Few studies have been performed on the link between mood and exercise, but recent research had some interesting findings:

A group of global public-health researchers, however, suspected that newer studies and a more rigorous review of the statistical evidence might bolster the case for exercise as a treatment of and block against depression.

So for the new analyses, they first gathered all of the most recent and best-designed studies about depression and exercise.

Then, for perhaps the most innovative of the new studies, which was published last month in Preventive Medicine, they focused on whether exercise could help to prevent someone from developing depression.

The scientists knew that many past studies of that topic had relied on people providing reports about how much they had exercised. We human beings tend to be notoriously unreliable in our memories of past workouts, though.

So the researchers decided to use only past studies that had objectively measured participants’ aerobic fitness, which will rise or fall depending on whether and how much someone exercises. Participants’ mental health also had to have been determined with standard testing at the start and finish of the studies, and the follow-up time needed to have been at least a year and preferably longer.

Ultimately, the researchers found several large-scale past studies that met their criteria. Together, they contained data on more than 1,140,000 adult men and women.

Among these million-plus people, the links between fitness and mental health turned out to be considerable. When the researchers divided the group into thirds, based on how aerobically fit they were, those men and women with the lowest fitness were about 75 percent more likely to have been given diagnoses of depression than the people with the greatest fitness. The men and women in the middle third were almost 25 percent more likely to develop depression than those who were the most fit.

In a separate study (some of the scientists were involved in each of the reviews), researchers looked at whether exercise might be useful as a treatment for depression. In that analysis, which was published in June in the Journal of Psychiatric Research, they pooled data from 25 past studies in which people with clinically diagnosed depression began some type of exercise program. Each study had to include a control group that did not exercise and be otherwise methodologically sophisticated.

The pooled results persuasively showed that exercise, especially if it is moderately strenuous, such as brisk walking or jogging, and supervised, so that people complete the entire program, has a “large and significant effect” against depression, the authors wrote. People’s mental health tended to demonstrably improve if they were physically active.

The final study review showed that exercise significantly reduced inflammation and increased hormones and other biochemicals associated with brain health. The study author had practical advice and:

 "encouraged anyone feeling overwhelmed by recent events, or just by life, to go for a run or a bike ride. “The main message” of his and his colleagues’ reviews, he said, “is that people need to be active to improve their mental health.”