You know that exercise can help prevent and treat breast cancer – but do you know how?
Guest Blog by: Bob Buresh, Associate Professor, Department of Exercise Science and Sport Management, Kennesaw State University
You’ve no doubt heard and read about the benefits of exercise in preventing and treating many of the chronic conditions that are plaguing our country – heart disease, obesity, diabetes, and, yes, several common forms of cancer. Many studies have shown that breast cancer is one form of the disease that is strongly associated with exercise. Nearly every investigation into the relationship between physical activity and breast cancer reports that exercise provides a protective effect, although the magnitude of the effect varies across studies. In general, studies suggest that exercise can reduce the risk of breast cancer between 20% and 80% in post-menopausal women, with a lesser effect in pre-menopausal women, and there appears to be a dose-response relationship, such that those who are more active acquire a greater degree of protection than those who engage in lesser amounts of activity.
Exercise is protective against most forms of chronic disease, largely because of the expectations of our genetics. We who live in the 21st century are walking around with a genome that is identical to that of our Paleolithic ancestors. In other words, very, very little “evolution” has occurred in our genetic machinery over the past 10,000 years. However, lifestyles have evolved enormously since Paleolithic times. Obviously, the epoch over which our genomes evolved was characterized by very active lifestyles among, mostly, hunter/gatherer cultures, and survival depended on a great deal of regular energy expenditure. By contrast, lifestyles in 21st century America are much less physically-demanding, and food availability is vast and constant. So, while our genetic machinery is unchanged from Paleolithic times, our lifestyles have “evolved” immensely, and we have reached a level of inactivity that our bodies are ill-equipped to manage. Remember, between 40-50% of your weight is comprised of skeletal muscle, the primary function of which is to move your body. We were never designed to be as sedentary as we are today, and the consequences include obesity, diabetes, heart disease, and several forms of cancer.
Exercise and Breast Cancer
There are several processes that are known to be linked with the development of breast cancer. Among them are 1) chronic, low-grade inflammation, 2) oxidative stress, and 3) insulin resistance.
Inflammation is a vital function of the immune system, involved in facilitating healing and removing infectious agents from the body. Inflammation is a process that our very lives depend on. However, sedentariness contributes to an environment in our bodies that is conducive to a constant, “simmer” of inflammation, and this “chronic, low-grade” inflammation has been shown to be involved in the development of a number of diseases, including breast cancer.
One source of inflammation is enlarged adipocytes – fat cells. As fat cells grow, at some point they begin to release a number of substances that are designed, in part, to recruit more fat cells. Each fat cell can store only so much, and as they approach their limits, they start sending out signaling substances that can result in the formation of new fat cells. Some of these substances also induce inflammation.
Another source of inflammation is sedentary muscle! It has long been known that active people exhibited lesser amounts of low-grade inflammation than their sedentary counterparts, but a fascinating discovery several years ago offered a partial explanation – exercising muscle releases a protein called IL-6, and this protein, when it is produced at levels consistent with exercise, appears to result in lower levels of inflammation throughout the body.
Regular exercise also appears to “reprogram” your immune cells, making them less inflammatory to any given stimulus. In other words, regular exercise makes your entire immune system less inflammatory, and consequently, less conducive to cancer initiation and growth.
You’ve perhaps heard the term, “free radicals.” Radicals are wildly reactive molecules that are capable of damaging body tissues by removing an electron from the molecules that make them up. Fat, protein, and even DNA can be targeted by radicals, and especially if the target is the latter, cancer can result.
We all produce radicals, as a by-product of being oxygen-consumers. As long as we are able to neutralize them with the antioxidants in our bodies, radicals are really just normal biological intermediates, and they even serve important roles as signaling molecules, meaning that they promote healthy adaptations to exercise and other stressors. However, when the radicals that we produce consistently “outnumber” our antioxidant defenses, we exhibit a condition called “oxidative stress,” and in that condition radicals are abundant, and are capable of doing a great deal of damage. Studies report that oxidative stress is not heavily involved with tumor growth, but is strongly associated with initiation of cancers.
So, how does exercise affect oxidative stress? Interestingly, it appears that exercise is capable of creating an environment that is very “radical-rich,” and our bodies respond to that by producing many antioxidants. So, regular exercise is a stimulus for producing more antioxidants, and if one never exercises, the size of the “antioxidant army” is never forced to increase, and oxidative stress is far more likely to develop.
It is also known that the mitochondria (the “power plants” in our muscle fibers) are more stressed when they are inactive. This stress is related to their function – mitochondria are responsible for producing energy, and if there’s no energy to produce because no energy is being consumed (e.g., as in one who is always inactive), they become stressed, and produce more radicals per unit of oxygen consumed than do mitochondria in exercising muscle. So, doing some regular exercise each day results in an increase in antioxidant production, while it also reduces the proportion of oxygen that is “radicalized.” Exercise, therefore, is a uniquely powerful antioxidant.
You may have heard of insulin resistance as it relates to diabetes, and it is certainly true that resistance to insulin is the most common “pathway” from healthy metabolism to diabetes. However, insulin resistance is also implicated in cancer, in that insulin, in addition to stimulating reductions in blood sugar, is an anabolic hormone – that is, it acts to stimulate tissue growth, even if that tissue is a tumor.
When you eat any carbohydrates, your body breaks them down into simple sugars and distributes them throughout the body in the blood so that those sugars can be taken up and used as fuel, or stored for later use. Insulin is the hormone that is responsible for signaling muscle cells to take sugar (glucose) out of the blood. In healthy people, a little insulin results in a rapid reduction in blood glucose. However, in metabolically impaired people, it takes much more insulin to lower blood glucose, so insulin levels stay higher, longer, and this can foster tumor growth.
Exercise can lower insulin levels in several ways. When you exercise, your muscles take glucose from the blood as a source of fuel, and do so without requiring insulin. This means that insulin levels can remain lower while blood glucose is successfully managed. In addition, exercise training can improve insulin sensitivity in a number of ways – by decreasing body fatness, increasing mitochondrial density and function, and lowering inflammation and oxidative stress (see above!).
The bottom line is that you were made to be active! Almost half of your body mass is skeletal muscle, and the implication is that your physiology expects you to move! Your genome was designed for regular, substantial doses of exercise, and chronic sedentariness is a lifestyle that your genetic machinery is not equipped to deal with. Some of the consequences of not exercising are elevated inflammatory activity, oxidative stress, and insulin resistance. If you’re sedentary, there is nothing – NOTHING – that you can do that will yield a greater increase in protection against a host of chronic diseases, including cancers of the breast and endometrium, than becoming more active.
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