About 10-11 hours after your last insulin response your body switches over to burning fatty-acids instead of glucose.
The longer you go without insulin the longer the mitochondria in your body will burn fatty-acids instead of glucose.
The end.
Edit: that means even the little sugar in your coffee and some day artifical sweeteners too. No tricks, no hacks.
The research shows we really need about 16 hours. Most important it seems to have a couple hours between eating and bed and also before we eat after we wake.
Also like the other response said, clean fasting means, clean “water only” some have black coffee or plain tea
If you only eat once per day, you will get the results you are looking for. I use to do 16:8 and lost 5 lbs in 3 years. On OMAD, I lose about 10lbs per month. I’m less hungry on OMAD after adjusting to the new schedule.
16:8 is the bare minimum you need to do in order to see weight loss results, imo. 12:12 is basically society’s standard eating window, so you’ll likely not make any progress or make very slow progress. Imo, 18:6 is the sweet spot for weight loss. I’ve lost about 1/2 the 90+ pounds I’ve shed thus far with that method. And if you can stretch it to 20 hours, it’s even better. 12:12 is really only okay once in a great while.
Yes, sugar in coffee will break your fast. It triggers an insulin response.