What can explain the presence of glucose in the urine of people with untreated diabetes mellitus?

The presence of glucose in the urine of people with untreated diabetes mellitus can be explained by the saturation of glucose transporters, specifically the sodium-glucose transporters (SGLTs) in the renal tubules. Under normal circumstances, these transporters reabsorb glucose from the filtrate back into the bloodstream. However, in individuals with untreated diabetes, high blood glucose levels (hyperglycemia) can lead to a situation where the concentration of glucose exceeds the transport maximum of these glucose reabsorption mechanisms. When blood glucose levels rise significantly, the renal threshold for glucose is surpassed, causing the transporters to become overwhelmed and unable to reabsorb all the glucose, resulting in excess glucose being excreted in the urine.

This phenomenon effectively reflects the body’s inability to manage blood sugar levels properly, primarily related to the underlying lack of insulin or insulin resistance, which is why glucose spills over into the urine as a consequence of insufficient reabsorption.