Here's a challenge for you. In the previous post, I said that examples of ineffective osmolytes were urea and glucose. That's because these two substances, although osmotically active, could easily distribute themselves across the various body compartments and so wouldn't cause fluid shifts from one compartment to the other. Although this is true in health, what happens in the case of diabetes?
The answer, of course, is that glucose becomes an effective osmolyte, capable for causing fluid shifts from the intracellular to the extracellular compartments. This is because diabetics have a (relative or absolute) lack of insulin, which is required for glucose entry into many cell types. Thus, for all intents and purposes, glucose becomes more confined to the extracellular compartment in diabetes.
This has serious implications, since the resultant fluid shifts are a major part of the pathogenesis of both diabetic ketoacidosis and the hyperosmolar non-ketotic state, which you've probably heard about.