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.