Every two weeks, the National Institutes of Health puts together a podcast examining – demystifying, really – the federal agency's latest research on everything from pelvic floor disorders to using MRIs to diagnose breast cancer.

Joe Balintfy, the producer, dutifully sends the podcasts to iTunes and the NIH Web site, where thousands tune in weekly. But he hasn't a clue as to his listening demographic.

In these challenging economic times, podcasts are serving as part medical consultant, part personal trainer, part workout partner. These audio broadcasts, which take their name from Apple's ubiquitous iPod, can be converted to MP3 files, downloaded and played back anywhere.

So, who's tuning in?

Fitness buffs tuning in while pumping iron? Hypochondriacs checking for potential new conditions? Physicians listening in their BMWs to keep up on research?

"I like to think of someone listening while on a treadmill," Balintfy says. "That way, it's doubly healthy. They are exercising and learning about the latest research at the same time."

The important thing, to Balintfy, is that people listen. The NIH Research Radio podcast consistently ranks among the top iTunes health and fitness downloads. Balintfy, though, has a picture in his mind of the ideal listener.

Podcasts are proliferating in cyberspace. In the health-and-fitness realm, podcasts conceived either by media companies, organizations or simply by users with a passion inform and entertain in equal measure.

Take, for example, the popular running podcast, "Phedippidations," produced by a middle-of-the-pack Massachusetts runner, Steve Walker. Each weekend, about 10,000 listeners download his musings on the running life, which Walker records literally on the run with a microphone clipped to his shirt collar.

"I don't like to use the word 'audience.' It's more like 'fellow runners,' " Walker says. "The sense is, I don't feel like I'm doing a radio show. I feel like I'm having a conversation.

"Most runners listen to me while they're running. Last summer, when I said I had to cut back because it was taking too much time, I got a lot of e-mails from people saying, 'Man, you just messed up my whole schedule. Friday night, the show shows up on iTunes. I download it for my Sunday morning long run and I go running with Steve.' "

Inspiration for recording – and, on the other end, downloading – podcasts can come from many sources, not to mention places.

David Jackson, a 43-year-old Akron, Ohio, man struggling to lose weight, listened to weight-loss podcasts and was so motivated that he decided to produce his own, called "Logical Weight Loss."

In little more than a year, Jackson has grown a base of 1,500 listeners (according to podtrac.com, which charts many downloads) and says he routinely hears from fellow dieters as far away as Saudi Arabia, Brazil and Italy.

Jackson's appeal is that he's not a buff personal trainer-cum-motivational speaker. He lost 20 pounds three years ago but "found the weight I lost." Now he's documenting how to lose weight and keep it off.

"People are finding inspiration in the fact that I am not quitting in the face of adversity," Jackson says. "Their faith and encouragement leads me to keep going with the podcast, which then encourages them. It's a great community."

Many times, people launch podcasts because they want to form an audio community where none existed before.

That was the case with Kevin Larrabee, a recent graduate from Keene State College in New Hampshire with a health science degree.

An admitted tech nerd who also is certified as a coach by the National Strength and Conditioning Association, Larrabee scanned cyberspace for a podcast that, in his words, "wasn't all about doing naked yoga or just running."

He came up empty. So he started his own, "The Fitcast," a weekly free- wheeling, round-table discussion featuring personal trainers, nutritionists and exercise physiologists musing about fitness and nutrition, with special emphasis on strength training.

Larrabee's mission is to cut through what he calls conflicting and sometimes "dead wrong" information provided by other podcasts. He wanted to give "the insight, real-world application and results that people are looking for."

Because anyone with the hardware and inclination can produce a podcast, Larrabee says, people need to check out the credentials of those giving the audio advice.

"Isn't that the way of the Internet?" he says. "Anyone can make a Web site, call themselves an expert and tell the world that they should do squats on a Swiss ball while doing dumbbell curls."

Larrabee also cautions that using a podcast as a stand-in for a personal trainer can be dangerous.

"Nothing can replace a set of eyes from a good trainer or coach," he says. "You can have someone who is very enthusiastic telling you to grind out that last rep through your headphones, but what if your deadlifting form sucks? Or if you cannot perform pull-ups without some help?"

Yet, there's no denying the motivational benefits of podcasts, says Walker of the running show "Phedippidations."

"Podcast or no podcasts, you go to a race or running club, you talk to people about what works for you," Walker says. "Better yet, the most important runners need to create a social group about is to ask the question: What should I avoid? It's in our mistakes that we learn."

Call The Bee's Sam McManis, (916) 321-1145.