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