I so get this one.
I have several grandmothers and one gramma in law, and when you show up they want to feed you. It's an old social thing. They had enough food to share and feed you. Granted they are all 95+, I get that. To them and the generation, refusing food was an insult. After all, they lived through the depression and food was a valuable commodity at that time and sharing it meant they were on the wealthy side.
As for today, it's a different story.
I have developed tons of allergies!
Even though my only truly medically diagnosed allergy is chocolate, I have become, theoretically allergic to sugar, MSG, gluten intolerant, and so on!
One thing I discovered is I don't say I can't eat this or that, I simply say, "I don't eat x,y,z"
Can't gives the control to someone else, "DON'T" gives the control to YOU!
Live it, love it, own it!
If the rest don't like it, they can go pound sand, or beat their laundry on rocks!
If you were diagnosed with diabetes, how would you handle the cake thing? Obviously cake is not a good option.
We all have choices, we just have to choose!
I have some relatives that are on board with me, some are not. The ones that are, I love, the that aren't, I still love, but blot out the noise, I am healthy and I feel great.