Apple's Health app gives users an "easy‑to‑read dashboard" for their health and ﬁtness data. Users can easily collect this information by allowing third-party health and ﬁtness apps to read and write data within the Health app. To do this you will need to individually give third-party apps permission to access your Health app. This is normally done somewhere in an app's settings if the app doesn't prompt you to give it access during the initial setup. This is no different than giving an app permission to use your location or access your photos.
iPhone Health App
Apple's Health app was created to help iOS 8 users store all their health data in one location, and to allow users to install third party Health-enabled apps to utilize this information. To do so third party apps need permission to access the data stored on the Health app, but you may not want some apps to access your private information. You can stop an app from repeatedly requesting access to your Health app data by navigating to Settings > Privacy > Health. From here you can manually disable apps that have requested access to your health data.