Answer to Question 1
Although the mechanisms are not empirically established, hypotheses fall into two camps, physiological and psychological. Physiological hypotheses mainly center on (1) facilitating synaptic transmission of brain monoamines or (2) boosting brain endorphin levels. The monoamines are the feel good neurotransmitters present in the brain such as serotonin, norepinephrine, and dopamine. If exercise facilitates monoamine action in the brain, then it should produce anti-depressant effects just as anti-depressant medications boost mood states. There is some suggestive evidence that serotonin metabolism increases after exercise due to an increase in the brain's supply of serotonin's precursor molecules, but more research is needed to confirm the monoamine hypothesis.
Answer to Question 2
Investigators found an inverted graded dose relationship between cardiorespiratory fitness as measured by a treadmill test and measures on a depression scale in 5,451 men and 1,277 women as well as a positive graded dose response between cardiorespiratory fitness and a self-report measure of emotional well-being. In other words, they determined that the greater the cardiorespiratory fitness, the lower the depression and the higher the emotional well-being. The peak beneficial dose was 11 to 19 miles of walking, jogging, or running per week. Strengths of the study included its use of large samples and objective measures of fitness; a weakness of the study is that it was cross-sectional rather than prospective. As a result, the investigators could not establish cause-effect relationships.
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