Inclusion Criteria:
* Histopathologic diagnosis of a glioma that has received prior irradiation and is presumed recurrent.
* Clinical indication for open biopsy or surgical resection.
* On MRI, a 2x2x2 cm enhancement absent of bleed, calcification, or other features that can obscure MRS signal.
* Karnofsky performance status ≥ 70.
* Age \> 18 years.
* Willing to submit archival tumor sample if available.
* Patients must have the following organ and marrow function:
* Absolute neutrophil count \>1,000/mcL
* Total bilirubin \< 1.5 x institutional upper limit of normal (ULN) OR total bilirubin \>1.5 × ULN with direct bilirubin \< 1.5 × ULN;
* AST (SGOT) and ALT (SGPT) \< 2.5 x institutional ULN
* PT or PTT \< 1.5 x institutional ULN
* Estimated glomerular filtration rate (eGFR) ≥ 50 mL/min/1.73 m2 calculated by the CKD-EPI equation
* Patients must be maintained on a stable or decreasing dose of systemic corticosteroid regimen of dexamethasone \< 4mg (no increase for 5 days) prior to start of study intervention (Day 1). Topical and inhaled steroid treatment is allowed.
* Patient must be able to provide written informed consent.
* Female participants of childbearing potential must have a negative serum pregnancy test prior to study start.
* Patients who have received prior ketogenic therapy (including supplements and diet) may enroll, but must have stopped prior therapy ≥ 7 days prior to day 1.
* Patient willing to minimize carbohydrate-containing medications.
Exclusion Criteria:
* Patients with a history of a metabolic disorder including documented defect in urea metabolism (including documented history of gout), carnitine deficiency (primary carnitine deficiency, carnitine palmitoyl transferase I or II deficiency, carnitine translocase deficiency), fatty acid metabolism, beta-oxidation defects, pyruvate carboxylase deficiency, mitochondrial function, porphyria, or treatment refractory nephrolithiasis.
* Active uncontrolled infection.
* BMI \< 20.0.
* Active bowel obstruction, ileus, or active or remote pancreatitis, known gastroparesis/delayed gastric emptying.
* Known history of clinically significant heart failure (NYHA \>2), recent myocardial infarction, or symptomatic atrial fibrillation.
* Diabetes mellitus, defined as HbA1c ≥6.5% or use of medications to treat diabetes (note, use of medications for weight loss that have been FDA approved for diabetes is acceptable)
* Conditions that may increase the risk of study intervention or significantly reduce compliance (i.e. cognitive impairment, frank dementia, etc.).
* Other concurrent experimental therapies.
* Food restrictions that would limit study diet adherence.
* Contraindication to receiving an MRI (cardiac pacemaker, insulin pumps, non-MRI-compatible stents/clips/plates/rods, non-removable glucose monitor, other non-removable ferromagnetic objects in the body, etc.).
