Psychosocial Mediators of the Fitness-Depression Relationship Within Adolescents.
BACKGROUND Studies of physical activity and incidence of physician-diagnosed depression have been limited to a single estimate of self-reported physical activity exposure, despite follow-up periods lasting many years. PURPOSE To examine longitudinal change in cardiorespiratory fitness, an objective marker of habitual physical activity, and incident depression complaints made to a physician. METHODS Cardiorespiratory fitness assessed at four clinic visits between 1971 and 2006, each separated by an average of 2-3 years, was used to objectively measure cumulative physical activity exposure in cohorts of 7936 men and 1261 women, aged 20-85 years, from the Aerobics Center Longitudinal Study who did not complain of depression at their first clinic visit in 1971-2003. Data were analyzed in August 2010. RESULTS Across subsequent visits, there were 446 incident cases in men and 153 cases in women. After adjustment for age, time between visits, BMI at each visit, and fitness at Visit 1, each 1-minute decline in treadmill endurance (i.e., a decline in cardiorespiratory fitness of approximately 1 half-MET) between ages 51 and 55 years in men and ages 53 and 56 years in women, increased the odds of incident depression complaints by approximately 2% and 9.5%, respectively. The increased odds remained significant but were attenuated to 1.3% and 5.4% after further adjustment at each visit for smoking, alcohol use, chronic medical conditions, anxiety, and sleep problems. CONCLUSIONS Maintenance of cardiorespiratory fitness during late middle age, when decline in fitness typically accelerates, helps protect against the onset of depression complaints made to a physician.