Apple Health is a feature of Apple’s iOS that manages the syncing of health and fitness data between apps, including MyFitnessPal on the iPhone.
Once MyFitnessPal and Apple Health are linked, MyFitnessPal and Apple Health will (with your permission) share these types of data:
Food - MyFitnessPal will update Meal Summaries (calories and nutrients) to Apple Health
Exercises - Exercises that are logged in Apple Health by other apps will adjust your calorie goal in MyFitnessPal. Activity added manually by the user to Apple Health does not sync to MyFitnessPal. If you have an integration partner synced with both MyFitnessPal AND Apple Health, MyFitnessPal will de-duplicate any logged exercise from that partner. Exercise simultaneously tracked by more than one app, however, will cause duplicates.
Weight - MyFitnessPal and Apple Health will update weight in both directions
You are always in control of which types of data are shared between MyFitnessPal and Apple Health. For information about adjusting these settings, please see this article.
IMPORTANT NOTE FOR MYFITNESSPAL USERS WHO LOG IN ON THE SAME IOS DEVICE
Apple Health does not distinguish between multiple MyFitnessPal accounts used in the same app on the same device.
If you share the MyFitnessPal app on your iOS device with others by logging in and out of multiple accounts, we do not recommend linking MyFitnessPal with Apple Health.
If you do, data from one MyFitnessPal account may overwrite existing data in Apple Health, and vice versa, resulting in mingled data in your individual MyFitnessPal accounts or no data transfer at all.