Glucose renal threshold

Glucosuria Occurs When the Renal Threshold for Glucose Is Exceeded... 

The processes of selective reabsorption of nutrients and xenobiotics goes on within the complex tubule system. 98-99% of filtered materials (salts, water, sugars, amino acids) are eventually reabsorbed by passive or active transport. Biomolecules such as glucose and amino acids are entirely reabsorbed if their concentrations are within the normal range in the blood. However, should the concentrations be higher than normal, those molecules might not be completely reabsorbed because they have exceeded the ability of the nephron transport systems to accommodate them. This is referred to as exceeding the renal threshold. Urine is therefore a convenient body fluid to assay for the initial assessment of metabolic or excretory system malfunctions. 

A striking symptom of diabetes is the high blood glucose level which may range from 8 to 60 mM. Lower values are more typical for mild diabetes because when the glucose concentration exceeds the renal threshold of 8 mM the excess is secreted into the urine. Defective utilization of glucose seems to be tied to a failure of glucose to exert proper... 

Whatever the site of the enzyme may be, Keston et al. have recently produced fairly conclusive evidence that glucose, which is reabsorbed by the kidney, is exposed to mutarotase at some stage of the process (117). Glucose infused into the renal artery spills into urine when the renal threshold is exceeded in the same anomeric form as that administered, whereas reabsorbed glucose in the renal vein is mutarotated. Hill has also shown that the anomer infused in excess is excreted in excess (73). 

Fructose is found in many plants and is an important portion of dietary carbohydrate. Most commonly it is ingested as free fructose or sucrose. Fructose is not actively transported by the intestinal mucosa, and variable proportions are converted to glucose in the process of absorption in man, about one-sixth is converted (Ml). Most of the metabolism of fructose occurs in the liver. If a renal threshold for fructose exists, it is very low. 

Because of increases in plasma volume and cardiac output, renal blood flow increases. The GFR ri.ses early in pregnancy, and creatinine clearance may be 150 ml/min or more by. 10 weeks. Serum urea and creatinine concentrations fall. Tubular function alters and. in particular, there is a reduction in the renal threshold for glucose. Glycosuria may be present in up to 70% of pregnancies. Tubular reabsorption of uric acid and amino acids alters, and their excretion in urine increases. 

Glucose tolerance declines with age even in the absence of diabetes mellitus. and the renal threshold for glucose rises. These observations can make the diagnosis of diabetes mellitus difUcult in an elderly patient. 

The normal renal threshold is between 9 and 10 mmol 1 but may show some individual scatter At this level, glucose is spilled over into the urine, resulting in a more or less strong correlation of blood... 

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