The short answer is carbohydrates.
The answer is diabetes is a disease where your blood sugar (aka blood glucose) is elevated. This glucose comes from your diet. Remember glucose is sugar and sugar is a carbohydrate so a dietician will want to monitor this macromolecule. Your glucose is controlled by insulin which is a hormone that helps get the glucose into your body to provide energy.
There are two type of diabetes, type 1 is when your body does not make insulin and type 2 when you body does not make or use the insulin well. Without insulin your body cannot synthesize the macromolecules you ingest and will result in high blood sugar levels.
In the long term too much sugar can affect your nerves, kidneys and even your eyes. It can lead to heart issues, stroke and cause circulation issues where limbs may even need to be amputated.