OVERVIEW

What is renal glycosuria?

Renal glycosuria refers to a condition where glucose appears in the urine despite normal blood sugar levels (below the renal threshold for glucose—the maximum capacity of the kidneys to reabsorb glucose from the filtrate). Normally, glucose in the filtrate is reabsorbed by the renal tubules and returned to the bloodstream, resulting in negative urine glucose. In renal glycosuria, impaired glucose reabsorption in the proximal tubules leads to positive urine glucose.

Clinically, it is classified into primary and secondary renal glycosuria. Secondary renal glycosuria is mainly caused by chronic kidney diseases, especially tubular dysfunction, and is relatively rare. This section focuses on primary renal glycosuria.

Primary renal glycosuria is a congenital genetic disorder caused by gene mutations that impair glucose reabsorption in the renal tubules. As a result, glucose appears in the urine even with normal blood sugar levels, leading to positive urine glucose (composed of glucose).

This condition is also known as familial renal glycosuria or benign glycosuria. Apart from positive urine glucose tests, patients typically show no other specific symptoms.

Is renal glycosuria common?

This condition is rare, with only a few reported cases in China.

Are renal glycosuria and diabetes with positive urine glucose the same disease?

No, they are different. Renal glycosuria presents with positive urine glucose but normal blood sugar levels. In diabetes, positive urine glucose usually occurs due to elevated blood sugar or the use of specific glucose-lowering medications.

What are the types of renal glycosuria?

Renal glycosuria is classified into three subtypes based on the renal threshold for glucose and tubular glucose reabsorption capacity: Type A, Type B, and Type O.

• Type A features a reduced renal threshold and decreased tubular glucose reabsorption capacity.
• Type B features a reduced renal threshold but normal tubular glucose reabsorption capacity.
• Type O features a complete loss of tubular glucose reabsorption. Patients may experience persistent nocturnal enuresis, polyuria, excessive hunger, or intermittent hyperphagia. Affected children may show delayed puberty and growth retardation, but adult height usually reaches normal levels.

Understanding renal threshold and tubular glucose reabsorption: The kidneys act as the body's "drainage system," retaining useful substances while excreting waste. This process involves two key structures: the glomerulus and the renal tubules.

The glomerulus is the first "filter," retaining beneficial components in the blood while allowing waste and some useful substances (including glucose) to pass into the filtrate (primary urine). The renal tubules then reabsorb essential substances like glucose to prevent their loss in the final urine (secondary urine).

Tubular glucose reabsorption has a limit. If blood sugar exceeds a certain level, the excess glucose in the filtrate surpasses the tubules' reabsorption capacity, leading to glucose in the urine. The blood sugar level at which this occurs is called the renal threshold for glucose, typically 8.88–10.55 mmol/L.

SYMPTOMS

What are the common symptoms and manifestations of renal glycosuria?

• Renal glycosuria has a familial clustering tendency, primarily presenting as glycosuria. Additionally, since certain protein molecules responsible for glucose reabsorption in renal tubules are also expressed in other organs (e.g., the intestines), corresponding gene mutations may lead to other clinical manifestations, such as intestinal malabsorption causing watery diarrhea in newborns, dehydration, and malnutrition.
• Glycosuria may be present at birth and persist throughout life.
• Patients with type O renal glycosuria may exhibit persistent nocturnal enuresis, polyuria, hyperphagia, or intermittent hyperphagia. Affected children may experience delayed puberty and growth retardation, but their adult height typically reaches normal levels, meaning it does not impact final stature.

What are the complications of renal glycosuria? Does renal glycosuria cause serious harm to the body?

Renal glycosuria has a favorable prognosis, with patients not exhibiting severe clinical consequences. It is considered a benign form of glycosuria.

CAUSES

What is the cause of renal glycosuria?

Renal glycosuria is a congenital genetic disorder caused by genetic issues. This condition results from gene mutations that reduce the reabsorption of glucose from the original urine by the renal tubules, leading to increased glucose in the final urine, which manifests as a positive glucose test in urine.

Which groups are commonly affected by renal glycosuria?

• Family members with a history of renal glycosuria are more likely to develop the condition.

• Some chronic kidney diseases that damage the renal tubules may also lead to secondary renal glycosuria, such as certain cases of nephrotic syndrome, multiple myeloma, secondary Fanconi syndrome, or renal tubular acidosis.

Is renal glycosuria contagious?

Renal glycosuria is not contagious; it is a congenital genetic disorder.

Is renal glycosuria hereditary?

Renal glycosuria is a hereditary disorder and can be passed down. It primarily follows an autosomal recessive inheritance pattern, though some families may exhibit autosomal dominant inheritance.

DIAGNOSIS

How is renal glycosuria diagnosed?

When diagnosing renal glycosuria, doctors primarily refer to the following diagnostic criteria:

• Persistent and relatively stable glycosuria (daily urinary excretion of 10–100 grams of glucose) even in a fasting state;
• Confirmed presence of glucose in the urine;
• Normal fasting blood glucose levels and oral glucose tolerance test results.

What tests are needed for renal glycosuria?

Generally, blood and urine tests as well as genetic analysis are required.

• Urine test: Primarily used to detect glycosuria.
• Blood test: Mainly used to measure fasting blood glucose and glucose tolerance test results.
• Genetic analysis: Helps confirm the diagnosis and identify the site of gene mutation.

Which diseases can renal glycosuria be easily confused with? How to differentiate them?

Since many conditions, including renal glycosuria, can present with positive urine glucose, it is important to distinguish renal glycosuria from diabetes mellitus and other glycosuric disorders.

• Diabetes mellitus: Can be differentiated using blood glucose and glucose tolerance tests. Diabetes shows elevated blood glucose and abnormal glucose tolerance results, whereas renal glycosuria exhibits normal blood glucose and glucose tolerance test results.
• Other glycosurias: Mainly distinguished using paper chromatography. Other glycosurias involve non-glucose sugars in the urine, while renal glycosuria involves glucose.

TREATMENT

Which department should I visit for renal glycosuria?

Nephrology, Endocrinology, General Internal Medicine.

Can renal glycosuria heal on its own?

No, renal glycosuria cannot heal on its own and is a lifelong condition.

How is renal glycosuria treated?

Renal glycosuria usually does not require special treatment. It is sufficient to moderately increase carbohydrate intake, but excessive exercise and physical exertion should be avoided.

Notably, during pregnancy or extreme hunger, patients with renal glycosuria may experience dehydration and ketosis. Therefore, regular follow-ups are necessary, especially for pregnant patients.

Does renal glycosuria require hospitalization?

Hospitalization is generally not required.

Can renal glycosuria be cured?

No, renal glycosuria cannot be cured and will persist for life.

DIET & LIFESTYLE

What should patients with renal glycosuria pay attention to in daily life?

Patients with renal glycosuria should moderately increase their daily carbohydrate intake and avoid excessive exercise and physical exertion.

PREVENTION

How can patients with renal glycosuria prevent complications?

The prognosis for renal glycosuria is favorable, and patients typically do not develop severe complications. Avoid extreme hunger, ensure regular prenatal check-ups during pregnancy, and prevent dehydration and ketosis.