There are a lot of variables, and no simple answer. In general, if you're admitted to hospital for an illness, and an IV is established in order to administer medications, antibiotics etc, it will most likely be 5% dextrose in water (D5W), since that is what most drugs are compatible with. Not all drugs are stable in saline

In most cases, the infusion rate will be minimal, no more than 500 ml per 24 hrs ( = 25g dextrose). Most often, after 48 hrs, the IV infusion is discontinued, and a special cap placed on the IV right at the skin. This way, only the medication will have to be infused, but again, chances are good it will be diluted in dextrose.

During surgery, or obstetric cases .. 0.9% "normal" saline is the most common solution used. If there's a chance you may require blood or blood products (albumin, plasma, clotting factors) these are ONLY compatible with saline. Ringer's Lactate is also used during surgery, and in emergency cases where rapid infusion of a large volume is required, such as severe dehydration due to prolonged vomiting, diarrhea or shock. Ringer's base solution is saline, with added potassium and calcium lactate (not the same as lactic acid ).
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My thought is, if you're very ill ... running a fever, or had surgery and are in the process of healing, your calorie/energy requirements are going to be very high. Any dextrose will be used for fuel. Since the IV is being infused at a slow, constant rate, any insulin release will also be slow and constant. Unlike eating a meal where there's a blast of glucose all at once. Since it's going directly in the vein, the digestive distresses caused by eating sugar, and the triggers caused by smell, taste etc will be by-passed.

Another thought ... a small amount of glucose is required by certain cells in the body, in particular the production of red blood cells, certain areas of the brain and heart muscle. When we're eating a lowcarb diet, this small amount can be met by eating a minimal amount of carbs AND glucose can also be made in the liver from the protein that we eat. But when we're sick in hospital, and not eating, if we don't get at least a small amount of glucose in our system (or eat protein), the body will break down the muscle tissue for the protien in order to make some glucose.

Sometimes life is too complicated. Here's my wish for good health for everybody; may you never get sick!

Doreen