Acute Oral Ketone Monoester Supplementation and Resting-state Brain Connectivity

Inclusion Criteria: * Adults aged 55 years or older * Subjective Cognitive Decline (SCD) or Mild Cognitive Impairment (MCI) SCD CRITERIA: * Self-experienced, persistent decline of cognitive abilities within the last 5 years in relation to a previously normal status * Normal results on demographically adjusted standardized cognitive tests * Accompanied by concerns associated with SCD and a feeling of worse performance than others in the same age group * Decline must not be related to a specific event or explained by a specific disease, medical disorder, medication, or substance abuse MCI CRITERIA * Fulfils criteria of the Diagnostic and Statistical Manual of Mental Disorders (DSM-5): * Demographically adjusted performance between -1 and -2 SD below average in one or more cognitive domains on standardized cognitive tests * Accompanied by concern from the individual, a knowledgeable informant, or clinician indicating mild cognitive decline * Capacity for independent daily activities is maintained Exclusion Criteria: * Clinical diagnoses of psychiatric disorders (e.g., anxiety disorders, depression) * Diagnosed major neurocognitive disorder (i.e., dementia) * Association of SCD or MCI with delirium * Diagnosed cardiometabolic disease (Uncontrolled hypertension (systolic blood pressure \>160 mm Hg, and/or diastolic blood pressure \>100 mm Hg, Type 2 diabetes) * Substance use disorder * History of heart attack or stroke requiring hospitalization in the past 3 years * MRI contraindications, including implanted medical devices * Experiencing residual fatigue, brain fog, or tiredness post-acute illness (i.e., long-COVID) * Severe literacy, visual, hearing, and/or speech impairment that would make participation in the study difficult or impossible * Individuals who are not fluent in Swiss German or German