By: Aarti Shahani

This holiday season, shelves are stocked with fitness devices that count calories, measure steps and even monitor how well you sleep. Just a few years back, these self-tracking devices were niche products for health nuts. Silicon Valley is now betting that self-tracking will grow into a multi-billion dollar industry that turns health data into a new commodity.

The Quantified Self Meetup, a monthly gathering that started with a handful of fitness junkies in 2008, has evolved to over 150 people at a Microsoft conference room in downtown San Francisco. They are not medical doctors. They're do-it-yourself technology lovers with homemade devices, playing doctor with people eager to measure, to quantify, some aspect of the human body.

Charles Wong straps a belt to me that vibrates when I slouch.

Wong: You should get a nice, gentle vibration that will remind you to...
Shahani: Oooh. It just happened. It vibrated.
Wong: How was that for you?
Shahani: It felt like a massage.

Jonathan Toomim slaps on a Velcro headband on me, to measure my concentration according to pre-frontal cortex activity.

"Your brain activity is increasing on average, at a fairly slow rate, but still substantial...until you start talking," says Toomin.

Heather Heine pokes my finger for a blood test.

Shahani: It's going to hurt right? It's a needle.
Heine: Just a little bit.
Shahani: Ah! Ok.
Heine: You did it.

Self-tracking is a popular new trend.

"FitBit's sales have exploded over the last two years," says Woody Scal, the chief revenue officer of FitBit.

FitBit is a brand name of tracking devices available at Amazon, BestBuy, Target, Apple, to name a few.

Scal says advances in motion sensor technology, extended battery life and bluetooth low energy have made it cheaper to create self-tracking devices. Smartphones make it possible to turn exercise into a real time game that customers play with friends.

"Graphs, and motivating badges, and views of a leader board. It actually motivates them to take those extra steps, climb those extra floors," says Scal.

FitBit's newest customers are businesses buying the tracking devices for employees, and paying to see data collected on physical activity. One insurance company is running a scientific experiment with FitBit, to test if employees who use devices get healthier than those who don't. Scal calls the six month results extremely promising.

"Employees increased their steps by over 40 percent. They absolutely lost weight. And we're quantifying how much they lost,"  he says.

Companies have to ask employees for consent to track their data.

"This isn't a grudging consent. This is a 'hey, I want to participate' consent," says Scal.

A couple years back, FitBit users found their sexual activity logs pop up in Google searches. Scal says those users did not restrict their privacy settings correctly.

"We've now made those privacy settings more prominent, so that people are more aware of what the privacy setting are. I think the area of privacy is an area that many companies like our have learned a lot over the past couple of years," he says.

Scal says his company also complies with federal regulations that limit the storage and sharing of health data.

Tim Chang, a venture capitalist at the Mayfield Fund, wants to sidestep regulators altogether.

"If any start-up requires FDA approval, I won't even touch it. I think it would be faster to watch paint dry than to wait for an FDA approval process for some start-up," say Chang.

Chang is among Silicon Valley's first investors to put millions of dollars into Quantified Self apps and devices.

Chang says he started tracking his own calories, heart rate and weight out of sheer vanity.

"In my mid-20s, flying back from a trip overseas and I was in the Hong Kong airport, and I noticed that in all of the GQ magazines and what not, there were never any Asian underwear models. And I was like, 'what is up with this?' And I thought, 'I better fix this. I'm going to try to be like that'," he says.

Chang, now 40, hasn't modeled yet.

"But I will say you derive a lot of lasting psychological value when you're happy with the way you look in the mirror," he says.

The vanity of Bay Area and New York super-athletes is just phase one in self-tracking as an industry.

"You might make a hundred million dollars a year. But that's not, you know, kind of making billions of dollars a year. And the market potential for this is vast. It's everybody!" says Chang.

It's people struggling to lose weight. It's parents, armed with tracking tools, deciding what groceries to buy.

"What supplements, or even when to turn off the XBox and tell the kids to go outside and ride a bike," says Chang.

Walter De Brouwer stumbled into the Quantified Self movement after a tragic accident with his son.

"He fell out of the window when he was five, and yeah, he had a very severe traumatic brain injury," says De Brouwer.

He spent a year watching his son's vital signs go up and down in the Intensive Care Unit.

"And you try to predict the next one. If this is going up to 63, we have a chance here. 'Come on, 63, 63.' All night," he says.

His son survived, and inspired De Brouwer to found Scanadu, a self-tracking device company with $2 million in venture capital.

De Brouwer takes out a diamond-shaped piece of plastic - the size of a palm, with a small sensor at one tip - and puts it in my left hand.

"To your temple, and now not talk," he instructs.

In 20 seconds, the sensor finishes scanning the pulses on my forehead. An iPhone 4 app displays vital signs it would have taken an hour to record at a hospital.

De Brouwer, a linguist with no medical training, reads them off.

"Healthy girl. 63 heartrate. 14 breathing rate. 98.7. Yeah, you're an example. Or you took a pill before you came here," he says.

De Brouwer says if enough people quantify themselves with enough frequency, and share their data with each other, patients can crowdsource, can teach each other, and rely less on doctors...even with serious diseases like cancer.

De Brouwer: I can give you a specialist doctor or ten specialist patients, who would you like to talk to first? I would like to talk to the ten patients first.
Shahani: Really, my immediate response is obviously the doctor. That's the expert.
De Brouwer: I don't think so. The doctor is one story. It may be biased.

Some doctors charge the self-tracking industry with dismissing the value of expertise. They say Wikipedia-type sites are no substitute for emergency rooms, and point out that patients can tamper with their data.

Still the growth of quantified self devices and apps demonstrates that flawed or not, this data is increasingly profitable.