We (Zaleski et al., 2015) and others (Al Mheid et al., 2011b; McGreevy et al., 2015) have previously shown high-dose 25(OH)D supplementation (i.e., 4000IU) to lower some, but not all indices of arterial stiffness among otherwise healthy adults with elevated BP and 25(OH)D deficiency. Pathophysiological alterations underlying the etiology of cardiac syndrome X, such as endothelial dysfunction, could partially explain why baseline 25(OH)D levels appeared to moderate the peak SBP to a GEST among women with cardiac syndrome X and not in the present study of otherwise healthy adults. BP regulation is complicated, redundant, and multifactorial; constantly responding to both internal (i.e., neural responses, hormonal influences, circadian rhythm) and external factors (i.e., temperature, noise).