Why can't you inject sucrose intravenously if a person has low blood sugar?

When a patient's blood sugar levels are extremely low, it is important to administer replacement sugar intravenously, rather than orally. This is because by injecting the sugar straight into the blood stream, you bypass the time-consuming and unreliably step of getting it absorbed by the intestine.

But we only ever administer glucose (the biological form is frequently known as dextrose - see this article). And, come to think of it, we only ever measure blood glucose levels; when we say a person's blood sugar is low, we mean that their blood glucose is low. Why?

The answer, of course, is that it is the common fate of any carbohydrate to become glucose. As we've covered here, although carbohydrates can range from massive polysaccharides to tiny monosaccharides, only monosaccharides are absorbed from the intestine. The body's enzymes simply breakdown any carbohydrate larger than this until its constituent single sugar units are left. (Of course, there are three common dietary monosaccharides - glucose, galactose and fructose - but the other two are converted into glucose by the liver, so any carbohydrate ends up as glucose.)

So why can't we administer sucrose intravenously? Well, sucrose is a disaccharide; it's two monosaccharides joined together (glucose + fructose), and since a disaccharide normally never reaches our bloodstream without being degraded into two monosaccharides first, our cells are simply unprepared for it. To put it bluntly, there isn't even a mechanism to get the sucrose inside a cell!

So, no, if your friend starts to feel dizzy from a low blood sugar level, you can happily fetch her some sucrose to eat (it's table sugar, by the way), but you can't inject it into her veins.

Alas.