Can new Health-app fields be created by 3rd parties? So, I've been using Apple's Health app, and I think it's wonderful. 

My Apple Watch frequent enters my heart rate and steps-taken into the database, and I can manually enter other common things like my blood pressure and weight. Cool. 

However, I don't see fields for everything that I may want to track. 

For example, there aren't any fields to keep track of so-called 'healthy' fats, like DHAs / Omega-3 fatty acids. 

I know that there are 3rd party apps that keep track of this sort of thing. But they seem to store that data in their own internal databases. I haven't found an app that can write data to the Health database. That is, unless the field already exists. 

So, that has led me to wonder: Is Apple the sole provider/decider of the fields that can be used in their Health app?  

Or, is it possible for 3rd party apps (or just a regular user like me) to create new fields that will work in the Health app?
Neither you nor third-party developers can add fields to Health.

Working with the HealthKit Framework
The HealthKit framework is designed to share data between apps in a meaningful way. The framework constrains the types of data and units to a predefined list, ensuring that all apps understand what the data means and how it can be used. Developers cannot create custom data types or units. Instead, HealthKit provides a wide variety of data types and units.

HealthKit | Apple Developer Documentation