Lets follow sugar as we eat it and how the body deals with this food.
Once eaten it is readly absorbed through the gut and transported by your blood.
Sensors in the pancreas monitor the blood sugar level and when it is too high they secrete insulin a hormone that talks to the body cells saying hey open up and take in this sugar to use as food..
Trouble starts when there has been high blood sugar levels for some time. The bodies cell become 'deaf' to the frantic calls of Insulin which the pancreas secretes more and more of - hey if a little does not work then why not a lot more?
Insulin being a resourceful hormone then signals the sugar to be laid down as fat stores.