Diet and fitness apps were used by 55.7 million American adults in 2013, up from 43.9 million in 2012, according to a study by Kantar Media's MARS OTC/DTC Study of 20,000 consumers.

The study found that 129 million American adults (55 percent of the population) own either a smartphone or a tablet or both -- 116 million (49 percent of the population) own a smartphone and 64 million (27 percent) own a tablet. By 2016, Kantar expects 60 percent of American adults to have a tablet or smartphone.

The survey asked respondents how they used their devices, and found that 34 percent of smartphone owners and 31 percent of tablet owners used their device to look for health-related information, while 25 percent of smartphone owners and 22 percent of tablet owners used their device to track their health, diet, or exercise.

Out of the survey respondents who said they had used the internet in the previous 30 days, 23 percent used mobile fitness apps and 20 percent used nutrition or dieting apps. Ten percent used symptom checkers or diagnostic tools and less than 7 percent used each of the following: health testing and tracking apps, health reference apps, or locator tools for clinics or pharmacies.

The number of adults using nutrition and dieting apps in this category doubled since 2012, according to Kantar, and those using fitness apps increased 33 percent.

On the other hand, 57 percent of adults who have used the internet in last 30 days reported that they had not used any type of mobile app to track health content, including 40 percent of smartphone owners in the internet group and 45 percent of tablet owners.

When Kantar looked at the subset of just smartphone and tablet owners who had used the internet in the last 30 days, they found slightly higher numbers: 28 percent of smartphone owners and 27 percent of tablet owners used nutrition and dieting apps, while 32 percent of each used fitness apps. Fourteen percent of smartphone owners and 12 percent of tablet owners in that category used symptom checkers.

In late 2012, the Pew Internet Project found that 21 percent of Americans use digital technology to track health indicators, a number which had stayed flat since 2010. The Kantar numbers, which express trackers as a percentage of smartphone owners only, puts that figure quite a bit lower.