Postural tachycardia syndrome (POTS) affects ≈3 million young people, characterized by chronic presyncopal symptoms characterized by dizziness, lightheadedness, and orthostatic tachycardia that occur while standing. Across-sectional survey found that 25% of these patients complains that meals rich in carbohydrates are among the factors that further exacerbate POTS's symptoms and cause a myriad of gastrointestinal symptoms. The splanchnic circulation is the largest blood volume reservoir of the human body, storing ≈25% of the total blood volume and contributing to sudden, and large, fluctuations in the stroke volume (SV). The orthostatic changes in systemic hemodynamics are particularly magnified after meals, due to increased blood volume sequestration triggered by the release of gastrointestinal peptides with vasodilatory properties. The purpose of this study is to determine if the worsening orthostatic tachycardia and symptoms after glucose ingestion in POTS patients are due to a greater increase in splanchnic venous capacitance and excessive blood pooling on standing as compare to Healthy controls. The study will also determine if glucose-induced GIP secretion increases splanchnic venous capacitance, orthostatic tachycardia and worsening POTS postprandial symptoms. For this purpose subjects will be further randomized to either saline versus GIP(3-30)NH2 acute infusion, to measure the changes their splanchnic venous capacitance and superior mesenteric arterial flow before and after a 75-g oral glucose challenge during supine and 45-degree head-up tilt positions (orthostatic challenge) for up to 3 hours.