I can’t pretend to be a scientist but protein right at the beginning and end of the eating window always feels great to me after being hungry/in preparation for being hungry. Sugars don’t feel great, I crash really badly and it feels like empty calories.
I do one longer weekly fast (24-42 hours) in addition to daily 16:8. For me, no carbs or sugar- nothing except bone broth with a little sea salt. Then, Wait an hour. Try Thinly sliced cucumber with olive oil and lemon juice, kalamata olives. Water. Wait for about 30 minutes. The intestines quickly empty out (gross but it happens with longer fasts). It might happen a few more times. Wait until it’s all done before you eat a big meal.