App data services company Flurry released statistics based on results from 15,000 iOS users in the US about how Millennials, defined as young adults aged 25 to 34 years old, and all age groups use smartphones. The data came from apps with Flurry's data-collecting software installed. All categories surveyed, including Sports, Health and Fitness, rose steadily during the day and peaked in the evening.

The data showed that Millennials were engaged with their smartphones every hour in the day and used fitness and health apps twice as much as the average of other age groups. In a gender split, women use health and fitness apps 200 percent more than men do.

While Flurry doesn't reveal an overal adoption metric for health apps, Pew's surveys from the past few years have.

In November 2012, The Pew Research Center's Internet and American Life Project's Mobile Health survey showed the percentage of mobile phone users who have downloaded a health app, 19 percent, has remained unchanged since 2010. Of people aged 18 to 29, 42 percent use their phone to look up health or medical information, while of those aged 30 to 49, 39 percent look up health information from their phone.

The Pew survey also reported that of the group that said they had seen "significant change...such as gaining or losing a lot of weight, becoming pregnant, or quitting smoking," 29 percent were health app adopters.

The Flurry data gathered came only from the apps that use Flurry's data analytics services. Without knowing which health and fitness apps Flurry is tracking, it's unclear to what extent this data is skewed.