Does lack of exercise increase the risk of depression in adolescents?

Depression explained:

While in everyday language usage the term depressive is oftenused to refer tonormal dysphoria (i.e., depressed mood usually without disease value), in a medical context depression is a common mental disorder. Symptoms of this affective disorder would include loss of appetite, guilt, low self-esteem, sleep disturbances, fatigue, and poor concentration.

Depression can occur over long periods of time or appear in recurring episodes, causing a person to struggle in work, learning, or life in general. At its worst, the condition can lead to suicide. While milder forms can usually be treated without medication, moderate to severe forms must be treated with medication or with the help of professional talk therapy. The disease often develops at a young age and, according to WHO statistics, affects women more often than men, with job seekers also being more affected.

According to the international classification system ICD-10 (i.e. International Classification of Diseases), there are three levels of severity:

• mild depressive episode: when there are at least two main symptoms such as depressed mood and lack of drive, and two additional symptoms such as guilt and sleep disturbances.
• Moderate depressive episode: when two main symptoms and at least three, at most four additional symptoms are present.
• Major depressive episode: when three main symptoms and at least four additional symptoms are diagnosed. In addition to this, the features must last for at least two weeks.

Other subtypes of depressive episodes can be further specified - as in chronic depressive disorder, psychotic depression, melancholic depression, seasonal dependent depression, or postpartum mental disorder. The following study does not include bipolar affective disorder.

Adolescents analyzed:

The study, published in February 2020, in the medical journal The Lancet, analyzed data from the ALSPAC study (i.e., Avon Longitudinal Study of Parents and Children), which has followed a cohort of nearly 15,000 children since 1991, since the mothers' pregnancy. Using accelerometers called accelerometers, sedentary behavior and physical activity were measured in children aged 12, 14, and 16. Total activity was assessed as Count Per Minute (CMP) and subsequently calculated as average CPM per day. The physical activity measure was then associated with the results of the CIS-R (i.e. Clinical Interview Schedule-Revised), which was evaluated in 4,257 participants aged 18 years. CIS-R is a structured diagnostic instrument used by psychiatrists to assess and score (i.e., value) mental disorders.

Physical activity accurately measured:

Physical activity was recorded using an MTI Actigraph 7164 or 71256 meter worn on the right hip for 7 days. Both models are of the same generation of single-axis accelerometers and there is no significant difference between the results, according to the study.

Except for washing or water sports, the study participants wore these meters during waking hours. Data was only taken from subjects who recorded more than 10 hours of wear time for at least 3 days.

This measurement defined the amount and intensity of physical activity performed into three categories:

• Moderate to vigorous physical activity: at a minimum of 3600 CPM. (e.g. brisk walking or jogging)
• light activity: 200-3599 CPM. (e.g. slow walking)
• sedentary behavior: at a maximum of 199 CPM. (e.g. lying still or sitting)

To account for differences in wearing time, time spent in each activity intensity was calculated as a percentage of total wearing time. The follow-up period was 6 years.

The older the more sitting:

Analyses showed that physical activity decreased between 12 and 16 years of age, which the study said was due to decreasing duration of light activity - from 5 hours and 26 minutes to 4 hours and 5 minutes. In addition to this, there was an increase in sedentary behaviour from an average of 7 hours and 10 minutes to 8 hours and 43 minutes.

There is a correlation between increased sedentary behavior and increased depression scores at age 18. Each additional hour in a sedentary position from the age of 12 years increased the depression score by 11.1%. On the other hand, each additional hour of light physical activity decreased the score by 9.6%. In general, depression scores were higher in subjects with persistently high and persistently average sedentary behavior than in those who were persistently less sedentary. Also, subjects with persistently high physical activity had lower depression scores than those with persistently low activity.

Around 15 minutes of daily exercise could reduce depression scores by 9% from as early as age 12. However, this link was not significant as this relationship was not found for moderate to vigorous physical activity at age 14 or 16.

Conclusion:

Exactly how exercise protects against depression is unfortunately not yet known. What is clear is that physical inactivity displaces light activity throughout adolescence and is associated with a higher risk of depressive symptoms at age 18. Increasing activity and decreasing sedentary behavior during adolescence could be a significant public health interest to challenge the rising incidence of depression.