Continually challenging your body with foods that are rich in fat and sugar can put you at risk for developing large blood sugar fluctuations and eventually, diabetes. If you already have diabetes, keeping your blood sugar levels stable can prevent or delay the onset of diabetic eye diseases including diabetic retinopathy, glaucoma and cataracts. Blood sugar fluctuations can also make the natural lens inside the eye swell, resulting in large prescription changes. You may be nearsighted one day, then farsighted the next. When this occurs, a possible diagnosis of diabetes is usually considered.