Sugar

The degu's Kryptonite, Sugar is present in a lot of food and care must be taken when treating your degus as too much sugar will cause Diabetes. The degus are unable to process (break down) sugar in their bodies and unprocessed sugar in anyone's body is dangerous.

The first sign of Diabetes is Cataracts (large white dots) in their eyes, and this is not reversible but it is a good sign that you need to seriously cut back on the sugar they're getting.

The degu's main source of sugar intake is from free sugars as degu owners would never actually feed refined sugar (or anything containing it) directly to their degu.