The Health app lets you manually enter and view blood glucose values in mg/dL (milligrams per deciliter). This unit of measurement is used by a number of countries, while some other countries use mmol/L (millimoles per liter). HealthKit supports both units of measurement. However, if you measure your blood glucose using a device that displays mmol/L, those values can't be manually entered or displayed in the Health app with that unit of measurement.
Apple's solution is to remove the ability to manually enter values via the Health app until an update is released that supports both types of measurements.
To prevent confusion in countries where mmol/L is commonly used, we'll soon release a software update that will temporarily remove the ability to manually enter and view blood glucose values in the Health app while we work on an update to support both units of measurement. If you have previously entered values manually in the Health app, you'll no longer see this data in the Health app after the update. However, your data won't be deleted, and other apps with permission to read health data will still have access to blood glucose values that you previously entered.
Notably, third-party apps that interact with HealthKit will still work.
Third-party apps will continue to be able to support both units of measurement and can continue to use HealthKit APIs to store blood glucose data.
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Read More [via CNET]