Obesity: A Cause or Consequence of Physical Inactivity?

obesityPhysical inactivity makes you fat, right? If you ask a dozen people on the street whether they agree with this supposition, the vast majority, if not everyone, will probably say that they do. If you then go on to ask why individuals who carry around a lot of excess body fat tend to exercise less than those who are lean, many respondents will likely say that it’s because the former are lazier and have less willpower and self-discipline.

These answers align with the conventional wisdom regarding exercise and obesity that’s imprinted in the public’s mind, and are based on the assumption that the road connecting Physical Activity Levels (PALs) with obesity is a one way street, in the sense that physical inactivity is a cause of obesity, not an effect/consequence.

But is this actually true?

A chicken and egg situation

Studies have shown that obesity is associated with physical inactivity (1, 2) Many people thus assume that physical inactivity causes weight gain and obesity, a seemingly logical conclusion.

What is important to remember, though, is that these studies aren’t able to establish a cause-effect relationship; they only show a correlation between two variables. In other words, it could be that physical inactivity is an effect of obesity, not a cause, or perhaps both a cause and an effect. It could also be that there is no causal connection between the two, but rather that other variables are confounding the relationship. Since there are so many factors to consider, it’s important to be cautious when we interpret the results of these types of studies.

That said, I don’t want it to sound like I don’t think physical activity and body fat regulation have anything to do with each other. Actually, I would say that there is solid evidence to show that physical inactivity can contribute to weight gain. By itself, a lack of exercise is not going to be sufficient to induce obesity; however, when it’s combined with other factors, such as a highly processed diet and disordered sleep, it can certainly contribute to making your body mass index creep over 30, which is defined as the threshold for obesity.

The weight of the evidence from systematic reviews and meta-analyses suggests that exercise by itself doesn’t make you lose a lot of weight (some people respond better than others) (3, 4, 5, 6). This isn’t necessarily surprising, given that many exercisers compensate for their exertions by eating more and/or being more sedentary outside the exercise period (4).

However, that doesn’t mean that exercise doesn’t have any role to play in the prevention and treatment of obesity. Regular exercise can help improve leptin and insulin sensitivity, increase lean muscle mass, improve appetite regulation, and elevate resting energy expenditure, among other things. It therefore shouldn’t come as a surprise that several studies have found that exercise can aid in the prevention of obesity (1, 7).

A case of reverse causality?

As explained in the previous section, many lines of evidence indicate that physical inactivity can factor into weight gain. However, as we’ll now see, there’s little doubt that it can happen the other way around as well, in the sense that physical inactivity can occur, in part, as a result of the accumulation of body fat.

This may seem counterintuitive to many people, as the general perception among the public is that the reason some people choose to exercise more than others is simply that they possess more willpower and discipline; it has nothing to do with how much body fat they carry or how their body is functioning.

This simplistic belief has caused a lot of heartache for a lot of people, because it has led many who are overweight to think that the reason they find it so hard to get off the couch and into the gym is that they are lazy and have a weak mind. This can then contribute to feelings of poor self-confidence and depression; particularly if they hear other – often leaner – people make mocking comments about the laziness of the obese.

Let’s be clear: willpower and self-discipline are fundamental to long-term adherence to an active lifestyle. However, a truth that eludes many folks is that these qualities aren’t fixed traits determined by genetic lottery; they can be learned and strengthened. Also, a lot of people are ignorant of the fact that our ability and will to exercise, as well as the enjoyment we get out of physical training, are determined not only by psychological factors, but also by biological ones. I strongly suspect that they key reason many obese people find exercise overly difficult and strenuous is that they are chronically inflamed, with compromised immunity and metabolism.

Chronic fatigue, physical inactivity, and suboptimal physical performance: Is inflammation the culprit?

Obesity is characterized by elevated levels of circulating inflammatory cytokines, a condition referred to as chronic low-grade inflammation. This inflammatory state is generally considered to arise as a result of obesity, due to the fact that fat tissue releases many inflammatory mediators.

However, we now know that this internal fire can also be a cause of obesity, in the sense that disruption of the gut microbiota and inflammation –resulting from factors such as antibiotic use, consumption of highly processed foods, and translocation of bacterial endotoxins from the gut into systemic circulation – set the stage for insulin and leptin resistance, impaired appetite regulation, cravings for unhealthy foods, overeating, and fat gain (8, 9, 10, 11) It’s a vicious cycle in which a perturbation of the microbiota and immune homestoasis leads to excessive caloric intake and fat gain, which then further exacerbate the inflammatory process.

Chronic low-grade inflammation is tightly linked with chronic fatigue and many other conditions that impair our physical performance and exercise tolerance (12, 13, 14). When the body is in an inflamed state, it doesn’t prioritize reproduction or athletic performance; rather, it allocates resources to cleaning up the mess. It doesn’t “want” to run, lift heavy things, or perform other activities that put a lot of stress on its musculoskeletal system; it prefers to take it easy, so it has a chance to recuperate.

The problem is that in obesity, the inflammatory cascade never stops. Fat tissue keeps pumping out TNF-alpha and other cytokines, and lipopolysaccharide continues to leak from the gut into the bloodstream unless steps (e.g., diet changes, manipulation of the microbiota) are taken to stop the deleterious process. Until we address the inflammation, physical activity will continue to feel like a chore.

I want to make it clear that although many lines of evidence indicate that something along these lines is what’s going on, there may be other yet unidentified factors involved. It could also be that we have some of the mechanisms wrong. What I am sure of however, is that the bodies of obese people don’t like to be in the gym.

Exercise shouldn’t feel like torture

I worked as a strength coach/personal trainer at a commercial gym for several years. During this period I spent quite a bit of time observing how people exercise, as well as their body language and the feelings they expressed when they were lifting weights, running, or doing other activities that stress their muscles. One of the things I noticed – which I’m sure a lot of other people have observed as well – is that people who carry a lot of fat mass in general seem to find it a lot harder to exercise than those who are lean.

For some, a session on the treadmill looks to be synonymous with being in a torture chamber. Of course, this may be due partly to the discomfort of carrying excess weight; however, I don’t think that’s the main reason. Rather, I think the aforementioned processes related to inflammation and perturbations of homeostasis may be the real culprits.

Keep in mind that chronic, systemic inflammation doesn’t just affect obese people. Actually, this condition is extremely common in our society today, and is at the root of many chronic diseases and health disorders. Even lean people, if inflamed, may be exercise-intolerant. Personally, my urge to exercise, as well as my physical performance, seem to vary with the levels of inflammation in my body.

A new understanding of an old problem

Many, if not most, of the studies that have investigated the relationship between PALs and obesity don’t show which came first, physical inactivity or obesity. However, there are some studies out there that have been able to connect the two in a causal way.

One example is a study out of the University of California, Los Angeles that was published in the journal Physiology and Behaviour (15). In the study, 32 female rats were allowed ad libitum access to unrefined rodent chow or a purified low-fat diet. After 6 months, the rats on the purified low-fat diet, which was high in sugar and highly processed foods, had gained significantly more weight than the rats on the diet of unrefined rodent chow.

When the researchers tested the rats’ performance by giving them a task in which they were required to press a lever to receive a food or water reward, they found that the rats on the junk food diet demonstrated impaired performance, taking substantially longer breaks than the lean rats before returning to the task. During a 30-minute session, the overweight rats took breaks that were nearly twice as long as the lean ones.

Lead researcher Aaron Blaisdell, in a press statement, framed the study results as follows:

“Overweight people often get stigmatized as lazy and lacking discipline,” Blaisdell said. “We interpret our results as suggesting that the idea commonly portrayed in the media that people become fat because they are lazy is wrong. Our data suggest that diet-induced obesity is a cause, rather than an effect, of laziness. Either the highly processed diet causes fatigue or the diet causes obesity, which causes fatigue.”

Blaisdell believes the findings are very likely to apply to humans, whose physiological systems are similar to rats’. (16)

Key points

Obesity can be both a cause and consequence of physical inactivity. Physical inactivity is associated with impaired sensitivity to hormones involved in metabolism and satiety, loss of lean muscle mass, and a reduction in total energy expenditure. It’s therefore not surprising that studies have found that physical inactivity can cause weight gain – and in combination with other factors, obesity. Physical inactivity can also be an effect of obesity, in the sense that the inflammatory state and metabolic disruption that accompany obesity set the stage for chronic fatigue, muscle weakness, impaired physical performance, and poor exercise tolerance.

It’s a vicious cycle: physical inactivity contributing to fat gain, leading to overweight and secretion of inflammatory cytokines, both of which may drive further reduction in physical activity levels.

The next time you see or talk to an obese person who finds it very difficult to adhere to a training program over the long term and seems to hate to exercise, you may want to think twice before jumping to the conclusion that he’s lazy and lacks discipline. Instead of making that simplistic assumption, offer him some good advice regarding how he can combat inflammation and lose weight in a healthy way.

Picture: Creative Commons picture by Marjan Lazarevski. Some rights reserved.

Comments

  1. It’s easy to understand it from a commercial standpoint. If diet is not perceived as the causal factor of disease, people go on with feeding the junk food business, and even fitness market may have some interest to put exercise as the primary way to get lean and fit. And things go on into the face of evidence.

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