Humans Are Not Zebras
Here's a vitally important point that we will come back to again and again: Human beings were simply not meant to carry around constant disturbances in our stress response (chronic stress); we were built to respond to stress quickly and then to have the stress hormones dissipate immediately (acute stress). When our bodies are exposed to wave after wave of stress (from our modern lifestyles), they begin to break down.
Animals don't normally harbor chronic stress the way humans do, but when they do (during stress experiments, starvation, injury, etc.) they get sick just like humans. In study after study, it quickly becomes obvious that the stress response, while acting as our friend in certain situations, turns against us when "everyday" events are perceived by the body as "stressful" events. Over time, stress-related diseases result from either an overexaggerated stress response (too much response to what should have been a small stressor) or an underexaggerated ability to shut down the stress response (which causes cortisol levels to remain elevated for longer than they should).
Stress, Cortisol, and Metabolism
This is probably as good a place as any to introduce the concept that cortisol is not a purely toxic substance—although in most cases too much of it certainly wreaks bodily havoc. In many ways, cortisol can be thought of as functioning like cholesterol or insulin: A small amount of each of these substances is needed for the proper functioning of the body. Cholesterol is needed for steroid metabolism. Insulin is necessary for blood-sugar control. And cortisol is needed for the restoration of energy stores following stress. However, if levels of any of these vital compounds exceed a certain small amount for a significant period of time, you run into problems (blocked arteries in the case of high cholesterol, diabetes in the case of elevated insulin, and obesity and a host of chronic diseases in the case of cortisol).
The whole point here is balance-keeping cortisol levels from falling too low or rising too high. In a condition called Addison's disease, people are unable to secrete glucocorticoids (of which cortisol is one) from the adrenal glands. As a result of this inability to mount an effective stress response, people with Addison's disease basically go into a state of shock when faced with a stressful event. They sustain a drop in blood pressure, circulatory collapse, and other such symptoms. So just as you do not want cortisol levels to rise too high, neither do you want them to drop too low.
Whenever you're exposed to stress, be it physical stress, such as exercise or emotional stress (e.g., the kind caused by that guy cutting you off in rush-hour traffic), your body begins a complex cascade of events that can alter metabolism in a number of significant ways. Think again about the fight-or-flight mechanism, wherein stimulatory hormones are secreted to prepare the body for rapid action against (fight) or away from (flight) a particular stressor. In a similar manner, upon exposure to everyday stressors the human body ramps up production of cortisol through a complicated series of events that involves both the hypothalamus and the pituitary glands in the brain (more on that later).
One of cortisol's many functions is to stimulate the release of glucose, fats, and amino acids for energy production. In the liver, cortisol stimulates the breakdown of glycogen into glucose. In the adipose tissue (where we store body fat), fatty acids are released in response to cortisol stimulation. (Fat breakdown? Sounds good, but the longer-term effect is fat gain.) In the skeletal muscles, cortisol promotes the release of amino acids, which are either used directly by the muscle for energy or sent to the liver for conversion into glucose. The main problem with this last scenario, however, is that if it continues for any prolonged period of time, a significant amount of muscle mass may be lost (bad for long-term weight maintenance).