When most people think of smartphones, they think of social apps like Facebook and Twitter, games like Candy Crush, and people taking selfies. However, there are a few surprising ways that smartphones can also help reduce obesity in America.

The obesity rate in America, defined as a body mass index of 30 or above, has hit 27.2% this year -- up from 26.2% last year, and the highest annual percentage since Gallup-Healthways started tracking rates five years ago. In other words, America -- the second fattest nation in the world after Mexico -- is now likely the fattest it's ever been.

Let's take a look at how Nike (NYSE:NKE), Weight Watchers International (NYSE:WTW), and VIVUS (NASDAQ:VVUS) apply three very different approaches to using smartphones to help Americans control their weight.

1. Fitness bracelets
Nike's FuelBand is a fitness bracelet that tracks a user's physical movement and records the results.

It connects to a personal computer or an Apple (NASDAQ:AAPL) iOS device via Bluetooth and uploads the data to its Nike+ social network. The bracelet was launched last February to compete against Jawbone's Up fitness bracelet, a similar device that initially lacked Bluetooth support.

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Nike's new Fuelband SE. Source: Company website.

Nike's iOS app and social network provide users with two reasons to get motivated -- they allow users to unlock video game-like "achievements" for reaching certain fitness milestones, and encourage users to compete against each other via a social network. Therefore, they tap into the gaming and social media appeal of smartphones to make personal fitness a social, rather than solitary, experience.

Jawbone has since caught up to Nike with the release last month of the Up24, which finally added Bluetooth support and real-time smartphone synchronization. Other newer products, such as the Fitbit Flex, Basis Band, and the Samsung Galaxy Gear smartwatch now also compete against Nike and Jawbone.

2. Traditional weight loss programs go mobile
Nike and Jawbone's success, however, has turned the traditional model for weight loss programs upside down.

Weight Watchers International, which was previously the undisputed king of weight loss programs, has been hit hard by the rise of fitness bracelets and other free calorie tracking apps. Last quarter, Weight Watchers' revenue fell 8.5% year over year as earnings plunged 10.5%.

The company's business model is based on selling point-based weight loss programs to customers. Unlike rival Nutrisystem, Weight Watchers doesn't deliver food -- it expects customers to follow its program guidelines to purchase healthy food, which is grouped by different point values. Customers also must participate in a face-to-face or online program to stay on track and achieve their fitness goals.

It's a business model that worked well before smartphone app developers gave away the same product for free. MyFitnessPal is one of these widely used free alternatives -- on its website, it offers calorie tracking tools similar to the paid ones from Weight Watchers, and it also offers a smartphone app that synchronizes daily calorie intake to its website.

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MyFitnessPal iOS app. Source: Company website.

By comparison, Weight Watchers Online costs $18.95 per month, with a one-time sign-up fee of $29.95. Its smartphone app has additional features like healthy recipes and barcode scanners for food information, but it is restricted to paid program members.

Free services like MyFitnessPal are only the tip of the iceberg that could eventually sink Weight Watchers -- other free apps like Fooducate (iOS, Android), Noom Weight Loss Coach (Android), and Run With Map My Run (iOS, Android) could all render Weight Watchers obsolete.

3. Can obesity drugs survive in a world of digital fitness?
Last but not least, we should mention the return of obesity drugs. Obesity drugs sold well in the 1990s, especially after two drugs -- fenfluramine and phentermine -- were combined to make fen-phen.

Unfortunately, fen-phen was found to cause potentially fatal heart problems, which led to its withdrawal from the market in 1997. As a result, major drug manufacturers were reluctant to launch new obesity drugs over the following decade.

Two companies, Arena Pharmaceuticals (NASDAQ:ARNA) and Vivus, gained FDA approval for two new obesity drugs last year, the first approvals in 13 years. However, neither drug -- Arena's Belviq and VIVUS' Qsymia -- has performed well, despite high analyst expectations.

Company

Drug

Peak sales estimate

2Q product sales

3Q product sales

Arena/Eisai

Belviq

$1.0 billion

$1.3 million

(31.5% of Eisai's sales of $4.1 million)

$1.7 million

(31.5% of Eisai's sales of $5.4 million)

VIVUS

Qsymia

$1.8 billion

$5.5 million 

$6.4 million 

Source: Quarterly earnings reports. Note: Eisai markets Belviq globally.

Anemic sales of Belviq and Qsymia could be caused by two main factors -- bad memories of fen-phen and the rising use of smartphone fitness apps for personal weight management.

Vivus apparently believes that tapping into smartphones could be the answer. The company recently launched a free smartphone app called "Q and Me" for iOS and Android devices. Q and Me is an app that resembles MyFitnessPal and Weight Watchers Mobile, with personal weight loss and fitness goals, calorie tracking, recipe suggestions, and personalized emails for motivation and tips.

Although Q and Me is available to anyone, not just patients taking Qsymia, the app promotes its own drug in a way that could be considered unethical, as seen on the right side of the following screenshot.

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Q and Me. Source: Company website.

Q and Me is obviously aimed at promoting a healthier image for VIVUS' Qsymia, but in my opinion people could be better off sticking with Nike's FuelBand, MyFitnessPal, or Weight Watchers Mobile instead. At least their apps don't advertise weight loss drugs in the middle of a workout routine.

The Foolish takeaway
Since smartphones are becoming an indispensable part of our daily lives, it's natural that they also become our virtual personal trainers, making sure that we exercise and eat right on a daily basis.

Nike, Weight Watchers, and VIVUS all represent different approaches to fighting obesity with smartphones. Which one do you think is the most effective? Please share your ideas in the comments section below!

Fool contributor Leo Sun has no position in any stocks mentioned. The Motley Fool recommends Apple and Nike. The Motley Fool owns shares of Apple, Nike, and Weight Watchers International. Try any of our Foolish newsletter services free for 30 days. We Fools may not all hold the same opinions, but we all believe that considering a diverse range of insights makes us better investors. The Motley Fool has a disclosure policy.