To appreciate why this matters, one must understand the biology of GLUT1 itself. The glucose transporter type 1, encoded by the SLC2A1 gene, is a facilitative uniporter embedded in the plasma membrane of mammalian cells, and at the blood–brain barrier it constitutes the near-exclusive gateway through which glucose enters the central nervous system. When one copy of SLC2A1 is lost or damaged, a condition known as haploinsufficiency, the result is GLUT1 deficiency syndrome, an autosomal dominant disorder of brain energy metabolism. Because neurons cannot compensate for reduced glucose delivery, a chronic cerebral energy deficit develops that impairs brain development and function. Patients classically present in infancy with epilepsy, decelerating head growth, developmental delay and complex paroxysmal movement disorders, and the biochemical signature is hypoglycorrhachia, an abnormally low glucose concentration in the cerebrospinal fluid. Symptoms evolve in an age-specific pattern, with ataxia and exertion-induced dyskinesia becoming prominent in adolescence and adulthood.

The disorder also reaches beyond the brain. A subset of patients displays a haematological phenotype, comprising mild haemolytic anaemia in which erythrocytes deform into echinocytes, a stomatin-associated shape change accompanied by increased intracellular sodium and decreased potassium. The red blood cell depends on GLUT1 as its principal glucose transporter, and when glucose uptake falters, the energy supply needed to maintain cation homeostasis collapses. The resulting cation leak causes red cells to swell and rupture. The mainstay of treatment, the ketogenic diet, sidesteps the transport problem altogether by supplying ketone bodies as an alternative cerebral fuel. Early initiation can improve seizure control and support cognitive development, but the response remains incomplete in a substantial subgroup of patients, particularly regarding movement disorders and cognitive outcome, underscoring a long-recognized need for complementary strategies.

The conceptual leap came from a recent biochemical discovery: GLUT1 is not only a glucose transporter. Work published by Ng, Sosicka, Xia and Freeze in 2023 demonstrated that GLUT1 is a highly efficient L-fucose transporter operating in the physiological micromolar range, and that it acts non-competitively with millimolar concentrations of glucose. Fucose is far from a biochemical footnote. It is a terminal sugar added to glycans on countless glycoproteins, and fucosylation plays critical roles in the central nervous system. The clinical relevance is vividly illustrated by SLC35C1-CDG, also known as leukocyte adhesion deficiency type II, in which loss of the GDP-fucose transporter causes a multisystem disorder with growth restriction, intellectual disability and recurrent infections, a condition that responds substantially to oral fucose therapy.

The patient at the heart of the new report is a boy, now three years and eight months old, born at 34 weeks and 4 days of gestation. His neonatal course was formidable: bilateral congenital cataracts requiring surgical correction at one month, a parieto-occipital haemorrhage with intraventricular extension despite normal coagulation, and haemolytic anaemia severe enough to require transfusions. Genetic testing identified a heterozygous de novo SLC2A1 variant, c.1306_1308del, resulting in the deletion of a single isoleucine residue, p.Ile436del, within the transporter’s major facilitator superfamily domain spanning twelve transmembrane helices. Cerebrospinal fluid glucose measured 1.96 mmol per litre with CSF-to-serum ratios of 0.34 to 0.53, consistent with impaired blood–brain barrier glucose transport, and the diagnosis of GLUT1 deficiency syndrome was made at the age of two months. Development was severely delayed, with motor function around the three-month level at sixteen months of age and a GMFCS level of V.

Treatment began promptly. A ketogenic diet, escalated from a 3:1 to a 4:1 ratio with exogenous ketone ester and sodium citrate supplementation, was initiated at two months and maintained blood β-hydroxybutyrate within the therapeutic range of 2 to 8 mmol per litre; continuous ketone monitoring over three consecutive days in May 2024 captured 863 measurements with a median of 5.9 mmol per litre and 90.5 percent of time in range. The child remained seizure-free. Yet despite sustained, well-documented ketosis, his development showed no progress whatsoever. This therapeutic plateau, in a child whose seizures were controlled but whose brain was not developing, is precisely the clinical scenario the ketogenic diet fails to address, and it set the stage for the metabolic investigation that followed.

Using a battery of analytical techniques, the team uncovered evidence of profound hypofucosylation. Cerebrospinal fluid free fucose, measured by UHPLC, was 0.48 mg per litre, below the reference range of 0.6 to 0.84 established in children aged one to five years, and gas chromatography–mass spectrometry confirmed reduced free fucose in erythrocytes, plasma and CSF. Quantitative lectin blotting using Aleuria aurantia lectin, which binds α-1,2, α-1,3, α-1,4 and α-1,6 linked fucose, showed significantly reduced glycan signal in erythrocyte lysates, at 81.4 percent of control, in plasma at 69.0 percent and in total CSF at 62.8 percent. Crucially, the investigators immunoprecipitated β-trace protein, a prostaglandin D synthase produced predominantly by cells of the central nervous system, and found its fucosylation reduced to 61.3 percent of control, with its abundance also diminished. The glycosylation defect was therefore not merely a peripheral reflection of blood abnormalities but extended to proteins synthesized within the brain compartment.

Oral L-fucose supplementation, initiated at 22 months of age under a German individual treatment trial, or Einzelheilversuch, with informed parental consent, began at 1 gram per day and was escalated stepwise to 1.5 grams per kilogram per day by day 480. The biochemical response was striking. Fucose concentrations rose above control levels in every measured compartment, and CSF fucose correlated linearly with administered dose, with a Pearson correlation coefficient of 0.98. Global protein fucosylation normalized in erythrocytes, plasma and total CSF, biologically plausible given that the cellular salvage pathway can incorporate exogenous fucose directly. One puzzling dissociation emerged, however: fucosylation of the CNS-derived β-trace protein did not significantly increase, remaining at 66.5 percent of control, even though its abundance rose. The authors suggest two possibilities: supplemented fucose may reach the cerebrospinal fluid but incompletely access the fucosylation machinery of CNS-resident cells, consistent with evidence that GDP-fucose exists in multiple differentially accessible pools, or β-trace protein may simply not represent the fucosylation state of the CNS as a whole. Mannose, trialed briefly at up to 10 grams per day, produced no change and was discontinued after two months because of diarrhoea.

Clinically, the changes were hard to miss. Within two months of starting fucose, the boy’s parents and physicians consistently reported accelerated development: he learned to roll, gained improved head control and showed increased social interaction and reciprocal play, with continued gradual gains raising his GMFCS level from V to IV. Serial EEGs recorded persistence of right posterior focal epileptiform discharges in February 2025, but by October 2025, roughly one year into therapy, the focal discharges had resolved and lateralized activity had disappeared. The patient remained seizure-free throughout, and fucose was well tolerated over eighteen months with no adverse effects. Metabolomics added nuance: several erythrocyte metabolites, including glucose, citrate, lactate and amino acids, shifted toward the control profile after therapy, while plasma showed a similarly selective response and CSF metabolites remained largely unchanged despite the elevated fucose, indicating a metabolite- and compartment-specific correction.

The authors are appropriately measured in their conclusions, noting that the single-patient design demands validation in larger cohorts. Nevertheless, the implications are considerable. Hypofucosylation may represent a previously unrecognized component of GLUT1 deficiency syndrome pathophysiology contributing to the neurological phenotype, and lectin-based measures of fucosylation could serve as candidate biomarkers complementing established diagnostics. The persistent hypofucosylation of β-trace protein raises important questions about the routes and efficiency of fucose delivery into the brain, and future work should test whether higher doses or alternative delivery routes can achieve CNS-intrinsic correction. A larger trial of L-fucose supplementation in GLUT1 deficiency syndrome is already ongoing under the identifier NCT07432490. If those results confirm what this single determined boy has revealed, a cheap, well-tolerated simple sugar, added to the ketogenic diet, could become the first therapy in decades to reach beyond seizure control and genuinely address the developmental burden of this devastating disorder.

Subject of Research: Fucose supplementation as a treatment for hypofucosylation in GLUT1 deficiency syndrome

Article Title: Fucose supplementation as a novel treatment approach for GLUT1 deficiency syndrome

Article References: Fucose supplementation as a novel treatment approach for GLUT1 deficiency syndrome. (n.d.). https://doi.org/10.1007/s00415-026-14211-3

Image Credits: AI Generated

DOI: 10.1007/s00415-026-14211-3

Keywords: GLUT1 deficiency syndrome, SLC2A1, L-fucose, glycosylation, blood-brain barrier, ketogenic diet, hypoglycorrhachia, beta-trace protein, congenital disorders of glycosylation, cerebral energy metabolism, hemolytic anemia, case report