By now, most people have heard that exercise is good for their health.

A new review suggests it can it also make a difference in major depressive disorder.

Researchers analyzed 15 existing studies with data on exercise and depression, finding an association between physical activity and depression risk. The investigators estimated that almost 12% of cases of depression could have been prevented with a certain amount of exercise.

It didn't take much.

Physical activity was associated with significant mental health benefits, even when someone wasn't exercising as hard or as often as public health recommendations, according to the researchers, led by Soren Brage and James Woodcock, from the University of Cambridge School of Clinical Medicine in England.

People engaging in brisk walking for 2.5 hours a week had a lower risk of depression than those who didn't exercise at all, the study found.

"Any movement, every movement, every step counts. It doesn't have to be as much as you need for physical health. You can get by with half of that, and this is very consistent with the literature," said Jennifer Heisz, a neuroscientist who was not involved in this study.

Heisz is an associate professor in the Department of Kinesiology at McMaster University in Ontario, Canada.

The 15 studies analyzed for this new paper included more than 191,000 participants in all.

The researchers found that people who accumulated half the recommended amount of physical activity had an 18% lower risk of depression compared to adults with no activity. Those who accumulated the recommended hours had a 25% lower risk of depression. Benefits diminished with exercise above that level.

Depression affects about 280 million people throughout the world and is the leading cause of mental health-related disease burden, the study noted. It is associated with premature death from suicide and health issues.

Estimating the dose of exercise needed can be challenging, the authors said.

A lot of people who have depression go undiagnosed, Heisz said. It can also be difficult to motivate people living with depression to get moving, so the information that any movement can add benefit may be helpful for those individuals, she said.

People should try to move a little every day, Heisz suggested. Maybe it's a five-minute or 10-minute walk. It could be a two-minute movement break every 30 minutes for people who sit all day.

"That's how simple we need to get, especially for people who are not moving at all, and to acknowledge that there is this additional barrier of motivation for people who are suffering from depression," she said.

"I think that the accumulating evidence is clear that we need to start having a conversation around the benefits of exercise for these individuals, either on its own or as an add-on therapy for medication," Heisz said.

Dr. Antonia Baum is an assistant clinical professor of psychiatry and behavioral sciences at George Washington University in Washington, D.C.

In treating depression, there is a role for exercise, for psychotherapy, for taking away drugs of abuse and for adding medications specifically targeted to be therapeutic, said Baum, who had no role in this study.

The reasons why exercise may benefit mental health are many, she said.

It can improve circulation to the brain and have an impact on inflammation and the body's immune response. There's a relationship between heart health and depression. There may also be intangible benefits, such as feeling empowered by getting stronger or having a sense of well-being, Baum said.

The authors of this new study amass a lot of data to support the relationship between physical activity and depression, though there can be many variables, including genetics, Baum said.

In her work, Baum has seen how over-exercise can lead to burnout in athletes or be a factor in an eating disorder, so she was glad to see the study also looked at what point the benefits of exercise might level off.

"They did at least allude to that inverse relationship at a certain crossover point, which of course is difficult to quantify," Baum said.

While many providers suggest to their patients that they would benefit from exercise, it's important to bolster that message, said Baum. Practicing what she preaches, sometimes she models exercise behavior in sessions with patients by walking or running with them.

"I think somewhere we physicians tend to fall down both in general internal medicine and then psychiatry and probably every other field; we might tell our patients to go exercise, but you have to just keep reinforcing it," Baum said.

The study findings were published online April 13 in JAMA Psychiatry.