don't know amd all cats react differently but, i would think 1 a day shouldn't hurt BG level much considering i have recently been giving my cat 2 pieces of his all time favorite crappy temptation milk treats full of carbs again and it hasn't shot up his bg by much at all (maybe around 2o i think) and it doesn't affect it for long..
this gnc supplement has a caution however that mentions asking vet before giving to animals with diabetes, don't see that on things often so personally i would ask my vet first just in case something in it interferes with insulin.