Whitney Shaw is training for a marathon — and her friends know every single detail.

She tracks the distance and time of every run on the social network DailyMile, posts her proudest achievements to Facebook and maintains a thoughtful account of her fitness and nutrition efforts on a Tumblr blog.

“My friends know my weekly mileage,” said Shaw, 26. “If I’m not hitting that, I hear about it.”

The Minneapolis resident is among millions of Americans counting on mobile apps, social media and wearable tech gizmos to lose weight or get fit. These digital tools promise a combination of high-tech habit tracking and positive peer pressure that users say helps them shed pounds faster. But the apps also make oversharing too easy, prompting eye rolls among those checking Twitter from the couch.

One in five American smartphone owners have downloaded at least one health or fitness app, according to a recent study by the Pew Research Center, and there are more than 13,000 apps in that category in the iTunes store. DailyMile, which draws data from different apps, claims users have logged more than 12 million workouts.

Altogether, this tech trend takes what once was mostly private — every calorie consumed, step taken or pound lost — and turns it into public discussion. Apps often link to Facebook and Twitter, automatically posting details of the latest run or even a user’s weight for all to see. That can be encouraging, annoying or embarrassing.

“My worst nightmare is that I will tweet my weight,” said Meghan Wilker, co-host of the Minneapolis-based Geek Girls Guide podcast and a co-worker of Shaw’s at Clockwork Interactive Media. “It’s too easy to share something embarrassing when you automate everything.”

Share with caution

Shaw, inspired by Wilker and other tech-savvy co-workers, started running a year ago. But she’s mindful of what she shares online. Not everyone is interested in her daily training updates, she said.

“I’m pretty sure I would’ve had many friends ‘unfriending’ me on Facebook if I published on Facebook everything I put on Tumblr,” Shaw said. “It can be too much.”

She posts milestones to Facebook with links to Tumblr and DailyMile, for those who want more detail. On DailyMile, for example, Shaw recently posted an easy 4.4-mile treadmill run that took 41 minutes with the note, “Finished my movie, got my miles. Week 6 of #marathontraining is already here!”

Other DailyMile members can leave comments (usually encouraging) and give “motivations,” which are the equivalent of “likes” on Facebook.

Yet even on this niche social network, Shaw thinks carefully before posting, lest she hurt someone’s feelings or appear to be bragging.

“I was really slow when I first started running,” she said. Now her times are much faster. “You don’t want to be an elitist.”

While she’s vigilant about turning off the autoposting function of her fitness apps, not all people do.

Lee Hersh, who teaches yoga and blogs about health and fitness at www.fitfoodiefinds.com, said sharing on the right social platforms can serve both as a personal reminder and an encouragement for others to make healthier choices. She regularly documents her meals and snacks on Twitter and ­Instagram.

“Instead of telling somebody ‘Don’t eat that,’ show people what you’re eating,” she said. “You’re putting it out there. Everybody is your accountability partner.”

Healthy support

Even if people aren’t sharing their health and fitness data on social media, just gathering the information can be helpful, experts say, especially for weight loss.

Dietitians like Beth Macias of HealthPartners say certain apps can provide valuable feedback about what people consume after just a few days.

“It gives you a very tailored reflection of what you’re eating and ways to make good changes,” she said.

That feedback has led to success for Kirsten Kaufmann of St. Paul, 10 pounds lighter since she started using an iPhone app called Lose It! She said data is helpful even though it’s strictly “between me and my phone.”

“I can see and track my goal,” said Kaufmann, 49. “It makes me very conscious of what I actually put in my mouth.”

Studies out of the University of South Carolina and Northwestern University both cited accountability as a key reason why social media and apps can further weight loss.

Still, it’s hard to say how many people are actually using the millions of health and fitness apps that have been downloaded. About half of the people who track their health and fitness told Pew they do so informally in their heads. One-third said they use pen and paper.

But Dr. Peter Eckman, a cardiologist at the University of Minnesota, said these apps can sometimes offer people the help they need.

One of Eckman’s heart transplant patients has been walking enthusiastically during his recovery because he found a social “walk to the moon” challenge online. The patient tracks his steps with a pedometer app on his smartphone and enters them online toward the collective goal of more than 477 million steps.

“The social part of this, for him, has really been critical,” Eckman said. “It’s another arrow in the quiver of ways to motivate people and help people maintain motivation themselves.”