I used to work next to the guys from mySugr. They went through the process of having their app certified as a medical device by the European Commission, (the CE or Conformité Européenne mark). Not easy, but doable. I'm not sure whether it gives explicit recommendations, but it let's you identify patterns based on what you're eating and the activities you're doing.
Regulations are a bastard, but there for good reasons. Imagine getting a med which wasn't tested and which is made in someones bathtub…
Exaggerated but you get what I mean?
https://mysugr.com/