Many things that we eat and drink show up in our blood for a long time after consuming them. If a doctor wants to get a clear picture of something like Glycosuria or Proteinuria, they will ask for a fasting blood sugar.
Anytime I have had to go for a fasting blood sugar test, I schedule it for as early in the morning as possible. This usually means you cannot eat or drink anything after midnight.
I love to eat breakfast, so if I schedule the test first thing in the morning, I can still eat breakfast when I am finished.