Galactosemia & Fructosemia
Biochemistry · Carbohydrates · lean revision notes
Galactosemia & Fructosemia
Inborn errors of monosaccharide metabolism are favourite NEET PG topics because they pivot on a few enzyme blocks, a classic clinical vignette (a neonate who deteriorates after milk, or an infant who collapses after fruit/sucrose), and one elegant lab clue — a reducing substance in urine that is NOT glucose. Master the enzyme, the accumulating metabolite, the trigger, and the dietary fix, and you can answer almost any question on this page.
High-yield: Galactose and fructose are both reducing sugars — they give a positive Benedict's / Clinitest but a negative glucose-oxidase (dipstick) test. This mismatch is the single most tested concept here.
The two pathways at a glance
Dietary lactose (milk sugar) = glucose + galactose. Dietary sucrose (table sugar) = glucose + fructose; fruits and honey supply free fructose. Each sugar is funnelled into glycolysis through dedicated enzymes, and a block at any step produces a distinct disease.
| Pathway | Substrate source | Key enzymes (in order) |
|---|---|---|
| Galactose | Lactose (milk) → galactose | Galactokinase (GALK) → Galactose-1-P uridyltransferase (GALT) → UDP-galactose-4-epimerase |
| Fructose | Sucrose / fruit → fructose | Fructokinase → Aldolase B → (triokinase) |
Galactose metabolism flow: Galactose → (GALK) Galactose-1-phosphate → (GALT) UDP-galactose / Glucose-1-phosphate → enters glycolysis.
Fructose metabolism flow (liver): Fructose → (fructokinase) Fructose-1-phosphate → (aldolase B) DHAP + Glyceraldehyde → glycolysis.
High-yield: GALT converts Galactose-1-P + UDP-glucose → UDP-galactose + Glucose-1-P. A GALT block causes toxic galactose-1-phosphate accumulation — the basis of classic galactosemia's severe organ damage.
Galactosemia
"Galactosemia" is an umbrella term for three enzyme deficiencies. The clinically dangerous one is classic galactosemia (GALT deficiency).
Classification of galactosemia
| Type | Deficient enzyme | Metabolite that accumulates | Severity |
|---|---|---|---|
| Classic (Type I) | GALT (galactose-1-P uridyltransferase) | Galactose-1-phosphate + galactose + galactitol | Severe — hepatic, CNS, renal, cataract |
| GALK deficiency (Type II) | Galactokinase | Galactose + galactitol | Mild — isolated cataracts |
| Epimerase deficiency (Type III) | UDP-galactose-4-epimerase | Galactose-1-P + UDP-galactose | Variable (benign to severe) |
High-yield: GALK deficiency → cataract only (no liver/brain disease). GALT deficiency → cataract PLUS hepatomegaly, jaundice, sepsis, intellectual disability. This pair is the classic exam differentiator.
Etiology & pathophysiology
All three are autosomal recessive. The damage in classic galactosemia stems from two mechanisms:
- Galactose-1-phosphate accumulation is directly toxic — it depletes inorganic phosphate, inhibits glycogen phosphorylase and other phosphate-dependent enzymes, and damages liver, kidney (Fanconi-like tubulopathy), and brain.
- Galactitol formation: excess galactose is reduced by aldose reductase to galactitol (dulcitol), a sugar alcohol that cannot diffuse out of the lens. It draws in water osmotically, causing lens swelling and cataract. (This is the same osmotic-polyol mechanism as diabetic cataract — sorbitol there, galactitol here.)
High-yield: Aldose reductase + galactitol is the unifying cataract mechanism. Because GALK deficiency still permits galactose → galactitol, it too causes cataracts despite being "mild."
A useful named association: E. coli sepsis in a jaundiced neonate. Galactose-1-P impairs neutrophil bactericidal function, so untreated classic galactosemia classically presents with gram-negative (E. coli) neonatal sepsis.
Clinical features — classic galactosemia
The infant is normal at birth and deteriorates after starting breast/formula feeds (lactose load):
- Vomiting, diarrhoea, poor feeding, failure to thrive within days.
- Jaundice (often direct/conjugated, progressing), hepatomegaly, then frank liver failure with coagulopathy.
- Hypoglycaemia.
- Bilateral cataracts ("oil-droplet" cataract).
- E. coli sepsis — a red-flag association.
- Renal tubular dysfunction → aminoaciduria, glucosuria (Fanconi syndrome).
- Long term, even when treated: intellectual disability, speech dyspraxia (verbal apraxia), and premature ovarian insufficiency in females.
High-yield: A jaundiced, vomiting neonate with hepatomegaly + cataract + E. coli sepsis + reducing sugar in urine = classic galactosemia until proven otherwise.
Diagnosis & investigation of choice
- Urine reducing substance positive (Benedict's/Clinitest) but glucose dipstick negative → suggests a non-glucose reducing sugar.
- Confirm the sugar by chromatography / specific assay (galactose).
- Newborn screening: measures blood galactose / galactose-1-phosphate, and enzyme activity.
- Confirmatory / investigation of choice: assay of GALT enzyme activity in erythrocytes (RBCs). Reduced/absent RBC GALT confirms classic galactosemia.
- Beutler test — a fluorescent screening test for GALT activity used in newborn screening.
High-yield: Definitive diagnosis of classic galactosemia = RBC GALT enzyme assay. Genetic confirmation = GALT gene mutation (commonest severe allele is Q188R).
Management / dietary principle
- Lifelong elimination of galactose and lactose — stop breast milk and standard formula; switch to soy-based / lactose-free (e.g. casein hydrolysate) formula.
- Early dietary restriction reverses acute liver disease, sepsis risk, and cataracts and prevents death.
- Important caveat: diet does not fully prevent long-term complications (cognitive deficits, speech apraxia, ovarian failure), because endogenous galactose is still produced from UDP-galactose turnover.
High-yield: There is no drug — treatment is galactose-free diet for life. Cataracts and acute hepatotoxicity are reversible with diet; neurodevelopmental and ovarian outcomes are not guaranteed.
Complications
Acute: liver failure, E. coli sepsis, hypoglycaemia, Fanconi syndrome, cataract, death if untreated. Chronic (despite treatment): learning disability, verbal dyspraxia, ataxia/tremor, and primary/premature ovarian insufficiency (hypergonadotropic hypogonadism) in females.
Fructosemia / disorders of fructose metabolism
Two enzymes, two completely different prognoses. The whole topic hinges on fructokinase vs aldolase B.
| Feature | Essential (benign) fructosuria | Hereditary fructose intolerance (HFI) |
|---|---|---|
| Deficient enzyme | Fructokinase | Aldolase B |
| Metabolite accumulating | Fructose (in blood/urine) | Fructose-1-phosphate |
| Inheritance | Autosomal recessive | Autosomal recessive |
| Severity | Asymptomatic / harmless | Severe, potentially fatal |
| Trigger | None (incidental finding) | Ingestion of fructose / sucrose / sorbitol |
| Urine | Reducing sugar (fructose) positive | Reducing sugar positive + other findings |
| Hypoglycaemia | No | Yes (profound) |
| Liver/kidney damage | No | Yes (hepatomegaly, hepatic & renal failure) |
High-yield: Fructokinase deficiency = essential fructosuria = BENIGN (just fructose in urine, found by chance). Aldolase B deficiency = HFI = DANGEROUS. Do not mix them up — this is the most common trap.
Essential fructosuria
- Fructokinase deficiency; fructose simply can't be phosphorylated, so it spills into blood and urine.
- Completely asymptomatic. No hypoglycaemia, no liver disease.
- Found incidentally as a urinary reducing substance (positive Benedict's, negative glucose dipstick).
- No treatment needed. Classic "do nothing" answer.
Hereditary fructose intolerance (HFI)
Pathophysiology — why it's dangerous: Aldolase B normally splits fructose-1-phosphate. When deficient, fructose-1-phosphate accumulates in liver and proximal tubule and:
- Traps inorganic phosphate → low intracellular Pi → inhibits glycogenolysis (phosphorylase needs Pi) and gluconeogenesis → severe hypoglycaemia unresponsive to glucagon.
- Falling ATP → increased AMP breakdown → hyperuricaemia / lactic acidosis and hypophosphataemia.
- Direct hepatotoxicity → vomiting, jaundice, hepatomegaly, coagulopathy, hepatic/renal failure.
High-yield: In HFI, fructose-1-phosphate sequesters phosphate, crippling glycogenolysis and gluconeogenesis → hypoglycaemia + hypophosphataemia + hyperuricaemia + lactic acidosis. Glucagon does not correct the hypoglycaemia.
Clinical clue — the weaning history: The breast-fed infant is well while on milk (lactose has no fructose). Symptoms begin when weaned onto fruits, fruit juices, sucrose-sweetened foods, or honey. Older, undiagnosed children develop a strong protective aversion to sweets and fruit and are notably caries-free (no sucrose) — a classic vignette detail.
Clinical features: vomiting, sweating, lethargy, seizures from hypoglycaemia after fructose; failure to thrive; hepatomegaly, jaundice; if untreated, progressive liver and renal (Fanconi) failure.
Diagnosis of HFI
- History of symptoms triggered by fructose/sucrose + biochemical triad (hypoglycaemia, hyperuricaemia, hypophosphataemia, metabolic acidosis after a fructose load).
- Confirmation: molecular genetic testing for the ALDOB gene (most common mutation A149P) — now preferred over the older risky fructose-tolerance test or liver biopsy enzyme assay.
- Avoid the IV fructose tolerance test unless monitored — it can precipitate dangerous hypoglycaemia.
High-yield: Diagnosis of HFI today = ALDOB gene testing. Historically = aldolase B assay on liver biopsy. The provocative fructose tolerance test is hazardous and avoided.
Management of HFI
- Lifelong elimination of fructose, sucrose, and sorbitol from diet (and from IV fluids/medications — sorbitol/fructose-containing infusions can be fatal).
- Treat acute hypoglycaemia with glucose.
- Vitamin C supplementation is often needed (fruit avoidance).
- Prognosis is excellent with strict dietary avoidance — unlike galactosemia, there are typically no fixed cognitive sequelae if diagnosed early and fructose is avoided.
High-yield: Never give a suspected/known HFI patient sorbitol- or fructose-containing IV fluids or syrups — acute liver failure and death can follow.
Putting it together — the reducing-sugar approach
A child with a positive urinary reducing substance and a negative glucose dipstick — work through it:
- Identify the sugar (chromatography): galactose vs fructose vs pentose.
- If galactose → GALT assay. Hepatomegaly + cataract + sepsis = classic galactosemia; isolated cataract = GALK deficiency.
- If fructose → is the child symptomatic? Asymptomatic + fructose in urine = essential fructosuria. Symptomatic (hypoglycaemia, liver disease, sweet aversion) = HFI → confirm with ALDOB testing.
Mnemonics:
- "Galactokinase → Goggles (cataract only)" vs "GALT → Grave (cataract + liver + brain)."
- Fructose enzymes alphabetical = severity ascending: Fructokinase (benign) before Aldolase B (severe)? Use instead: "Aldolase B = Bad (HFI); Fructokinase = Fine (fructosuria)."
- "Milk makes galactosemia sick; fruit makes fructose intolerance sick." (Trigger = the dietary sugar.)
Key differentials
| Disease | Triggered by | Hallmark | Reducing sugar | Severity |
|---|---|---|---|---|
| Classic galactosemia (GALT) | Milk (lactose) | Cataract + hepatomegaly + E. coli sepsis | Galactose | Severe |
| Galactokinase (GALK) deficiency | Milk | Isolated cataract | Galactose | Mild |
| Essential fructosuria (fructokinase) | Fructose | Asymptomatic | Fructose | Benign |
| HFI (aldolase B) | Fruit/sucrose/sorbitol | Hypoglycaemia + liver failure + sweet aversion | Fructose | Severe |
| Pentosuria | — | Incidental | L-xylulose | Benign |
| Diabetes (glucose) | — | Hyperglycaemia | Glucose (dipstick positive) | — |
High-yield: The only dipstick-positive reducing sugar in this list is glucose. Galactose, fructose, lactose and pentoses are all dipstick-negative, Benedict's-positive.
Recently asked / exam angle
- Enzyme matching: "Cataract + hepatosplenomegaly + galactose in urine — deficient enzyme?" → GALT (galactose-1-phosphate uridyltransferase).
- Isolated cataract in an infant on milk → galactokinase deficiency; cataract mechanism = aldose reductase → galactitol.
- Vignette of an infant who vomits and becomes hypoglycaemic after fruit/juice, well on breast milk, avoids sweets, caries-free → hereditary fructose intolerance (aldolase B).
- "Most appropriate confirmatory test for classic galactosemia" → RBC GALT enzyme assay (not urine test).
- "Investigation/diagnosis of HFI" → ALDOB gene mutation analysis; mutation A149P. Galactosemia's commonest mutation = Q188R.
- Biochemical triad in HFI after fructose → hypoglycaemia + hypophosphataemia + hyperuricaemia (± lactic acidosis), glucagon-unresponsive.
- "Which condition needs no treatment?" → essential fructosuria (fructokinase deficiency).
- Diet questions: galactosemia → galactose/lactose-free (soy formula); HFI → fructose/sucrose/sorbitol-free + vitamin C.
- Cataract polyol pathway linkage: galactitol (galactosemia) and sorbitol (diabetes) both via aldose reductase.
- Neonatal E. coli sepsis with conjugated hyperbilirubinaemia → think classic galactosemia.
Rapid revision
- Lactose = glucose + galactose; sucrose = glucose + fructose — know the building blocks.
- GALT deficiency = classic galactosemia (severe): cataract + hepatomegaly + jaundice + E. coli sepsis + intellectual disability.
- GALK deficiency = isolated cataract only (mild); galactose-1-P does NOT accumulate.
- Cataract mechanism in both = galactose → galactitol via aldose reductase (osmotic lens swelling).
- Confirm galactosemia by RBC GALT assay; commonest mutation Q188R; treat with galactose-free (soy) diet for life.
- Galactosemia diet reverses liver disease and cataract but not verbal dyspraxia or premature ovarian insufficiency.
- Essential fructosuria = fructokinase deficiency = benign, asymptomatic, fructose in urine, no treatment.
- HFI = aldolase B deficiency = dangerous: hypoglycaemia + hepatic/renal failure after fructose/sucrose/sorbitol.
- HFI pathophysiology: fructose-1-P traps phosphate → blocks glycogenolysis & gluconeogenesis → glucagon-resistant hypoglycaemia + hyperuricaemia + hypophosphataemia.
- HFI clue: well on breast milk, sick on weaning, aversion to sweets, caries-free; confirm by ALDOB gene (A149P); never give sorbitol/fructose IV.
- All four disorders are autosomal recessive; all give Benedict's-positive, glucose-dipstick-negative urine.
- Glucose is the ONLY dipstick-positive reducing sugar — the key discriminator from galactose/fructose/pentose.