But it's no miracle cure: On average, six people would need to hit the gym or go for a jog for one person to see a mood improvement.
"It's a nice piece of evidence and I'm pleased because I like the concept," said Dr. Alan J. Gelenberg, who chairs the department of psychiatry at Penn State University in Hershey.
Gelenberg, who wasn't involved in the new work, said the findings jibe with guidelines from the American Psychiatric Association, which recommends regular exercise against the blues.
"There is some evidence for its use to prevent depression, and there actually is evidence for exercise as a treatment in itself," he told Reuters Health.
With the new study, published in the Archives of Internal Medicine, researchers wanted to weigh the evidence that training can also help chronically ill people who don't have a diagnosis of depression, but nonetheless may feel down.
That's important because depressive symptoms could make people less likely to take their meds, could increase their use of health services and decrease their quality of life, said Matthew Herring of the University of Alabama at Birmingham.
He and his colleagues combed through 90 previous studies including more than 10,000 people with health problems like cancer, heart disease, chronic obstructive pulmonary disorder (COPD), fibromyalgia, chronic pain or obesity.
In each study, people had been randomly chosen to do exercises -- on average, three times a week over 17 weeks -- or not.
According to Herring, people's depressive symptoms, as rated on a variety of psychological scales, dropped about 22 percent with exercise overall. That's similar to the effects on fatigue, anxiety, pain and other mental health outcomes.
"The magnitude of the effect of exercise training on depressive symptoms among patients found in our review is small but significant," he told Reuters Health by email.
Herring added that moderate -- at least 150 minutes of moderate intensity exercise per week -- and vigorous -- at least 75 minutes of vigorous intensity exercise per week -- seemed to help the most."

INSPIRATIONAL STORY OF THE DAY
And now...to inspire you, and to remind you how much YOU can do...read this story of a young lady who is once again swimming---even though she only has one leg:
"Denise Castelli is one of seven people chosen to be a part of Dr. Sanjay Gupta's Fit Nation Triathlon Challenge. As a recent amputee, Denise is searching for a way to reclaim the feeling of being a competitive athlete that she cherished before her accident.

Getting in the pool has forced me to face a number of fears. The first being the obvious fear - ditching the doggie paddle and actually learning how to swim. The other fear is not so obvious and much more personal.
The swim is the only leg of the race that I’ll be doing, well, legless. Prosthetics aren’t made to be submerged in water and I can imagine it would be quite difficult to swim with a heavy piece of carbon fiber attached to my body.
My prosthetic has been my safety net ever since I learned to walk again. It has essentially become my super hero cape. When I wear it, I know I can do anything. I have the world in the palm of my hand. Without it, am I handicapped?

That is a word I seldom use to describe myself. But whenever I catch a glimpse of myself without my prosthetic on, it really hits me. That’s when I’m forced to face the fact that this is me and this is how it’s going to be forever.
This isn’t the bad dream where you’re in your high school in nothing but your underwear. There is no magical unicorn blood that I can drink to help regenerate my leg. This is real life. I am an amputee. And that is a lot harder to swallow than the occasional mouthful of pool water.
Facing this realization, I jumped in the pool for the first time and, much to my surprise, it was not as bad as I expected it to be. My coaches, Mickey Cassu and Kristin Cacicedo of Start-Tri.com, had me swim the length just to see where my knowledge base was. By the time I reached the end, I was panting. All I could think was, “How am I supposed to swim half of a mile... in the Pacific Ocean?!?!”
Then Mickey assured me that if he swam the way I did, he would be out of breath too. That’s when I learned how to work with the water instead of against it.
Since that first lesson, I’ve been swimming on my own at least three times a week. Along with these solo swim workouts, I’ve met with Mickey and Kristin two other times for some one-on-one training. I can already feel the improvements that I’ve made, which makes me hungry for more. Every time I’m at the pool I “swim stalk” other people and I’m mesmerized by their perfect form and ability to glide through the water. I want that.
Helping me get there is my new best friend – the pull buoy. It helps me keep my focus on my breathing and where my arm entry is.
“Head in the water, rotate every third stroke to breathe, repeat.” Trying to form muscle memory is a lot harder than I remember. It’s going to take a lot of repetition. It’s going to take a lot of repetition!
So, for now, I’m just going to keep baby steppin’ until it becomes second nature. I’m going to keep clicking my leg off and hopping to the edge of the water to dive in. Overcoming all of my fears, one stroke at a time."

"Do what you feel in your heart to be right. You'll be
criticized anyway." -Eleanor Roosevelt