Inulin is not digested because of the linkages ( beta-1,2) present in it.

Inulin as a Test for Kidney Functioning

Inulin is a naturally occurring polysaccharide that is commonly used to test kidney functioning. It is a polymer of fructose, meaning it is composed of multiple fructose molecules joined together. Inulin is particularly useful for this purpose because it is not digested or metabolized by the body, allowing for accurate measurement of kidney filtration.

How Fructose is Used in the Test

While it is true that fructose is digested and can release ATP (adenosine triphosphate) during metabolism, the use of inulin as a test for kidney functioning does not involve the digestion of fructose. Instead, inulin is administered to the patient as a solution, typically intravenously.

Measurement of Kidney Filtration

Inulin is chosen for kidney function tests because it is freely filtered by the glomerulus, a network of capillaries in the kidney. It is not reabsorbed or secreted by the renal tubules, making it an ideal substance to measure the glomerular filtration rate (GFR).

Administration of Inulin

Inulin is typically administered to the patient as a bolus injection or as a continuous infusion. Once in the bloodstream, it circulates freely and is filtered by the glomerulus. As it is not metabolized or reabsorbed, the amount of inulin excreted in the urine can be used to calculate the GFR.

Measurement of Inulin Clearance

To measure the GFR using inulin, a blood sample is taken to determine the concentration of inulin in the plasma. Simultaneously, a urine sample is collected over a specific period of time, typically 1-2 hours, to determine the amount of inulin excreted. By comparing the plasma concentration with the amount excreted, the GFR can be calculated using the formula:

GFR = (Urine flow rate x Urine inulin concentration) / Plasma inulin concentration

Advantages of Inulin as a Test

1. Accuracy: Inulin provides an accurate measurement of kidney filtration as it is not metabolized or reabsorbed by the body.
2. Sensitivity: Inulin clearance is highly sensitive to changes in renal function, allowing for early detection of kidney dysfunction.
3. Specificity: Inulin measures only the glomerular filtration rate, providing valuable information about the kidney's primary filtration function.
4. Safety: Inulin is considered safe for use in kidney function tests and does not have any known adverse effects.

Conclusion

Inulin, a polymer of fructose, is used as a test for kidney functioning due to its non-digestible nature and accurate measurement of glomerular filtration rate. It is administered intravenously and its clearance from the body through urine allows for the calculation of GFR. Inulin provides a reliable and sensitive assessment of kidney function, aiding in the diagnosis and monitoring of renal diseases.