New evidence suggests that even a small amount of exercise – 15 minutes of jogging a day, for example – could lower your risk of depression.

A recent study by Massachusetts General Hospital looked at more than 600,000 adults, assessing their genetic makeup, medical history and physical activity levels. Researchers determined that participants who were genetically more likely to exercise may be less likely to develop depression.

"Previous studies have shown an association. People who exercise have less depression," Dr. David Agus said Thursday on "CBS This Morning." "But this study, in a very clever genetic way where they actually were able to use genetics to randomize people … showed causality. That exercise itself, no matter who you are, can reduce the risk of depression."

The key is to be active every day. "Fifteen minutes of jogging, an hour of gardening. Get outside and do something," Agus said. His rule of thumb? Get your heart rate up 50 percent higher from where it started for that 15 minutes of activity.

"If your heart rate starts at 60, get it to 90. That's not a crazy amount of exercise. That means you don't even need to be sweating or out of breath, just move is critical," Agus said.

In 2016, an estimated 16.2 million adults in the U.S. had at least one major depressive episode, according to the National Institute of Mental Health.

Agus said he hopes this latest research will impact how doctors treat depression.

"My patients now, when they come in and say, 'I'm starting to feel a little depressed and not do as much,' my first thing is not prescribe a pill, but get outside and do something," Agus said.