3 and 6 METs, or 7.5-15 METs-h/wk) or at least 75 minutes a week performing intense physical activities (above 6 METs, ie > 15 METs-h/wk). This means that to follow these health recommendations, you can do for example 5 sessions of 30 minutes of running (6.5 km/h corresponds to 6 METs) per week or 3 sessions of 25 minutes of running at 10 km/h (10 METs).

However, being physically active (ie, fulfilling the WHO criteria) does not mean that you are not sedentary. One is not the opposite of the other. Many people spend a lot of their day sitting but doing a workout every day. These people follow the public health recommendations, and are therefore physically active, but they are also sedentary. However, sitting between 4 and 6 hours a day decreases the amount and activity of lipoprotein lipase, an enzyme found in the blood which allows the hydrolysis of blood triglycerides so that free fatty acids enter the muscle. This decrease has an effect of increasing the blood concentration of triglycerides, low density lipoproteins (LDL), decreasing that of high density lipoproteins (HDL), and decreasing insulin sensitivity. And this phenomenon has been observed in non-athletes and endurance athletes after 2 weeks without training. Extended sitting periods also have deleterious effects on glucose metabolism, with a decrease in glucose transporter protein (GLUT-1 and GLUT-4), bone mineral density, and blood pressure. It is these adaptations of the body to a sedentary lifestyle that are strongly correlated with noncommunicable diseases such as type 2 diabetes, obesity, cardiovascular diseases or cancers.

Many studies have focused on stopping smoking, better nutrition, low alcohol consumption and their effects in the prevention and treatment of noncommunicable diseases. But much less attention has been paid to the importance of an active (and less sedentary) lifestyle in this prevention. Physical inactivity is said to be responsible for about 5.3 million deaths a year. Its international economic cost is estimated at $67.5 billion in medical care and lost productivity. Thus, one question remains, is it possible to attenuate, or even eliminate, the risks associated with a sedentary lifestyle by having sufficient physical activity ?

The Study

To answer this question, an international team of researchers carried out a study on the links between sedentary life, that is, sitting time, physical activity and mortality. For this, the researchers conducted a meta-analysis of 16 studies involving 1,005,791 people followed over a period of 2 to 18.1 years, during which 84,609 people died. Based on these studies, the researchers collected the daily sitting time they divided into 4 groups (<4 h/d, 4-6 h/d, 6-8 h/d and > 8 h/d) , the level of physical activity, divided into 4 quartiles (<2.5 MET-h per week, which is equivalent to 5 minutes of moderate intensity physical activity each day; 16 MET-h per week, which is equivalent to 25-35 min of moderate intensity physical activity each day, 30 MET-h per week, which is equivalent to 50-65 min of moderate intensity physical activity each day, and > 35.5 MET-h per week, which is equivalent to 60-75 min of moderate intensity physical activity each day) and causes of death. For the analysis, the reference group consisted of those who sit the least (<4 h/d) and who are the most active (> 35.5 MET-h/wk). They also collected information on the time spent watching television thanks to 3 studies that included 465,450 people (43,740 deaths).

Results & Analyzes

The main results of this study show that in comparison with the reference group (those who sit the least and who are the most active), mortality at follow-up was 12 to 59% greater for the two least active groups (Fig. 1). In the second most active group, only those who sat more than 4 hours had higher mortality than the reference group. Among the most active people, the data did not show any significant relationship between sitting time and mortality, suggesting that a significant amount of physical activity eliminates the increased risk of sitting-related mortality. People who were the most physically active and who sat more than 8 hours a day had a lower risk of death than the least active people who sat less than 4 hours a day.

Association between sitting time, physical activity level and mortality.

Figure 1. Association between sitting time, physical activity level and mortality.

Regarding sitting time spent in front of the television, the analyzes showed similar results with a mortality rate increased from 16 to 93% for those watching more than 5 hours of television a day (Fig. 2). In the group of the most active people, only people watching more than 5 hours of television a day had a higher mortality rate (+ 16%). In comparison, the least active people who watched less than an hour on television each day had a significantly higher mortality rate (+ 32%).

Association between TV-viewing time, physical activity level and mortality.

Figure 2. Association between TV-viewing time, physical activity level and mortality.

The higher mortality risks observed when people are watching television can be explained by the associated eating behaviors and by the time of day when this occurs. In fact, watching television is more often associated with snacks (salty or sweet) and with sweetened and/or alcoholic beverages. In addition, the most opportune time to watch TV is often the evening after dinner, which for many people is the richest meal of the day. This long period of post-prandial inactivity is generally detrimental for glucose and lipid metabolism. In addition, food advertising generally do not help to change bad eating habits.

Practical Applications

In 1994, epidemiologist Jerry Morris described physical activity as the best buy for public health. It must be noted, however, that sedentary behaviour and physical inactivity have continued to grow ever since. This meta-analysis highlights the strong potential of physical activity in the global prevention of noncommunicable diseases that eat away at modern societies. It is encouraging to see that if long sitting periods cannot be avoided during the day, the negative health effect associated with this sedentary time can be counteracted by enough physical activity during other times of the day. Conversely, even if your sitting time is low, if your level of physical activity is insufficient, the risks to your health increase. In addition, for comparison purpose, people who sit more than 8 hours a day and who are not very active have the same mortality risks (+ 58%) as smokers or obese people.

Of course, in the studies used for this meta-analysis, physical activity was not measured but reported by the people themselves who responded to standardized and validated questionnaires. However, studies have shown that people tend to report more physical activity than what they actually do. This fact should be kept in mind when reading many epidemiological data on physical activity and sedentary lifestyle.

Note that the highest level of activity here is more than twice the amount of physical activity recommended by WHO. But when you look at the numbers closely, intensities considered moderate and intense have very low thresholds. It is therefore quite possible for a large number of people to do 60 to 75 min per day of walking at a fast pace (3-6 METs), or even more intense. If resistance training sessions are added in the week, the goal of the 35.5 METs-h weekly is largely attainable. It is also important to implement in parallel a strategy to reduce sedentary time, or at least to interrupt it as much as possible (after 4 hours sitting up without getting up, the lipoprotein lipase is already impacted, for example). As you understand it, we must literally move our ass !

References

  1. Ekelund U, Steene-Johannessen J, Brown WJ, Wang Fagerland M, Owen N, Powell KE, Bauman A and Lee I-M. 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. The Lancet 388 : 1302-1310, 2016.

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