I've done 18 days while working. My job is mixed, on my feet/at a desk throughout the day. No heavy labor. My biggest issue wasn't lack of mental clarity, I did fine in that department. I just had to be careful getting up from a sitting position because of orthostatic hypotension, which is a common side effect of a long fast due to drop in B.P.
Most people in the imperialist core don't have the luxury of not working during a fast. I would just play it by ear and see how you feel. If it hampers your productivity, you can always eat.