A large number of studies indicate that there is a positive association between exercise and good health, particularly good cardiovascular health. Researchers are now focusing their efforts on identifying the physiological and molecular mechanisms underlying these beneficial effects.
A study conducted among 580 young Finnish men shows that aerobic fitness (also known as cardiorespiratory capacity) is associated with levels of several metabolites that are beneficial to health. The approach used in this study is referred to as “metabolomics”, i.e. an approach that aims to identify metabolic differences, for example in the blood of people with a disease (diabetes, cancer) compared to people in good health. Most of the metabolomic studies conducted to date have focused on diseases, but this approach has also been applied recently to determine which metabolites are indicative of good health, particularly with regard to exercise.
Of the 66 metabolites selected in the Finnish study, 48 were at different levels between the group of participants who had the highest aerobic fitness and the one with the lowest aerobic fitness (see Figure 2 and Figure 3 of the original article). These differences include a 44% lower concentration of low-density lipoprotein (LDL, the “bad cholesterol”), an 81% higher concentration of high-density lipoprotein (HDL, the “good” cholesterol), a 52% lower total of triglycerides (Figure 1 below, orange bars). On the other hand, greater muscular strength of the participants was not associated with favourable levels for these same metabolites (Figure 1 below, blue bars).
Figure 1. Main differences in blood metabolites between the participants who had the highest aerobic fitness and those who had the lowest (orange bars) or between the participants who had the highest muscular strength and those who had the lowest (blue bars). * Significant difference (P <0.001 or P <0.002); NS: Not significant difference. From Kujala et al., 2019.
The more detailed analysis (see Figure 2 in the original article) shows that all LDL and VLDL particles of different sizes (small, medium, large, very large, extremely large) are present in lower concentrations in the blood of participants who have a good aerobic capacity than in participants who have a lower aerobic capacity. On the contrary, all HDL particles (very large, large, or medium-sized), except for small ones, are present in higher concentrations in the group with the highest aerobic fitness. Large-size HDLs are particularly beneficial for good cardiovascular health.
Participants with good cardiorespiratory capacity had 80% less apolipoprotein B (ApoB) in their blood than those who were less fit. ApoB is a protein found in very low-density lipoproteins (VLDL) and low-density lipoproteins (LDL). The measurement of the ApoB makes it possible to estimate the number of particles of cholesterol, which is a good indication of the risk of developing cardiovascular disease. High blood levels of ApoB are therefore a risk marker for cardiovascular disease, independently of the level of LDL-cholesterol.
A high concentration of triglycerides in the blood is a risk marker for coronary heart disease and is associated with obesity and type 2 diabetes. Excessive consumption of sugars and alcohol (not fat) is generally the cause of a high level of triglycerides in the blood.
Other important metabolites that are present in lower concentrations in individuals with good aerobic capacity include total fatty acids (-60%), glycerol (-64%), lactate (-34%), pyruvate (-36%), branched-chain amino acids (BCAA) isoleucine (-37%) and leucine (-55%), and amino acids phenylalanine (-54%) and tyrosine (-55%). Interestingly, theunsaturation degree of fatty acid of participants in better aerobic fitness was 59% higher than in less fit participants; asituation conducive to good cardiovascular health knowing that it is saturated fatty acids that, in excess, increase theconcentration of LDL-cholesterol and are atherogenic.
High levels of BCAA, phenylalanine and tyrosine are found in obese people and they have been associated with a 5-fold increased risk of developing type 2 diabetes in two separate cohorts. Lower levels of glycerol and ketone bodies (acetylacetate, 3-hydroxybutyrate) in individuals with a high aerobic capacity suggest an increase in fat degradation.
Several metabolites (19) remain associated with a high aerobic fitness after adjustments to account for age and percentageof body fat. After making the same adjustments, muscular strength was associated with only 8 measures of the “metabolome” and none of these associations related to cholesterol or other blood lipids.
This study found more favourable associations between aerobic fitness and certain metabolites that are risk factors for cardiovascular disease than for high muscular strength. It should not be concluded, however, that muscular endurance exercises are useless, quite the contrary. Indeed, muscle training increases aerobic fitness and is an important component of maintaining and improving the condition of people with chronic diseases and the elderly. It is therefore necessary to combine aerobic and muscular exercises to optimize the benefits for cardiovascular health and overall well-being.
Although regular physical activity is associated with a marked decrease in the risk of several chronic diseases and premature mortality, the proportion of sedentary people, i.e. who do not reach the minimum of 150 minutes of physical activity recommended per week, continues to increase worldwide. According to recent estimates, a sedentary lifestyle is directly responsible for about 10% of premature deaths worldwide, a negative impact similar to that of smoking and obesity. Several factors contribute to this decline in the level of physical activity, the most important of which are probably the major social transformations that accompanied industrialization and, more recently, the technological revolution: motorized transport modes make it possible to travel great distances easily, computing is at the centre of many professions, and the constant arrival of new electronic devices continues to reduce the energy costs of most of our activities, both at work and at home. While this progress can generally be viewed as positive in terms of improving quality of life, it is important to remain aware that the resulting decrease in physical activity can be detrimental to health.
Canada is not immune to this trend, as studies that have measured physical activity levels accurately (using an accelerometer) show that only 15% of adults are active enough to reach the minimum of 150 minutes of physical activity per week. This is very little, and although lack of time is the main reason given by sedentary people for their low participation in physical activity, this excuse does not really hold up: 29% of Canadian adults spend 15 hours or more per week (more than two hours a day) in front of the TV and 15% are frequent computer users (11 hours or more per week) during leisure time. There is therefore a window of time available to reduce the sedentary lifestyle, especially since it is possible to be more active without having to dedicate a specific period of the day to exercise: several studies show that the simple fact of integrating light or moderate physical activities in our daily routine, whether walking, gardening or doing housework, is enough to enjoy the benefits of exercise on health (see our article on this subject).
The time spent commuting to and from work represents another interesting opportunity to increase the level of physical activity. In Canada, the use of motorized means of transportation to get to work is widespread, with 8 out of 10 workers using a motor vehicle for daily commuting (75% as drivers, 5% as passengers) (Figure 1). Conversely, active modes of transport such as walking and cycling are used by only 8% of workers, a proportion that is much lower than in most European countries: in Holland for example, no less than 50% of all daily trips are made by walking and cycling, while in all Nordic countries (Denmark, Sweden, Norway and Finland) this proportion is around 30%.
Figure 1. Distribution of the modes of transportation used by Canadians to get to work. From Statistics Canada (2013).
This low use of active transportation in North America likely contributes to the negative impacts of a sedentary lifestyle, particularly with respect to the high proportion of people who are overweight. It has been estimated that each hour spent in a car is associated with a 6% increase in the risk of obesity, while every kilometre walked is associated with a 5% reduction in the risk of being overweight. For example, an Australian study that followed 822 people over 4 years found that those who travelled every day by car tended to have greater weight gain (2.18 kg) than those who did not use a car (0.46 kg). This is in agreement with the results of a British study showing that people who stopped using their car to get to work and replaced it with active transportation or public transit lost weight (- 0.30 kg/m2). Conversely, those who stopped using active modes of transportation and instead opted for the car saw their body mass index significantly increase (+ 0.32 kg/m2).
In addition to promoting the maintenance of a healthy weight, several studies have shown that active transportation also has several positive effects on health, particularly in terms of prevention of cardiovascular disease, type 2 diabetes and premature mortality. One of the best examples is a large study of 263,540 participants living in 22 communities in the United Kingdom. By analyzing the modes of transportation used by this population to get to work, the researchers found that cyclists had a 41% lower risk of premature mortality than those who used motorized transport (public or cars). A positive effect was also observed for walkers, with a 27% reduction in the risk of myocardial infarction and a 36% reduction in the risk of dying from a cardiovascular event. Overall, a meta-analysis of 23 prospective studies that examined this issue (531,333 participants in total) shows that commuters who use active transportation (cycling, walking) have a significantly lower risk of cardiovascular disease (9%), type 2 diabetes (30%), and premature mortality (8%). As with any form of physical activity, these benefits associated with active transportation come from the many positive effects of this type of exercise on all cardiovascular risk factors, including hypertension, dyslipidemia, diabetes and obesity (Table 1).
Table 1. Major cardiovascular risk factors influenced by active transportation.
The benefits of cycling
The positive impacts of active transportation are particularly striking for those who travel by bicycle, with higher reductions in the risk of several chronic diseases and premature mortality than those who walk (Figure 2). These benefits associated with regular cycling are in line with Danish and British studies indicating that premature mortality rates are about 30% lower for cyclists compared to non-cyclists. This superiority of cycling over walking is probably due to the fact that cyclists in these studies showed a higher level of overall physical activity than walkers, with 90% of the people travelling by bicycle who reached the minimum recommended activity against 54% for those who travel on foot.
Figure 2. Reduced risk of several chronic diseases depending on the mode of active transportation. Adapted from Dinu et al. (2019).
Intervention studies carried out among sedentary people show that the positive effects of active bicycle transportation on health can be observed in the first six months following the adoption of this mode of transport. For example, people who cycle 20 minutes daily to commute to work for 8 weeks already show a noticeable improvement in their maximum aerobic fitness (VO2max), one of the best markers of good health. More recently, a British study showed that only 6 months of active bicycle transportation resulted in abdominal fat loss and significant improvement in insulin sensitivity in a group of sedentary people who were overweight or obese. The positive impact of active transportation is therefore very rapid, and these modes of transport can really contribute to improving the health of sedentary people.
It is also interesting to note that the benefits of active bicycle transportation on the reduction of all-cause mortality and type 2 diabetes are similar to those observed among people who cycle in their leisure time (Figure 3). For people who lack the time to exercise during the week or the weekend, riding a bike to work compensates for this lack of time by integrating physical activity into their daily routine. Not to mention that some studies suggest that the bicycle is the mode of transport that makes you happiest!
Figure 3. Risk of premature death for people who use bicycles in leisure time or as an active means of transportation. Adapted from Østergaard et al. (2018).
Walking or cycling to work is obviously not within everyone’s reach, especially for people who have to travel long distances from home to their workplace (in Canada, for example, over half of workers live more than 8 km from their work). On the other hand, even without completely replacing the use of motorized vehicles, it is still possible to be more physically active by walking or cycling for a portion of the journey (using bicycle-sharing systems), for example by parking the car at a certain distance from work or by getting off the bus or the metro a few stops earlier. For commuters using motorized transportation, the mere fact of walking the last kilometre (about 10 minutes) to and from work is equivalent to 100 minutes of physical activity per work week, a good proportion of the minimum recommended exercise. This is a worthwhile investment of time: in the British study mentioned earlier, even if the maximum protection (41%) is observed among commuters who make the entire trip by bike, people who used this mode of transportation for only part of the journey still had a 24% lower risk of premature mortality compared to those who travelled exclusively by motorized transport.
In North America, land use planning is unfortunately not very favourable to active transportation. In most cases, the cities developed after the invention of the automobile, which resulted in the construction of infrastructure focused mainly on the use of the car as a means of transport and therefore favoured urban sprawl. In Europe, the cities are older and were built before the advent of motorized transport, leaving in place infrastructure more conducive to non-motorized travel. For example, the proportion of workers who use bicycles as a means of transportation to work is 10 times higher in some European cities such as Copenhagen and Amsterdam than in North America (Figure 4).
Figure 4. Comparison of active bicycle transportation in various international cities. Adapted from Buehler and Pucher (2012). It should be noted that the data for Montréal are from 2006, so before the implementation of the bike-sharing system (Bixi). The proportion of bike users has increased since then to 3.2% in 2011, and Montréal is now ranked 18th cycling metropolis in the world.
To promote active transportation, we must therefore rethink what is called the “built environment”, i.e. the overall infrastructure that is part of our daily life, such as buildings, parks, schools, the road network, food sources (grocery stores, restaurants) or recreational facilities. Several studies have shown that people who live in cities where distances are reduced, streets are well connected, shops easily accessible and where areas for walking and cycling are well demarcated and safe are more physically active and in better cardiovascular health. It is therefore to be hoped that this type of built environment will become the norm in the near future.
It is well established that regular physical exercise improves lipid levels, glucose tolerance, and insulin sensitivity, all of which are cardiovascular risk factors. One question researchers have been asking in recent years is whether a single exercise session can, after a period of prolonged physical inactivity, have a positive impact on the risk factors associated with a sedentary lifestyle. This is an important issue as more and more workers are sitting for long hours at the office or in their car, and many of them do not have time to exercise more than once a week.
In a randomized controlled trial published in 2019 in the Journal of Applied Physiology, participants (n = 10) first spent four days without exercise, sitting for much of the day (≈13.5 hours/day). At the end of the fourth day, half of the participants did one hour of intense treadmill exercise (60–65% VO2max) and the other half remained inactive. On the morning of the fifth day, after fasting for 12 hours, all participants consumed a meal high in fat and glucose. Blood samples were taken before and every hour (up to 6 h) after the meal, and triglycerides, glucose and insulin were measured. After a rest period of several days, the experiment was repeated by swapping the groups (crossover study design).
No significant differences in plasma levels of triglycerides, glucose or insulin were found between the two groups. The authors conclude that prolonged physical inactivity (e.g., sitting about 13.5 hours/day and walking fewer than 4,000 steps/day) creates conditions where people become “resistant” to the metabolic improvements that are normally achieved after an aerobic exercise session. It is therefore important to develop good habits at work and at home (take active breaks, work standing, etc.), in order to fully benefit from the positive effects of exercise during leisure time.
A similar study published in 2016 came to comparable conclusions. The researchers randomized the participants into three groups: 1) sitting >14 h/day and a high-calorie diet; 2) sitting >14 h/day and a balanced diet; 3) active: standing, walking, sitting 8.4 h/day and a balanced diet. In addition to being randomized, the study had a crossover design, i.e., subjects participated in three five-day interventions (one week of rest between each intervention), changing groups each time. On the evening of the fourth day, the participants did treadmill running for 1 hour (67% VO2max). On the third and fifth day, participants consumed a high-fat meal (high-fat tolerance test) and blood tests were taken before and every hour (up to 6 hours) after the meal. Triglycerides, free fatty acids, glucose, and insulin were subsequently assayed in the plasma of the various samples collected.
After two days of sitting for long hours, participants had 27% higher triglyceride levels after consuming a high-fat meal, compared to participants who were more active. In participants who spent four days sitting for more than 14 hours, the 1-hour aerobic exercise did not decrease triglyceride levels in the blood or increase fat oxidation. On the other hand, in participants who were active during the previous four days, aerobic exercise decreased triglycerides by 14% (a non-significant decrease, p = 0.079) and significantly increased fat oxidation (p <0.05). The authors concluded that sitting for a good part of the day for 2 to 4 days was sufficient to increase postprandial triglyceride levels (after a meal) and that this increase cannot be reduced by sustained exercise.
A meta-analysis of 13 population studies assessed the ability of physical activity to eliminate or reduce the association between sitting time and all-cause mortality. These studies were conducted with more than 1 million people who were followed from 2 to 18.1 years. The results (Figure 1) show a very clear dose-effect relationship between the amount of exercise and the reduction in the relative risk of mortality associated with sitting or watching television. High levels of moderate physical activity (i.e., approximately 60–75 min/day) appear to eliminate the increased risk of mortality associated with long periods of sitting (Figure 1A). However, high levels of physical activity significantly alleviate, but do not eliminate the risk of mortality associated with long periods (> 5 h) spent watching television (Figure 1B).
Figure 1. Meta-analyses of joint associations of sitting time and amount of physical activity with all-cause mortality (A) and television-viewing time and amount of physical activity with all-cause mortality (B). 2.5 MET-h/week is equivalent to about 5 minutes of moderate activity per day; 16 MET-h/week is equivalent to 25–35 minutes of moderate activity per day; 30 MET-h/week is equivalent to 50–65 minutes of moderate activity per day, and 35.5 MET-h/week is equivalent to 60–75 minutes of moderate activity per day. From Ekelund et al., 2016.
The amount of physical activity in the highest quartile (> 35.5 MET-h/week) is equivalent to approximately 60–75 minutes of moderate-intensity exercise per day. This is much more than the minimum recommended by public health organizations (150 min/week). However, among those who do 16 MET-h/week, which is equivalent to 25–35 minutes of moderate-intensity physical activity, the increased risk of mortality associated with long periods of sitting (> 8 h/day) is less marked than for people in the least active group (<2.5 MET-h/week, equivalent to about 5 min of exercise per day). The increased risk of mortality (58%) for those who are less active and who sit more than 8 hours a day is similar to that associated with other risk factors such as smoking and obesity.
Why are the results for sitting time and television-viewing time different? This may be due in part to differences in the accuracy with which these behaviours are reported, but the authors of the study propose other plausible explanations: 1) people usually watch television in the evening, normally after dinner, and a prolonged sedentary episode after a meal could be particularly harmful to the metabolism of lipids and glucose; 2) people likely take more active breaks during work than during the viewing of TV programs, and it appears that these breaks are beneficial in reducing several cardiometabolic risk factors; 3) It is also plausible that people who watch television consume more “snacks” or obesogenic food.
Spending long periods of time in a sitting position is very common in our modern societies and this will only increase with future technological and social innovations. In addition to promoting regular physical exercise, public health organizations will likely also need to include a reduction in sedentary time in their guidelines and emphasize the importance of taking “active” breaks. Taking short active breaks may mean walking for a minute, drinking water, and getting up when talking on the phone. We must develop strategies to avoid sitting for long periods of time. In addition, we need to find the time to do at least 150 minutes of moderate exercise a week, up to 300 minutes a week to get maximum health benefits.