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If I were to ask you, why does cycling prevent disease, you would probably answer with some statement related to the fact that it makes you healthier. If you think about that answer though, you would realize that it is redundant or a circular argument because health is the absence of disease. I could then ask you again, how does cycling make you healthier? Perhaps then you would provide an answer such as it prevents obesity, heart disease or diabetes. However, that is still not very insightful. The question is really how does cycling (or exercise in general) help prevent disease? What is it about the act of moving that helps fend off disease?
Chronic diseases (heart disease, cancer, diabetes, etc.) are the leading causes of death and disability in the United States. In fact 7 of the top 10 causes of death in the US are chronic diseases. I often state that if shows like ER, Grey’s Anatomy, and House focused on what really put people in the hospital, no one would watch the show because it would be the same boring conditions over and over. It is no mystery that exercise helps prevent chronic diseases and can often be used to treat chronic diseases. Those interested in the exercise and chronic disease have even gone so far as to view exercise as a “drug” in that it should have a prescription of frequency and dose. If we know that exercise works, why doesn’t everyone do it? Well, that is a question for the behavioral psychologists to answer and not me. Many of us, however, are very interested in how exercise works physiologically so that those same mechanisms can be targeted by other means to slow the progression and enhance the treatment of chronic disease.
The answer to how exercise works to prevent disease is a complicated one because of the huge diversity of mechanisms. The diversity of mechanisms is what makes exercise so powerful – it has vast systemic effects at multiple points of control and regulation. For now, as a general overview, I will focus on one consideration, known as energy flux. Energetic flux is the movement of energy in and out of the body. Low flux is a rather stagnant system, while high flux is dynamically changing. Nutrition represents entry into the system and physical activity can be a large exit from the system. In the athlete, flux can be very high. In opposition, conditions such as obesity are characterized by low energetic flux. It is thought that humans evolved during a period when large changes in flux rates were common (e.g. periods of high and low food availability, migrations to follow animal food sources) and that these changes were responsible for the gene expression changes to keep one metabolically healthy. In other words, by not being physically active we are not triggering the pathways that evolved to keep us healthy.
Highly trained cyclists have some of the highest recorded human energetic flux rates. On the flip side, chronically low flux rates (i.e. sedentary behavior) are clearly related to the dramatic rise in obesity and chronic diseases. Low energy expenditure that is not matched by an equally low intake leads to energy surplus, fat storage and, ultimately, obesity. Although there are other important factors involved (genetics, epigenetics, environmental exposure, social factors), the links between obesity and chronic diseases are inarguable. In addition, it is now clear that low flux itself, in the form of inadequate exercise and/or excessive sedentary behavior (e.g. sitting), causes changes to physiology that dramatically increase the risk for chronic diseases independent of obesity. Decades of careful research suggest that many of the key physiological processes that mediate the effects of low flux rate/energy surplus on human disease are captured in several broad categories including (but not limited to)mitochondrial function, production and removal of reactive oxygen species and insulin sensitivity. These examples are some of the “how” exercise prevents disease.
I want to return to the concept that humans evolved during a period of time when food was scarce and we required physical activity to get food (while also trying to avoid being food). Because our bodies evolved for large changes in flux, some have argued that those who are active are actually the “normal” and those that are inactive are the “abnormal”. By this reasoning, if you were to design a scientific study related to human function, the active people would be the controls (what we are supposed to be) while sedentary people would be the experimental group. Sadly, that is not the case today since we know that inactive people far outnumber active. The fact that we study exercise is evidence that it is treated as the experimental, not the control.
What I provided here was just a broad overview of why exercise (cycling) contributes to a healthy state. In reality, the mechanisms behind this go far deeper and require additional columns to explain the intricacies of cellular signaling and tissue specific effects. Rest assured, that those of you who practice a healthy lifestyle, including exercise, are doing the best you can to fend of chronic disease. No pill or supplement can yet give you what exercise can.