where does it come from?

[Robbity]

I kept on readng and see I had found out what it meant a month or so ago - 'alternate day fasting'. Maybe this time I will remember.

If I help other readers by unzipping unusual/rare acronyms, I am pleased for the service.

I was just making a guess what "adf" means when your post confirmed it for me, so my decrepit old brain offers you a big thanks!