More than half of smartphone users (58 percent) in the United States have downloaded a health-oriented app while 42 percent had downloaded five or more, according to a new survey published in the Journal of Medical Internet Research. But they don't always continue using them after they download them.

According to the study, analyzed by researchers at NYU Langone Medical Center, the most popular health apps focus on fitness and nutrition. Those who had such apps used them almost daily, according to the survey. Calorie-monitoring and calorie-burning far outpaced other themes covered by the more than 40,000 health-related apps available on iTunes, including provider directories, self-diagnosis and prescription filling.

The most popular reasons for downloading health apps were to track physical activity (52 percent), to track eating patterns (47 percent), to lose weight (46 percent) and to learn new exercises (34 percent).