Study of Vitamin D and Uric Acid Lowering on Kidney and Blood Vessel Function

We have demonstrated that lower levels of 25-hydroxyvitamin D (25[OH]D) and higher concentrations of uric acid are both potentially modifiable factors that are independently associated with an increased risk of developing hypertension (high blood pressure) in humans. Other investigators have shown that vitamin D supplementation, or lowering uric acid with allopurinol, may reduce blood pressure. Animal experiments suggest that activation of both the systemic and local kidney-specific renin angiotensin systems (RAS) may be the principal mechanism linking 25(OH)D and uric acid with hypertension. In human parallels to these animal studies, we have shown in cross-sectional analyses that non-hypertensive individuals with lower 25(OH)D and higher uric acid levels have increased activation of their systemic and kidney-specific RAS, independent of other factors. However, whether vitamin D supplementation or uric acid lowering attenuates RAS activation has never been demonstrated in humans. Both lower 25(OH)D and higher uric acid concentrations are also associated with endothelial dysfunction in humans, and endothelial function may modulate the RAS and provide an alternate mechanism for the development of hypertension. It remains unclear, however, whether an intervention to increase 25(OH)D or decrease uric acid levels among non-hypertensive adults improves endothelial function; furthermore, it is unknown whether treatment of these individuals would lower blood pressure. Determining whether treatment of 25(OH)D and uric acid concentrations, per se, can attenuate RAS activation, improve endothelial function, and lower blood pressure among nonhypertensive individuals is critically important, with implications stretching beyond hypertension prevention, since RAS activation, endothelial dysfunction, and blood pressure are also implicated in the pathology of cardiovascular and chronic kidney disease. Individuals who are overweight and obese (two-thirds of US adults) represent an important population who are known to have lower 25(OH)D levels, higher uric acid concentrations, activation of the RAS, endothelial dysfunction, and an increased risk of hypertension, cardiovascular disease, and chronic kidney disease. Interestingly, our preliminary data demonstrate that among overweight and obese individuals with normal 25(OH)D or low uric acid levels, adiposity is no longer associated with activation of the RAS, suggesting that low 25(OH)D and high uric acid concentrations might be mediators of the adverse consequences of overweight and obesity.

Inclusion criteria

• 25(OH)D < 20 ng/mL OR Uric acid ≥ 5 mg/dL
• Age ≥ 18, ≤ 75 years
• Body Mass Index (BMI) ≥ 25 kg/m^2

Exclusion criteria

• Hypertension, or on BP-lowering medicine
• Diabetes
• Coronary Heart Disease
• estimated glomerular filtration rate (EGFR) <60 mL/min
• Kidney stones
• Active cancer (except non-melanoma skin cancer)
• Pregnant
• Taking vitamin D supplements and unwilling to stop
• Osteoporosis
• Hypo- or hypercalcemia
• Hypo- or hyperphosphatemia
• Known allergy to angiotensin-converting enzyme (ACE)-inhibitors
• Taking medication for hyperuricemia
• Gout, anemia, cirrhosis, active/chronic hepatitis, abnormal aspartate aminotransferase (AST), alanine aminotransferase (ALT) or total bilirubin levels, or anemia
• Known allergy to either allopurinol or probenecid
• Current use of didanosine, azothioprine, methotrexate, ketoprofen, ketorolac, mycophenolate, or ACE-inhibitors