This is a tough question because insulin responds to virtually everything you put in your body. But we will briefly describe insulin’s functions: insulin is a hormone that promotes the body’s use of glucose (blood sugar), protein synthesis (the making or repairing of tissues) and storage of nutrients, some of which you may not immediately need. Insulin is the primary hormone the body uses to maintain your blood sugar within a healthy range. Therefore when you consume foods, and especially carbohydrates (because they are the first foods converted to blood sugar), you illicit an insulin response in order to keep you alive.
Inadequate secretion of insulin (poor response) results in the improper metabolism of carbohydrates, protein & fats, leading to diabetes characterized by high blood sugar that then causes many other health problems.
If you eat pure sugar on an empty stomach, you will get large quantities in the blood very fast, which will then trigger an immediate insulin response that might “overreact” and send your blood sugar to the low side of normal, making you feel temporarily weak or dizzy. If you are basically healthy this feeling passes quickly or you may want to eat something that contains carbohydrates, but less sweet to improve your mood until everything normalizes. To avoid this, don’t eat pure “sugar” by itself on an empty stomach or other very simple carbohydrate treats in this manner. It's always best to eat meals containing all 3 macronutrients, protein, carbohydrates and fats, because it helps maintain steady blood sugar.