The top 20 health apps for mobile devices, including MapMyFitness, iPeriod and Web MD Health, have been sending sensitive user information to around 70 third-party companies, according to privacy group Evidon.
The third parties, which are primarily advertising and analytics companies, use the information gathered from consumers who are tracking diseases, diets exercise routines and eveb menstrual cycles to build profiles and target ads.
Jeff Chester, executive director of the Centre for Digital Democracy, said that it meant some of the "most sensitive details of your life"were available to others.
The apps companies, including Web MD, have denied that the information is personally identifiable or that it is being sold.
They added that the information was being used for site analysis and advertising within the apps, and companies are not allowed to combine the information with the gathered elsewhere.
There are regulations in place to prevent the tracking and selling of an individual’s specific prescription or medical records, but some companies have been accused of trying to bypass these restrictions by building health profiles from information on apps.
The health and fitness apps have seen an increase in popularity, and some companies offer them for free.
But Scott Mayer, chief executive of Evidon, warned: "If there is a lot of content that is being provided to you for free, data is driving the economy of that content."
Data tied to mobile phone apps is catching up with the internet, he added.
