Among the early changes seen when individuals engage in an aerobic training program are mood elevation, heightened energy levels, enhanced self-confidence and self-esteem, lower anxiety levels, resistance to depression and improved coping ability. Changes in blood pressure and heart rate, which are, to a large extent, mediated by the central nervous system, occur soon afterward. Heart rate is slowed, and hypertensive blood pressure (systolic and diastolic) is reduced toward normal.

These physiological changes are a function of the rebalancing of the sympathetic (fight and flight) and parasympathetic (rest and repair) halves of the autonomic nervous system. A few studies have found increased parasympathetic tone in aerobically trained subjects, and ascribe the slowing of heart rate and reduction in blood pressure to this increased tone. Others have recorded lower plasma catecholamine levels associated with lower blood pressure following aerobic exercise. Resting heart rate is largely controlled by the parasympathetic fibers of the tenth cranial nerve (vagus) to the heart's pacemaker (SA node). But blood pressure is much more complex, and more body chemistry, especially hormonal chemistry, is involved. The bottom line is that aerobic exercise reduces hypertensive blood pressure, and that the response is distance/intensity-graded.

Returning to the neurotransmitter connections with exercise, higher levels of serotonin and dopamine have been recorded following aerobic exercise training. These would account for the mood elevation and antidepressant effects of regular aerobic exercise. Keep in mind that changes in GABA, endorphins and other neurotransmitters may well contribute to these psychological effects. There have been improvements in the physical capabilities of Parkinson's disease patients following six to eight weeks of aerobic walk training. (Dopamine levels are commonly low in people with Parkinson's disease.) In one patient, a 69-year-old female, anti-Parkinson medication was discontinued after aerobic training levels were achieved.

Also related to dopamine changes, some cigarette smokers can quit with few, if any, signs and symptoms of withdrawal. Ordinarily, nicotine addiction is difficult to break because high dopamine levels drop precipitously upon smoking cessation. Aerobic exercise can also elevate dopamine levels, and cases of smokers who quit easily may be taken as initial evidence that aerobic exercise can prevent a drop in dopamine with smoking cessation.

Continuing in the realm of psychological effects, a number of cognitive improvements have been documented in older adults who train aerobically. These include quicker mental reaction time and improved fluid intelligence quotients. It has been proposed that such changes may be the result of improved acetylcholine levels. Acetylcholine is a universal nerve transmission chemical in both the brain and somatic nerves. If acetylcholine is responsible, aerobic exercise should also benefit Alzheimer's disease, which exhibits chronic acetylcholine depletion.

At the base of the brain is the small pineal gland, which releases melatonin, a hormone that influences such widely diverse functions as sleep-wake cycles and immune system integrity. The production of melatonin, related chemically to serotonin, is upset when people travel across several time zones. A marked reduction in jet lag can be achieved when an exerciser's training schedule is optimal for frequency, distance and intensity.

Thyroid and parathyroid glands. The next stop in the body is the neck, where the thyroid and parathyroid glands are located. The thyroid controls metabolic rate, and the parathyroids are involved in calcium metabolism. Metabolic rate is also influenced by aerobic exercise, and calcium metabolism by both aerobic and resistance exercises.

Lungs. Chronic obstructive pulmonary disease (COPD) results from years of exposure to particulate and chemical pollutants in the air. The result is breathlessness (dyspnea) with mild to moderate physical exertion, and reduced functional respiratory volume. There is less elasticity of the air sacs and of the entire chest wall. Aerobic training results in less dyspnea and increased respiratory capacity.

Another chronic respiratory disease is asthma, but asthma, with its three components of allergy, inflammation and anxiety, is more complex. Asthma is characterized by constriction of the bronchioles, the smallest tubular passages before the air sacs, and expiratory wheezing. Asthmatic distress has been widely noted in exercises of shorter duration and higher intensity.

Occasional asthmatic individuals on medication have participated in aerobic walk training programs I have instructed. I have observed the medical progress of eight such individuals as they reached and maintained aerobic levels of exercise. Without exception, they reported reduced incidence and severity of symptoms, and less need for bronchodilator medication.

Heart and blood vessels. The working muscle of the heart, the myocardium, is structurally and functionally different than the voluntary muscles used for movement. Heart muscle looks different under a microscope, uses a different mix of biochemical energy cycles and responds to exercise differently. One thing that the myocardial and somatic muscles have in common in response to aerobic exercise is an increased blood supply. Even in coronary heart disease, where one (or more) of the coronary arteries is partly blocked by lipid deposits, aerobic exercise, in combination with a low-fat diet, results in increased opening of the blocked vessel(s).

Without going into what is known about the complex biological mechanisms involved, here are some heart benefits of aerobic exercise: regularity of heart beat at a slower rate; improvement of blood lipid factors (decreased total cholesterol, low-density lipoproteins and triglycerides, and increased high-density lipoproteins); diminished atherosclerosis of coronary and carotid arteries; increased stroke volume; greater total blood volume with decreased viscosity; decreased platelet aggregability; and increased blood flow to cardiac and somatic musculature on physical effort.

Gastrointestinal tract. For the gastrointestinal tract, exercise shortens transit time for food as it enters the stomach and then passes through the colon and rectum. The reduced incidence of colon cancer is doubtless a consequence of decreased transit time, combined with increased immune system competence.

Liver. The liver, in the upper right quadrant of the abdomen, serves several functions, including participation in the regulation of carbohydrate metabolism. Foods digested in the small intestine -- carbohydrates, fats and proteins -- are absorbed by a network of veins and carried to the liver. When the liver receives a fresh supply of carbohydrates in the form of simple sugars, it has a few choices. It can (and generally does) release some glucose into circulation, it can store some as glycogen and/or it can convert a generous amount to fat for storage. The capacity for the storage of liver glycogen is greatly increased in aerobically trained individuals.

Pancreas. Just across from the liver is the pancreas, which functions as a digestive organ supplying enzymes to the small intestine, and as an endocrine organ with its specialized islet cells, which produce the hormones insulin and glucagon. Insulin can activate receptors in all cells of the body to metabolize glucose; glucagon, conversely, acts to release glucose from glycogen storage. Aerobic training increases sensitivity in insulin receptors throughout the body.

Adrenal glands. A little lower in the abdomen are the paired adrenal glands, one atop each kidney. The adrenals are the source of two classes of hormones, the gluco- and mineralo-corticoids. The former, or cortisol group, can be released in response to stress -- physical, chemical, bacterial, viral, radiation and intensive exercise. Long-term stress may result in chronic, high levels of cortisol, followed by depletion, resulting in lowered resistance to infection. Adequate, but not excessive, aerobic exercise training keeps resistance levels high, and hastens recovery from injury or illness. The adrenals and the kidneys have a strong hand in blood pressure regulation, and aerobic exercise is known to reduce hypertensive blood pressure.

Mid-body muscles. Aerobic exercises such as cross country skiing and aerobic walking improve the tone of three muscle groups: the pelvic-support muscles, the lower-back muscles and the gluteal muscles that splint the neck of the femur. Three disparate conditions, incontinence (especially in older women), chronic low-back pain and the risk of "hip" fracture, are thus improved.

Calcium metabolism. Exercise also improves the body as a whole. Calcium metabolism, a complex balance of many influences, is improved by strength and aerobic training. In women young enough to have adequate estrogen levels, both types of exercise increase bone mineral density. In post-menopausal women, such exercise will inhibit the bone density decline that commonly occurs with passing years.

Connective tissue. Another whole-body effect is on connective tissue, since aerobic exercise creates more physiologically active fibroblasts and a more youthful balance of collagen and elastin fibers.

Body fat. Still another whole-body influence of aerobic exercise is the strong effect on body fat percentage. Optimal levels of aerobic training have consistently resulted in a lowering of fat-to-lean ratios. Many people think of whether they are too fat in terms of weight, but the effect of aerobic exercise is on fat storage, rather than on weight, per se. Individuals who are relatively lean before starting an exercise program often report losing inches (thighs, waist, hips, waist, chest, upper arms) without change in weight.

Aerobic exercise does not bring about its fat-loss effect merely by caloric expenditure. It also involves multiple biochemical changes, including changes in lipoprotein lipase, brain cholecystokinin, glucocorticosteroids, leptin, c-reactive protein and other peptides, as well as an increase in resting metabolic rate.

Immune system. Another generalized effect of aerobic exercise is on the immune system. Aerobic exercise affects both the cellular and humoral processes of this complex defense system. Different changes occur during a workout, after a workout and long term, if exercise is practiced on a regular basis. New balances are achieved among the various immune mechanisms and chemicals.

The immune system reacts differently depending on whether the exercise is at optimal aerobic levels, exhaustive distance and intensity, or at overtraining levels. The overall effect of exercise on the many components of the immune system can be judged by the clinical picture. That bottom line is that ideal levels of aerobic exercise translates into greater resistance to infection (bacterial and viral) and to lower risk for breast cancer and colon cancer. An indirect path to these benefits is the increased ability to tolerate stressors. Overtraining -- generally acknowledged as more than 90 minutes at a hard pace for one exercise bout, or 35 miles (or equivalent) per week at workout pace -- can result in an opposite effect. Overtraining, like chronic stress, results in a reduction in immune system competence.

Exercise as medicine

Exercise affects the great majority of the body's tissues, organs and systems to bring about homeostatic stability and normal function. Aerobic exercise at optimal levels of frequency, distance (time) and intensity can markedly reduce the risk of developing many of the chronic diseases commonly seen. As such, the public health implications of establishing widespread aerobic exercise programs are important for society as a whole.

By Mort Malkin