Vitamin D Deficiency and Dysautonomia

The investigators approach will combine HUT testing coupled with autonomic testing that includes continuous blood pressure and HR measurements, Baroreflex Sensitivity and Hear Rate Variability, to establish objective autonomic profiles, along with vascular testing including Pulse Wave Velocity, Ankle Brachial Index at rest and measures of blood volume of different compartments, Systemic Vascular Resistance and cardiac output at rest and in response to hand-grip stress. This will allow the treating physician to provide patients with a specific diagnosis, and ultimately develop data for more focused, rational treatments than currently achieved. The full vascular profile is also novel and has the potential to improve therapeutic management of the participants independent of the outcomes with the vitamin D supplementation. This study is designed to recruit 80 participants into 4 groups of 20 each. The 4 groups represent non OI (those recruited from the clinics for nausea but without a positive tilt test) or those showing orthostatic intolerance (POTS alone, OH, and syncope). The general objective of this proposal is to address this gap in knowledge by determining vascular function, the neurohumoral profile and autonomic status supine and in response to HUT in OI subjects with low vitamin D levels in comparison with subjects who test negative for OI on the HUT. The investigators aim to examine the effect of vitamin D replacement on these measures, providing the possibility of therapeutic use of vitamin D to treat or ameliorate the symptoms associated with OI.