Children and adolescents with high aerobic endurance - the ability to sustain exertion for a long time, such as running - may face clinical depression less often by middle age. This follows from a study published in the Journal of Affective Disorders. Scientists observed more than a thousand Australians over more than three decades.
Depression is a mood disorder with prolonged low mood, loss of interest and feelings of worthlessness. Antidepressants do not help everyone and can have side effects, so scientists are looking for modifiable prevention factors. Physical fitness is an objective measure of how well the heart, lungs and muscles are adapted to exertion. It is divided into cardiorespiratory (endurance of the heart and lungs) and muscular (strength and power).
Previously, a link between good fitness and mental health had been found in adults. For example, in Britons with low endurance and strength, the risk of depression seven years later was almost twice as high. A Swedish study of conscripts showed that low fitness at age 18 was associated with a higher risk of severe depression decades later. But it remained unclear whether this works in girls and when measured in childhood.
The authors from the University of Tasmania analyzed data from the 1985 Australian Schools Health and Fitness Survey. At that time, 8,498 children aged 7-15 were tested. Later, some underwent repeat examinations in 2014-2019. The final analysis included up to 1,080 people (519 men and 561 women), who were followed for an average of 32.5 years. Participants underwent a diagnostic interview for depression.
In 1985, the children took a test: a 1.6 km run or walk, and speed was calculated from the results. A subgroup aged 9, 12 and 15 took a cycle ergometer test to assess endurance. Muscle strength and power were measured by standing long jump and handgrip dynamometer - squeezing a hand device. The measures were standardized by age and sex.
During follow-up, depression occurred in 190 of 1,080 people (17.6%), including 117 women (20.9%) and 73 men (14.1%). After adjustment for age and sex, a one standard deviation increase in childhood running speed - roughly the difference between an average child and one fitter than five out of six peers - was associated with a 15% lower frequency of depression. After adjustments for standing long jump, area socioeconomic status and body mass index, the estimate remained 15% but was no longer statistically significant.
In women, the association appeared stronger: a one standard deviation increase in running speed was associated with a 19% lower frequency of depression, and cycle ergometer endurance with a 28% lower frequency in models adjusted for age. After full adjustment, the estimates remained similar but significance disappeared. In men, the reduction was 10% and 6% respectively, also not significant. Standing long jump and handgrip strength showed no significant association with depression.
The authors emphasize that this is an observational study; it does not prove causation. "The biggest misconception is to conclude that fitness prevents depression or that low fitness causes it," said co-author Brooklyn Fraser. Limitations include loss of participants (the analysis included about one in eight of those originally tested), possible recall errors when remembering the onset of depression, and the influence of motivation on test results. The association may also reflect other factors: activity habits, health, social conditions or genetics.
Nevertheless, according to Fraser, fitness is one factor that most people can improve with regular exercise. The researchers call for further work to understand whether the association is causal and what mechanisms underlie it.