Inclusion Criteria:
* Adults (age ≥ 18 years)
* Newly or recently diagnosed glioblastoma. Patients may enroll in the study from the time of diagnosis (prior to the initiation of standard-of-care chemoradiation) up until the initiation of post-radiation adjuvant chemotherapy (typically approximately 3-4 weeks after the completion of radiation therapy). Overall, this provides a window of approximately 3 months from the time of diagnosis for patients to enroll in this study. No recurrent glioblastoma is allowed on trial.
* Being seen for treatment of glioblastoma at Cedars-Sinai Medical Center. Note: Okay if patients receiving chemotherapy or radiation therapy at another facility
* Patients on low and managed doses of steroids are permitted. Up to 8 mg dexamethasone or steroid equivalent.
Exclusion Criteria:
* Karnofsky Performance Status of \<70 as deemed by physician or equivalent
* Body Mass Index of \<22 kg/m2 (rounded to nearest integer)
* Patients with disorders that affect lipid metabolism such as pyruvate carboxylase deficiency, porphyria, primary carnitine deficiency, carnitine palmitoyltransferase I or II deficiency, carnitine translocase deficiency, Beta-Oxidation Defects
* Medical comorbidities that in the opinion of the investigator limits the patient's ability to complete this study
* Pregnant, nursing, or implanted with an intrauterine device (IUD) that is not MR compatible
* Patients who are vegan or vegetarian will be excluded, as the ketogenic diet includes foods that may interfere with these preferences
* Patients on high doses of steroids will be excluded
* Patients currently receiving experimental therapeutic therapy will be excluded from the trial. Note: Off-label therapy use is permitted
* Inability to adhere to the protocol
* Patients with history of allergic reactions to surgical steel or elastomer/rubber are excluded from the activity monitor portion of the study. Because the activity monitor uses a Bluetooth low energy Transceiver, individuals using pacemakers, implantable cardiac defibrillators, neuro-stimulation devices, cochlear implants, hearing aids, or other electronic medical equipment should also be excluded from wearing the activity monitor. They can still use the Aria scale to record weight.
* Patients with mechanically, magnetically, or electrically activated implants, such as cardiac pacemakers, neurostimulators, and infusion pumps, that are not magnetic resonance imaging/positron emission tomography (MRI/PET) compatible.
* Patients with ferromagnetic implants and ferromagnetic foreign bodies, such as intracranial, aneurysm clips, shrapnel and intraocular metal chips as these could become dislodged.
* Patients unable to tolerate MRI/PET imaging secondary to an inability to lie supine or severe claustrophobia as assessed by the treating physician or principal investigator.
* Patients unable to lie still, hold their breath, or follow imaging procedure instructions as assessed by the treating physician or PI.
* Patients whose most recent renal function test does not meet Cedars-Sinai Medical center standard of care MRI contrast protocol requirements (glomerular filtration rate \<45ml/min).
* Patients with a known allergy to Fludeoxyglucose F18 (F18-FDG) radioactive tracer or gadolinium-based contrast agent.
* Patients with allergy to animal dander or animal-instigated asthma.
