Managing Kidney and Bone Health

In Fanconi-Bickel Syndrome (FBS), the kidneys and bones are deeply interconnected. Because the kidneys “leak” essential minerals into the urine, the body is forced to pull those same minerals from the bones to keep the blood chemistry stable ^[1]. This can lead to softened bones and long-term kidney stress. Managing this aspect of FBS requires a dedicated “replacement therapy” strategy and careful monitoring.

Managing the Kidney “Leaks”

The primary kidney issue in FBS is Proximal Tubulopathy, often called Fanconi Syndrome. The proximal tubule is the part of the kidney responsible for reabsorbing most of the filtered glucose, amino acids, bicarbonate, and phosphate before they reach the bladder ^[2]. In FBS, this mechanism fails, leading to several specific imbalances that require daily intervention:

• Metabolic Acidosis: Because the kidneys leak bicarbonate (an alkaline substance that neutralizes acid), the blood can become too acidic ^[3]. To correct this, doctors prescribe alkali supplements, such as sodium bicarbonate or citrate solutions ^[4][5]. These help keep the blood’s pH in a healthy range. Note that supplements replace what is lost; they do not repair the underlying GLUT2 defect in the kidney cells.
• Electrolyte Depletion: In addition to bicarbonate, the kidneys may leak potassium and other electrolytes ^[6]. Low potassium (hypokalemia) can cause muscle weakness or fatigue, so supplements are often necessary to keep these levels steady ^[3][7].

Protecting Bone Health and Treating Rickets

One of the most visible impacts of the kidney leak is hypophosphatemic rickets. When the kidneys lose too much phosphate—a key building block for bone—the skeleton becomes soft and can bow under the child’s weight ^[1][8].

To heal and strengthen the bones, the standard toolkit includes:

• Phosphate Supplements: These are given multiple times a day to replace what is lost in the urine ^[4].
• Active Vitamin D: Typical over-the-counter vitamin D is often not enough. Doctors usually prescribe “active” forms like calcitriol or alfacalcidol ^[4][9]. These forms are already processed so the body can use them immediately to help the gut absorb more calcium and phosphate ^[8].
• Calcium: While children with FBS usually have normal calcium levels, they may need extra calcium to support bone rebuilding, but only when explicitly indicated by diet and laboratory findings ^[3].

The Balancing Act: Monitoring and Risks

While supplements are essential, they must be perfectly balanced. Both the disease itself and the treatments (like high doses of phosphate and vitamin D) can lead to hypercalciuria, where too much calcium ends up in the urine ^[1][3].

If this calcium builds up in the kidney tissue, it can cause nephrocalcinosis (calcium deposits in the kidneys), which can eventually lead to chronic kidney disease ^[10][1]. Note that tubular disease can be significant even when creatinine or estimated GFR is normal, so your team will track specific serum and urine measurements. To prevent calcification, your medical team will perform regular check-ups:

1. Lab Work: Frequent blood and urine tests to monitor phosphate, bicarbonate, and alkaline phosphatase (a marker of bone activity) ^[3][10].
2. Renal Ultrasounds: Periodic imaging to screen for any early signs of calcium buildup or stones in the kidneys ^[10].

The “New Frontier”: SGLT2 Inhibitors

Recently, researchers have begun investigating a new class of medications called SGLT2 inhibitors (such as empagliflozin or dapagliflozin) for use in FBS ^[9][2].

What the Research Shows (So Far):

• The Rationale: These drugs are normally used for diabetes because they block the kidneys from reabsorbing glucose. In FBS, the goal is slightly different: by blocking glucose from entering the kidney cells in the first place, these drugs may reduce the “logjam” of glycogen that causes cell damage ^[2][9].
• Early Findings: A small case series of seven patients (including three children) showed that those who took empagliflozin had improved markers of kidney function ^[9]. Remarkably, the three children in the study were able to reduce or stop their phosphate and alkali supplements while on the medication ^[9].
• A Safety Paradox: While these drugs lower blood sugar in typical patients, the small FBS studies reported no episodes of symptomatic low blood sugar (hypoglycemia) during the follow-up period ^[9].

What You Should Know:
SGLT2 inhibitors are currently off-label and are not yet the standard of care for FBS ^[9]. The evidence comes from very small groups of patients and animal models, and long-term safety data in FBS is not yet available ^[2]. Risks include severe dehydration, genital or urinary infections, and ketoacidosis. These drugs must be paused during illnesses, and families must never start, stop, or pause these medications without a written plan from the prescribing team. Any discussion about these medications should involve a highly specialized metabolic team, and they are typically only considered after conventional therapies have been optimized ^[11][9].