The body uses what's available. If you eat no sugar, it will make it's own.
Gluconeogenesis - Wikipedia
That part of your metabolism is dedicated to getting rid of it. Your blood sugr is not supposed to be high, it's basically poison for your organs. The only reason you can eat a bunch of fruit or processed carbs/grain is because your pancreas is holding up in spit of it. If it stops, boom, Diabetes and other problems. The modern diet is not good for you. A lot of these conditions people suffer from basically didn't exist before industrialization, factories making bread and processed foods, etc. People ate meat and veggies, and fruit when they had it, and not all the time. Fruit was a treat, not staples.
