Exercise is usually seen as a way to strengthen the body or prevent disease, but it actually fits the model of complementary and alternative medicine treatments (CAM). In the past some students have reacted negatively to my characterization of exercise as a form of CAM, thinking that perhaps “complementary” meant “less good than,” and I acknowledge that in some medical care settings the term does have this meaning. Here is my reason for thinking of exercise in this way: Exercise is different from other medical interventions because it involves the patient doing some activity rather than having something done to them. Even more fundamentally, it differs in that it is intended to produce broad changes in functioning rather than more constrained or system-specific ones. Besides being a source for a broad spectrum of changes, exercise is also a behavior that people may or may not engage in. Exercise is therefore often seen as the dependent variable in psychological intervention studies, but as the independent variable in physiological ones.
The idea that exercise is a form of medical treatment goes back to antiquity. For instance, Indian physicians recommended daily exercise for good health as early as 700 BC, and Chinese medicine noted its benefits even earlier. Greek and Roman physicians recommended exercise from the time of Herodicus (500 BC), who was the teacher of the better-known Hippocrates (460-370 BC, who formulated the Hippocratic Oath to "first, do no harm"). The Greek physician Galen (130-210 AD), whose "four humors" theory of medicine dominated treatment during the Middle Ages, noted the benefits of vigorous activity in particular.
Exercise has a very wide range of benefits for the body and the mind (summary below is from Lavin et al, in press). In fact, it’s probably more appropriate to ask what physical systems exercise doesn’t affect. Some of the best-studied benefits are cardiovascular, increasing overall heart and lung size while also making the heart work more efficiently. These benefits can be seen with simple physiological measures using at-home sensor devices, including improved heart rate variability, lower resting heart rate, and more efficient oxygen consumption (VO2 max).
All of these changes in heart and lung physiology translate into a lower chance of cardiovascular disease with aging. In a 2013 study, exercise was found to produce greater reductions in mortality than the best-available classes of medication for patients with stroke disorders, and was just as effective as drug interventions for patients with coronary heart disease and diabetes. The treatment of heart failure was the only exception, where diuretic medication was more effective. Besides stroke prevention, exercise is effective in reducing blood pressure, preventing coronary heart disease, and protecting against heart failure.
Building muscle tissue is usually noted as the other main physical benefit of exercise: It leads to increases in the size and strength of skeletal muscles. It also increases the muscle tissue's responsiveness (i.e., speed of movement) and reduces the amount of fatigue when the same effort is made over time. Dramatic muscle changes are more often seen in men than in women, however. In women, the main benefits of exercise may be to prevent a sharp drop in bone and muscle mass at the time of menopause, which can lead to increased risk of falls, fractures, and general frailty. Specific benefits have been noted for the treatment of musculoskeletal diseases like osteoarthritis, rheumatoid arthritis, and osteoporosis.
A less-commonly noted benefit of exercise involves the endocrine system, where exercise can reduce the production of stress hormones and inflammatory compounds, improve digestive processes, and enhance neurotransmitter activity. People who exercise regularly are better able to regulate their blood glucose levels, which prevents complications associated with diabetes. Even when exercise doesn't directly reduce weight or prevent obesity, it improves insulin sensitivity, maintains muscle mass, and prevents complications like heart disease, high cholesterol, and diabetes among people who are overweight.
On the mental health side of the ledger, exercise alone is just as effective a treatment for depression as either psychotherapy or antidepressant medication. It's not just a preventative treatment for mild depression, either: A recent meta-analysis found an effect size of about 3/4 of one standard deviation for exercise's effect on symptoms among patients diagnosed with major depressive disorder, which is approximately the same as the benefit produced by the best available psychotherapeutic modalities or antidepressant drugs. That's a clinically meaningful impact for people with a genuinely debilitating mental health condition. Additionally, exercise has few risks and many positive side effects like better heart and lung functioning, which should lead us to routinely recommend it as a first-line treatment for depression. These mental benefits of exercise are likely at the Intuitive-system level, affecting how we perceive the world, our immediate reactions to stressful stimuli, and our ability to calm down after a stressor. One possible idea is that better cardiovascular efficiency simply makes us react less strongly to stressors, and our bodies interpret that lack of a strong reaction as a feeling of calm! But there are other possible mechanisms for exercise's mental health benefits, including things like the production of oxytocin (a "bonding" or "calming" hormone) or increased utilization of serotonin (a "happiness" hormone) by the brain. Or it could be simply that feeling better physically (e.g., less pain or fatigue) makes us likely to feel better mentally as well.
One interesting hypothesis is that exercise stimulates the growth of new neurons in adults, which may facilitate its antidepressant effects. Perhaps for this reason, people who exercise regularly are less likely to develop Alzheimer's or Parkinson's diseases than people with similar risk factors who don't exercise. Movement therapies and martial arts (e.g., tai chi, qi gong, yoga) tend to focus on balance and range of motion, in addition to physical conditioning as in other forms of exercise. But these practices also may differ from other forms of exercise when the training incorporates philosophical elements associated with mindfulness. For example, tai chi was examined in a small-scale pilot RCT with older nurses, and showed statistically significant benefits for work productivity and reduced absence rates, not just physiological benefits.
Exercise also can reduce pain and directly improve functional ability, as indicated in current clinical practice guidelines for the treatment of pain in osteoarthritis. In cancer treatment, exercise is used to treat symptoms like pain and fatigue, and for its mental health benefits. Emerging areas of research are focused on whether exercise could also improve outcomes when used in combination with anti-cancer drugs, for example by slowing the growth of tumors or preventing cancer recurrence. Overall, exercise can prevent or delay people's loss of functioning as they age, leading to more independence, lower health care costs, and biological functioning that looks "younger" than one's chronological age.
The exact mechanisms by which exercise achieves some of these benefits are still not clearly known, although we can be relatively certain about the cardiovascular changes. Current research is looking at changes in protein production or metabolism by cells in the heart, muscles, gut, and brain, as well as changes in gene expression (i.e., epigenetics, in which some genes are "turned on" or "turned off" to produce various effects). Aerobic and resistance forms of exercise likely have different effects on various body systems, although both have shown positive effects. Some day, we might imagine that these lines of basic science research could produce some of the benefits of exercise without someone having to actually exercise! In the meantime, however, exercise is probably the single best thing that can be done to improve overall health. It has many well-documented benefits for the body and the mind, and likely has additional benefits in areas that we are not even aware of.
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