REN and Hypertension: However, it should be noted that the antihypertensive effect of vitamin D was not observed in some trials, which could be due to a suboptimal study design, the knowledge that a deficiency of vitamin D could promote the sustained activation of RAAS (renin–angiotensin–aldosterone system), increasing angiotensin, arterial hardening, and endothelial dysfunction, could support the hypothesis that vitamin D deficiency contributes to the development of hypertension [199].