Present your diet as a get “Healthy Plan” instead of a weight loss plan.
People are pretty excited about someone pursuing health but the we often get jealous of someone potentially losing more weight than themselves (there is an oddly sabotaging attitude about this issue). Also as a doctor I would much rather people think about making dietary changes for healthful reasons than weight being the motivation.
Tell people what you are eating and what you have to avoid so they won’t think you are insulting their famous cookies or family pot roast recipe. People are more likely to feel important and validated if they know up front you won’t be eating so they don’t focus on you not eating their food.
Bring your own foods especially if your diet change is due to finding some new food allergies. If you suddenly can’t eat dairy then some holiday favorites may not be available to you. Coming prepared with your own food/drink will help you enjoy your holidays all the more.
Remember that your family loves you and just wants to share.
Especially grandparents like to cook for people they love and this may be upsetting to them if you refuse their food, but it isn’t about them. Your diet is important to you and they should respect that; they may give you a hard time only because they feel hurt you aren’t trying their food. Remember that you love your family always.