Are inactivity and exercise points on a continuum?Over the last couple of decades, researchers have been trying to identify whether being inactive and exercising are merely points on a continuum. If you are inactive for most of the day, can adding in some moderate to vigorous physical activity average out that inactivity and improve the negative effect you get from sitting all day long? Most of the epidemiological data shows that exercising is healthy and reducing sedentary time is healthy, but are they both just manipulation of the same physiological response?
The problem with using epidemiological data to determine something like this is that when someone engages in one form of health promotion such as exercise, they are more likely to engage in other activities that are health promoting and avoid activities that are health compromising. In other words, someone who exercises is more likely to also pay attention to their diet and avoid smoking when compared to someone who doesn't exercise at all. This causes the data to be corrupt because you really can't determine from a questionnaire whether or not the healthy person is seeing an effect from a single healthy activity or from their many healthy activities. This is why you typically see conflicting data from study to study.
It becomes very difficult to figure out what is healthy when you get so much conflicting data, such is the inherent risk of relying on epidemiological data. A great case in point is that this study shows that sedentary time is associated with the metabolic syndrome independent of exercise, this study shows that exercise but not sedentary time increases the risk of Type 2 diabetes in at risk people, and this study shows that sedentary time is a better predictor for Type 2 diabetes than exercise time. If that doesn't make your head spin, those last 2 articles were published a month apart in the exact same journal and essentially say the polar opposite of one another.
Current research suggests a difference between inactivity physiology and exercise physiologyWhile there is conflict in the epidemiological data, the bulk of epidemiological evidence coming out now point to exercise and reducing sedentary time both being important for health, but for different reasons(1). In other words, moderate to vigorous exercise does not offset sitting down all day because they work through different channels. This has directed researchers to observe the effects of sitting time and exercise on how genes are expressed in animals, which has confirmed that there is a difference between exercising and reducing sedentary time, particularly with the way fats are metabolized in the body(1, 2, 3, 4). Further research in humans has confirmed these results and identified a few other potentially important cellular responses to high levels of sedentary time.
The amount of time people spend being sedentary has been associated with poor health and obesity, primarily through higher levels of inflammation(5, 6). As mentioned in Evidence against the Western Lifestyle, three conditions that are associated with every chronic disease are chronically high levels of inflammation, high blood glucose levels, and high levels of oxidative stress. Since the epidemiological data is conflicting but animal studies have shown that inactivity physiology and exercise physiology are different, studies in humans have been conducted to determine what happens when humans sit for prolonged periods of time.
Inactivity and changes in gene expressionOne of these studies found that sitting for prolonged periods of time without getting up induced changes in gene expression that negatively impacted glucose regulation as well as antioxidant and anti-inflammatory pathways(7). When people broke up these periods of inactivity by getting up and walking around, a positive response in gene expression was seen. Another study looking at the effect of loading one leg while causing the other leg to be unloaded showed that the expression of genes in each leg was different, with the unloaded leg showing the less desirable pattern of gene expression(8). This poor pattern of gene expression persisted even once the unloaded leg was reloaded again.
Since these studies are not comparing people who exercise but are mainly sedentary to those who don't exercise but are on their feet most of the day, they cannot be used to say that there is a significant difference between the two conditions. At this point there is no perfect study such as that to prove one way or another whether exercise makes up for being seated all day long. However, there is evidence that reducing sedentary behavior and performing exercise lead to different physiological outcomes.
Lipoproteins in inactivity and exerciseA study looking at blood lipids found that reducing sedentary time improved VLDL-P, LDL-P, and triglycerides while exercise had no effect. Exercise had an effect on HDL-P, HDL size, Apo A1, and total HDL-C while reducing sedentary time had no effect(9) on these variables. All of these variables are related to how fat and cholesterol are carried in the blood and metabolized, and this evidence points to there being distinct benefits from reducing sedentary behavior and performing exercise that do not overlap with one another. Another study backed up this evidence by showing that 1 hour of exercise did not compensate for the negative changes in glucose handling and blood lipids caused by sitting the rest of the day despite both groups burning the same total number of calories(10).
Another study that compared the effects of sedentary time to exercise found that levels of interleukin 6(IL-6) were affected by reducing sedentary time independent of whether the person exercised or not(11). This is an interesting finding because high levels of IL-6 are a very strong risk marker for the diseases associated with chronic levels of inflammation(12).
ConclusionThese studies point to there being a difference between inactivity physiology and exercise physiology. On top of their effect on overall health, high levels of inflammation and altered lipid metabolism will also negatively impact your ability to burn fat. Many people like to push exercise as the end all be all in fat loss, but these studies suggest otherwise. There is also a fundamental flaw in the logic that 1-2 hours of exercise 5-6 days per week can make up for being sedentary all of the time. Sitting down all day long and running or strength training for a couple of hours should not be considered the default condition because it is not the condition we evolved in and were selected for. The condition we evolved in would have included very little sitting time, walking 5-7 miles per day, and intermittent bouts of intense to moderately intense exercise when necessary. While just a couple of years ago it was believed that sitting down all day long and exercising were merely a continuum, the bulk of the evidence out today points to reducing sedentary time and exercising being physiologically different from one another. This does not mean one is more important than the other, but if you are doing one or the other and not seeing the results you would like to see from your wellness plan, start working on the other variable.
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