Recent research on the link between physical activity and depression risk in adults has suggested that exercise may offset the genetic tendency toward depression. Adults with genetic risks who exercised regularly were no more likely to develop depression than those without the genetic propensity.
There’s good evidence that this same association holds in adolescents, a group with a generally high risk of depression, and with concerningly high suicide rates. But adolescence is also a time when physical activity often becomes less common, especially among girls.
The World Health Organization recommendation is for an hour of moderate to vigorous exercise every day for adolescents, in addition to whatever they do in school — that means activity that gets you sweating and breathing hard. But many adolescents don’t exercise anywhere near that much.
So people who study adolescent mental health and suicide prevention are interested in the possible protective effects of more limited exercise or more moderate physical activity — and also, of course, in the question of how to help adolescents get moving.
A prospective study published in the March issue of the journal The Lancet Psychiatry found that even light activity — and a corresponding decrease in the amount of time that kids spent being sedentary — was linked to better mental health as they got older.
The researchers looked at the activity of adolescents at the ages of 12, 14 and 16, who were then assessed for depression at around 18. The participants wore devices called accelerometers, which continuously measured their activity during the day.
The first author on the new study, Aaron Kandola, a Ph.D. candidate in physical activity and mental health at University College London, said that many of the older studies had relied on memory, asking people what they had done in their leisure time. While this captures exercise and other moderate activity, he said, it misses light activity, which actually makes up the bulk of many people’s movement during the day — walking at a casual pace, shopping, playing an instrument, doing chores around the house.
The study found that total physical activity dropped between ages 12 and 16, mostly because of decreases in that light activity, and sedentary behavior increased. And the activity levels when kids were younger were linked to their mental health later on; the depression scores at 18 were lower for every additional 60 minutes per day of light activity at 12, 14 and 16, and higher for every additional sedentary hour.
Mr. Kandola said that by age 16, young people in the study were spending an average of close to nine hours a day on sedentary activity, and that it would be hard to decrease that behavior significantly through exercise, because it would require lots of exercise to significantly reduce that amount of sedentary time. If the goal is to reduce sedentary time significantly (say, by two hours a day), it might be useful to build in more light activity — for example, by reshaping the school environment, he said.
“We have this time at school where we have put out an agenda of sedentary behavior,” he said, citing the emphasis on sitting down, with rare breaks. “It’s time for reforms to target the school system.” He suggested that activity breaks be scheduled regularly in schools, and that schools try standing desks, which have been successful in workplaces.
Xihe Zhu, an associate professor of human movement science at Old Dominion University in Norfolk, Va., said that research shows that “some exercise is going to be much better than no exercise at all.” Dr. Zhu was the first author on a 2019 study of 35,000 children and adolescents from 6 to 17 in the United States, which found that those who reported no exercise were twice as likely to have mental health problems, particularly related to anxiety and depression, compared with those who met the exercise guidelines.
Even if children exercised only one to three days a week, he said, there was a strong correlation with lower rates of anxiety and depression — and there was no significant difference between them and those who exercised four to six days a week.
Good sleep duration and extracurricular activities were also associated with better mental health. In fact, physical activity may improve sleep quality, which is closely linked to mental health.
Elaine McMahon, a research fellow at the National Suicide Research Foundation and the School of Public Health, University College Cork in Ireland, was the lead author on a 2017 study coming out of a large research project on European adolescents.
They studied more than 11,000 13- to 15-year-olds in 10 countries, and found that only 13.6 percent of them met the recommended guidelines of 60 minutes of moderate to vigorous exercise a day.
There was a clear association between more frequent physical activity and lower levels of depression and anxiety, but the most significant difference was between the least active group (active for 60 minutes or more on zero to three of the past 14 days) and the somewhat active group (four to seven of the past 14 days). The most active group (eight to 14 of the past 14 days) had the highest levels of well-being and the lowest levels of depression and anxiety, though within that group, daily activity conferred no special benefit.
The study also found that being on a sports team was associated with an extra improvement in mental health, beyond what was associated with the physical activity — and it was particularly strong for girls.
There were striking differences among the 10 countries. In Slovenia, 66.9 percent of the boys and 49.7 percent of the girls were in the most active group; in Italy it was only 27.8 percent of the boys and 9.6 percent of the girls.
“What’s going so right in some countries in terms of keeping girls active through adolescence?” Dr. McMahon asked. “Is it about offering a larger variety of activities, or school schedule, or a culture of outdoor activity?” The variety of activities available matters, she said, and so does building sports into the curriculum and making sure there is free time in an adolescent’s week, and encouraging them to walk or bike to school.
The cross-sectional studies that show an association between exercise and better mental health cannot actually show causality, and being depressed or otherwise affected by mental health problems might stop a person from exercising. “When you look at populations with mental health issues, they typically have low physical activity or exercise,” Dr. Zhu said. In adults, those populations also typically have high levels of obesity and cardiovascular health problems, he said.
But recent prospective studies, including this new one looking at adolescents, build on what we already know to suggest that there is a strong relationship in the other direction: Regular exercise lowers your risk of developing depression. A 2019 review by Mr. Kandola and his colleagues cited a number of possible ways exercise may affect depression, including biological mechanisms like stimulation of neurological pathways and processes, and reducing inflammation, but also that “exercise promotes self-esteem, social support and self-efficacy.”
So the message is that exercise is good, activity is important, but you’ll start seeing benefits long before you get to that solid hour a day. “Moderate activity of any kind, getting out and doing something, is associated with improvements, lower levels of depressive symptoms, lower levels of anxiety, better well-being,” Dr. McMahon said.
