Some people do “dirty fasts” which means taking a little calories anyway during a fast. That would be a splash of milk in your coffee or anything below maybe ~50 calories ( the number is debatable)
Generally, taking ANY calories will break your fast for the purpose of autophagy. But if you just want to lose weight feel free to experiment. I found that taking calories in my fast makes me hungrier, but plenty of ppl say it helps them get through it.
Also i found things full of sugar, like fruits, make me SUPER hungry :p
Fung says a small dollup of cream in coffee is ok. Smallest effective portion. So if coffee with cream is life for you, this is ok.
But it’s better not to. It sort of poking the bear. A little nutrition is more likely to make the fasting harder to maintain, than just bring clean and eating during your window.