Acute Oral Ketone Monoester Supplementation and Resting-state Brain Connectivity

The primary objectives of this pilot study are to test if a single dose of an oral ketone monoester supplement will increase blood flow and connectivity in the brain in adults with subjective cognitive decline (SCD) or mild cognitive impairment (MCI). The main questions it aims to answer are: Does a single dose of an oral ketone supplement increase global cerebral blood flow (CBF)? Does a single dose of an oral ketone supplement improve resting-state functional connectivity in the Default Mode Network (DMN)? Does the increase in CBF positively correlate with improved functional connectivity in the DMN? Participants will: * Attend one 2-hour session, which includes: * Neurocognitive assessment * MRI Scans (two, each 15 Minutes) * Capillary blood ketone level measurements * Hemodynamic assessment (blood pressure, heart rate)