Depends on who you ask. Many say only water, or water, tea, coffee or bouillon, but Dr. Fung allows 1T total of CO/butter in coffee if that is what it takes to keep you on track (and also water, tea, coffee, bouillon). The rationale is that pure fat does not raise insulin.

I don't chew until ~noon too, but need only 1 tsp of butter in my coffee to keep me full until then.