Vitamin D deficiency predisposes to up-regulation of the renin-angiotensin-aldosterone system and hypertrophy of both the left ventricle and vascular smooth muscle cells (2) (Fig. 3). In vitamin D-deficient animals there is an increased incidence of hypertension, left ventricular hypertrophy, and atherosclerosis (29). Human studies indicate that 1,25(OH)2D inhibits renin synthesis, which may lower blood pressure (30). Krause et al. (31) showed that increased exposure to UVB radiation in a tanning bed 3 times per week for 3 months led to a 180% increase in 25(OH)D levels and a 6-mm Hg reduction in both systolic and diastolic pressures. A small, randomized, placebo-controlled study of patients with type 2 diabetes and low baseline 25(OH)D levels showed that a single dose of 100,000 IU of vitamin D2 reduced systolic blood pressure by a mean of 14 mm Hg and significantly improved endothelial function as measured by forearm blood flow (32). In the NHANES III study, the mean systolic blood pressure was about 3 mm Hg lower in those in the individuals in the highest quintile of serum 25(OH)D levels compared with those in the lowest quintile (22).