Pathogenesis of acidosis in hereditary fructose intolerance

An 18-yr-old man with a classical history of hereditary fructose intolerance (HFI) developed typical biochemical changes following an oral fructose load: fructosemia, hypoglycemia, hypophosphatemia, hyperuricemia, and metabolic acidosis. Hypokalemia (3.1 meq/liter) was also noted. Three aspects of this case expand the published literature on this syndrome: (1) Metabolic acidosis was found to be due to both lactic acidosis and proximal renal tubular acidosis (RTA). We could quantitate the relative contribution of each, and found that urinary bicarbonate loss due to proximal RTA accounted for less than 10% of the fall in serum bicarbonate. The major cause of the metabolic acidosis was lactic acidosis. (2) Hypokalemia was found to be due to movement of potassium out of the extracellular space rather than to urinary loss. Potassium may have entered cells with phosphate or may have been sequestered in the gastrointestinal tract. (3) The coexistence of proximal RTA and acidemia made it possible to study the effect of acidemia on the urine-blood partial pressure of carbon dioxide (PCO2) gradient in alkaline urine (U-B PCO2). The U-B PCO2 measured during acidemia was much higher at the same urine bicarbonate concentration than in normal controls during alkalemia, providing evidence in humans that acidemia stimulates distal nephron hydrogen-ion secretion.