Overview
One of the most obvious differences between modern life and life in the past for humans can be found in the level of physical exercise. For the majority of people living in developed countries today, heavy physical exercise does not occur as a part of ordinary daily life but must be deliberately sought out. Compare this to most of human history, in which heavy daily exercise was a requirement for survival. Even among the upper classes in nineteenth-century Europe, going for a ten- to twenty-mile walk by way of recreation was not unusual.
The human body was designed to use its physical capacities. However, modern life has become a sedentary affair, in which “exercise” involves moving from couch to car to office cubicle. While decreasing strenuous exercise does have some benefits, such as reducing injuries, it also presents major drawbacks. Inadequate exercise is a major contributor to the current epidemic of obesity, which in turn leads to diabetes, heart disease, and osteoarthritis.
Conversely, increasing one’s level of exercise provides a wide variety of benefits. Besides enhancing strength and endurance and improving physical attractiveness, exercise is thought to enhance overall health and to reduce symptoms in a number of specific ailments. However, while the many benefits of exercise appear self-evident, they can be quite difficult to prove in a scientific sense. The primary problem is that it is difficult, if not impossible, to design a double-blind study of exercise.
In a double-blind, placebo-controlled study, neither participants nor researchers know who is receiving a real treatment and who is receiving a placebo. Consider the following scenario: a study (technically, an observational or epidemiological study) may note that people in a given population who exercise more develop heart disease at a lower rate than those who exercise less. From this, it is tempting to conclude causality: that exercise reduces heart disease risk. However, such a conclusion might not be correct.
Observational studies show only association, not cause and effect. Studies of this type had long shown that women who used hormone replacement therapy (HRT) were less likely to develop heart disease. Furthermore, the use of HRT was known to improve one’s cholesterol profile. It seemed like an obvious case. However, to researchers’ surprise, when a giant double-blind study compared hormone replacement therapy with placebo, the results showed that the use of HRT actually increased heart disease risk.
It is now hypothesized that this apparent contradiction may be due to the fact that women who use HRT are generally of higher socioeconomic status than women who do not use HRT, and that it is this socioeconomic status, and not the HRT, that was responsible for the apparent benefits seen. Whatever the reason, it is now clear that HRT does not prevent heart disease, and that the conclusions drawn from observational studies were exactly backwards. Based on this, one must at least consider the possibility that people who engage in more exercise have other qualities that protect them from heart disease, and that it is these qualities, and not the exercise, that protects them. The problem here is that while it is possible to give a placebo that convincingly resembles HRT, it is difficult to conceive of a placebo form of exercise that participants and researchers would not immediately identify as different from real exercise.
Besides observational studies, other forms of scientific research involving exercise remain similarly inadequate. For example, numerous studies have attempted to prove that exercise is helpful for depression. In these studies, people who are made to exercise improve to a greater extent than those who are not interfered with. However, this finding does not prove that exercise per se aids depression. It might be, for example, that simply being enrolled in a study, and being motivated to do anything at all, might aid depression. (This suspicion is given further weight by findings that improvement in depression is not related to the intensity of the exercise done; if it were the exercise itself, one would think that more intense exercise would provide greater benefits.)
Double-blind, placebo-controlled studies eliminate all of these potential confounding factors and many others. However, it is not feasible to design a double-blind study in which people are unaware (“blind” to the fact) that they are exercising. Therefore, all results regarding the potential benefits of exercise must be taken with caution.
Scientific Evidence
The benefits of exercise with the most solid scientific foundation include the
following: preventing falls in the elderly, slightly reducing blood pressure,
mildly improving cholesterol profile, enhancing survival in people with heart
disease, and improving metabolic syndrome. Regarding
blood
pressure, aerobic exercise has the best supporting evidence,
but resistance exercise (weight training) has also shown promise. One study found
that four ten-minute “snacks” of aerobic exercise per day were as effective at
lowering blood pressure as forty minutes of continuous exercise. Aerobic exercise
can also raise levels of HDL (good) cholesterol and reduce levels of
triglycerides.
Other conditions for which exercise has some meaningful supporting evidence of
benefit include asthma, depression, type 2 diabetes (improving blood sugar
control, even in the absence of weight loss), fibromyalgia,
and, osteoarthritis. Regarding osteoporosis, the general scientific
consensus is that exercise does help, but the supporting evidence is surprisingly
weak.
Inconsistent or otherwise weak evidence suggests potential benefit for back pain, chronic fatigue syndrome, cognitive impairment (mild dementia), colon cancer prevention, insomnia in the elderly, stroke prevention, and weight loss. It is widely believed that exercise improves immune function, but there is no meaningful supporting evidence for this belief. High- intensity exercise (such as marathon running) is known to temporarily weaken the immune system, increasing the likelihood of respiratory infection.
Evidence conflicts on whether exercise is helpful for reducing menopausal symptoms. However, it is known that heavy exercise causes increased calcium loss through sweat, and the body does not compensate for this by reducing calcium loss in the urine. The result can be a net calcium loss great enough to present health concerns for menopausal women. One study found that the use of an inexpensive calcium supplement (calcium carbonate), taken at a dose of 400 milligrams twice daily, is sufficient to offset this loss.
Bibliography
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Elavsky, S., and E. McAuley. “Physical Activity and Mental Health Outcomes During Menopause.” Annals of Behavioral Medicine 33 (2007): 132–42. Print.
Elley, R., et al. “Do Snacks of Exercise Lower Blood Pressure?” New Zealand Medical Journal 119 (2006): U1996. Print.
Fagard, R. H. “Exercise Is Good for Your Blood Pressure: Effects of Endurance Training and Resistance Training.” Clinical and Experimental Pharmacology and Physiology 33 (2006): 853–56. Print.
Hauer, K., et al. “Effectiveness of Physical Training on Motor Performance and Fall Prevention in Cognitively Impaired Older Persons.” American Journal of Physical Medicine and Rehabilitation 85 (2006): 847–57. Print.
Larun, L., et al. “Exercise in Prevention and Treatment of Anxiety and Depression Among Children and Young People.” Cochrane Database of Systematic Reviews (2006): CD004691. Available through EBSCO DynaMed Systematic Literature Surveillance at http://www.ebscohost.com/dynamed.
Martin, B. R., et al. “Exercise and Calcium Supplementation: Effects on Calcium Homeostasis in Sportswomen.” Medicine and Science in Sports and Exercise 39 (2007): 1481–86. Print.
Thomas, D. E., E. J. Elliott, and G. A. Naughton. “Exercise for Type 2 Diabetes Mellitus.” Cochrane Database of Systematic Reviews (2006): CD002968. Available through EBSCO DynaMed Systematic Literature Surveillance at http://www.ebscohost.com/dynamed.
Van Uffelen, J. G., et al. “The Effect of Walking and Vitamin B Supplementation on Quality of Life in Community-Dwelling Adults with Mild Cognitive Impairment.” Quality of Life Research 16 (2007): 1137–46. Print.
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