The fasted state is not an on/off switch
I often encounter questions that goes a little something like this
"...can I use milk/cream in my coffee during the fast?"
"...will I break the fat if I have a 1/2 cucumber during the fast?"
"...I accidentally drank a mouthful of regular Pepsi during the fast, is this considered a fast breaker?"
A: The fasted state is not an on/off switch that immediately gets turned off once you have a tiny amount of calories, like a lot of people seem to believe. The research is not clear on exactly what it is with fasting that causes the positive effects seen in the clinical trials, but some of the hypotheses revolve around
a) letting insulin levels drop below a certain threshold, rather than a semi-elevated state (which would be the case with a higher meal frequency approach)
b) creating an acute energy deficit (which you enter during the fast)
Note that I'm really dumbing this down to get my point across.
So, it's a question of a dose-response effect. Can you have some milk in your coffee? Sure, I wouldn't worry about it and I have it myself. Life would just be too damn boring with only black coffee, especially if you're used to having some milk with it. How much milk/cream?
I would put the limit at 50 kcal total used throughout the fast. That's about 1 deciliter or 1/2 cup 2% milk.
Can you eat half a cucumber during the fast? Sure, I wouldn't worry about it, though I don't see the reason as to why you would do so. A cucumber won't fill you up, and eating something, anything, may actually stimulate hunger during the fast in some people. Not to mention the fairly strong hunger suppressive effects of fasting, which makes the need for eating something quite pointless.
In conclusion, use common sense and understand that the fasted state and the positive effects that may result from it isn't an on/off switch, or an 'all-or-nothing' kind of deal.
It's the same thing with the pre-workout meal I recommend for fasted training. For clients doing AM sessions, I recommend something along these lines
pre-wo meal of 10 g BCAA 5-15 min prior to session
Training at 8-9 AM
Break fast with pwo meal at 10 AM or as soon as you're done training: the 8-10 hr feeding window starts now (and not with the pre-wo BCAA's).
Some might argue that "oh, well the BCAA's are insulinogenic/they provide calories and that will take you out of the fasted state" etc. Yeah sure, but then again exercise has the effect of dropping insulin levels. And coupled with what is known from research on pre-workout protein and the effects on protein synthesis, the pro's of the 'fasted' pre-wo meal certainly outweighs whatever miniscule effect it has on insulin or the tiny amount of calories they provide.
http://www.leangains.com/2009/07/questions-answers.html