With some people it does affect ketosis in the first week or so but in some it doesn't. Personally it didn't affect it for me, when I did Cambridge for the very first time I lost a stone the first week and couldn't have stuck to it without all those black coffees! I have just started again after losing the weight and then putting it back on again during pregnancy. At the minute I'm adding a sweetener and some skimmed milk to coffee but I plan to cut the sweetener next week and the milk the following week.