Swimming greats Michael Phelps (23 Olympic gold medals), Allison Schmitt (eight-time Olympic medalist), and Ian Thorpe (five Olympic golds) have all been in hot water. They've each battled major depression.

“It's like a weight is pressing down on you. There are days when you just can't get out of bed,” explains Thorpe.

They're not alone: About 10% of people in the U.S. will experience major depressive disorder at some point. And 50% of the time, the cause is genetic — meaning multiple genes predispose people to depression.

When your parent or sibling has the condition, your risk for depression is two times greater than for those without a family connection. And it's four to five times greater if that family member has had recurrent depression that started before their 30s.

The good news is that there's a lot you can do to reduce or eliminate the risk, even with a genetic predisposition.

Research published in the journal Depression & Anxiety found that 35 minutes of additional physical activity daily reduces the risk of gene-associated depression, and helps protect against future episodes.

What kind of activity works? Research shows that overtraining (that may be the case with some super-athletes) is associated with depression, but moderate amounts of higher-intensity aerobics and dance, as well as lower-intensity yoga and stretching, are very effective. Walking was also protective.

The most important factor is consistency.

So be sure to take steps (10,000 a day is good, with interval walking that provides intensity) to make exercise a part of your feel-better plan. But don't overdo it.