Glucose

In a first step, our kidneys function like a filter. Some substances are not filtered and remain in the blood, others pass through the filter and enter the primary urine, the first form of later urine. Glucose is such a small molecule that it passes through the kidney filter and first enters the primary urine. In a second step, the formed primary urine flows through a long system of tiny tubes, the tubules, and is further modified. Some substances and large amounts of water are absorbed back into the blood or tissue, resulting in a much more concentrated final urine, the final version of urine. These substances also include glucose. Therefore, it is not excreted with the final urine, but remains in the body.

However, if there is an excessive amount of glucose in the blood, such as in diabetes mellitus, the kidneys no longer manage to transport all the glucose that was initially filtered back into the blood. The glucose transporters in the tubules are supersaturated. This is called exceeding the kidney threshold. Consequently, glucose remains in the urine and is excreted. Up to a blood sugar level of 160-180 mg/dl (= 8.9 – 10.0 mmol/l), the kidneys can usually return the entire glucose back to the blood. If the blood sugar level continues to rise, the kidney threshold is exceeded.

However, glucose is not exclusively found in urine with diabetes. Even with diseases of the kidneys, it can lead to the excretion of glucose. Here, the kidney threshold is not exceeded in the classical sense, but it is reduced by the damage to the kidney. Even glucose levels, which normally do not lead to exceeding the kidney threshold, now ensure that the pathologically lowered threshold is exceeded. Then, even at normal blood sugar levels, glucose is excreted in the urine.