External Carotid Blood Flow in Individuals With Post-Bariatric Hypoglycemia

Inclusion Criteria: * Age 18-70 years of age, inclusive at screening. * Individuals able to provide informed consent and follow all study procedures, including complete daily food, activity, and symptom diaries, and willingness to provide access to medical record review. * For healthy control group: individuals without a history of upper gastrointestinal surgery and without reported symptoms of hypoglycemia. * Individuals with access to personal iPhone running operating system 16 (iOS) or later versions (not feasible for Android platform users) and a password-protected home Wi-Fi network. Exclusion Criteria: * History of coronary artery disease or presence of 2 or more risk factors including diabetes, uncontrolled hypertension, uncontrolled hyperlipidemia, and active tobacco use. * History of myocardial infarction, unstable angina or revascularization within the past 6 months. * History of syncope (unrelated to hypoglycemia) or diagnosed cardiac arrhythmia. * Documented hypoglycemia occurring in the fasting state (\> 12 hours fast). * Concurrent administration of β-blocker therapy. * Vigorous physical activity within 24 hours prior to assessment. * Seizure disorder (other than with suspect or documented hypoglycemia). * Hematocrit \< 33% (women) or \<36% (men) within the past 6 months. * History of insulinoma. * Use of central nervous system-active medications, including: antidepressants, anxiolytics, antipsychotics, stimulants, antiepileptics, sedatives and opioids. * History of severe neurologic vascular disease (e.g. cerebrovascular disease/stroke). * History of previous diseases associated with dysautonomia (e.g. multiple system atrophy, multiple sclerosis, Parkinson's disease). * Impaired hepatic function (aspartate amino transaminase and/or alanine amino transaminase \>2x upper limit of normal range), as determined within the past 6 months. * Impaired renal function (serum creatinine \>1.5 mg/dL), as determined within the past 6 months. * Serious psychiatric or substance abuse disorders (e.g. alcohol, opioid, cocaine use disorder), as judged by the study investigator. * Non-consent or inability to comply with study procedures. * Current pregnancy and/ or lactation. * Vulnerable individuals (e.g., fetuses, children, prisoners) and those judged by the investigator to be unsuitable for the study. * Individuals wearing earrings or other jewelry in the left ear who are unable or unwilling to remove them for the duration of the study. * Unwillingness to wear study masked CGM for duration of study