Why does a test for diabetes mellitus be performed in a fasting sample?

Because if you take the food and go to the tests then you will not get the accurate result of the test's that the reason doc's say that dont eat before a diabetic test i am telling this because my grand father is a diabetic patient so he takes insulin daily due to that and after every month he will test the sugar level.

I am not a medical expert but I am very interested in diabetes. Perhaps because older people tend to catch type 2 diabetes. I think a person fasting will have no "extra glucose" from ingested food. I think this means that the insulin level should be at its minimal since there is no need to reduce blood sugar levels. Perhaps the insulin level at fasting has to fall in a normal range... Comments please. :) Cheers :)