Relationships between serum and urine phosphorus with all-cause and cardiovascular mortality: The osteoporotic fractures in men (MrOS) study

Background: Serum phosphorus is associated with cardiovascular disease (CVD) in the general population, but may not comprehensively reflect phosphorus homeostasis. Whether urine phosphorus-creatinine ratio (a marker of intestinal absorption) or urine fractional excretion of phosphorus (FE_Pi; a marker of urinary phosphorus handling) is associated with risk of mortality or CVD is uncertain. Study Design: Prospective observational study. Setting & Participants: 1,325 community-dwelling men 65 years or older participating in the MrOS Study. Predictor: Serum phosphorus, urine phosphorus-creatinine ratio, and FE_Pi. Outcomes: All-cause and CVD death. Results: Mean age was 74 ± 6 (SD) years, estimated glomerular filtration rate was 75 ± 16 mL/min/1.73 m², and serum phosphorus level was 3.2 ± 0.4 mg/dL. During a median follow-up of 9.3 years, there were 364 (120 CVD) deaths. After adjustment for demographics, CVD risk factors, and kidney function, the risks of all-cause death in the highest quartiles of serum phosphorus (≥3.6 mg/dL), urine phosphorus-creatinine ratio (≥0.55), and FE_Pi (≥18%) were 1.63 (95% CI, 1.23-2.17), 1.22 (95% CI, 0.90-1.65), and 0.88 (95% CI, 0.64-1.23), respectively, compared to the lowest quartiles of each. Results were similar for CVD death. Results also were similar in those with estimated glomerular filtration rate ≥60 and <60 mL/min/1.73 m². Limitations: Older all-male cohort. Few had advanced chronic kidney disease. Spot urine specimens were used. Conclusions: In community-living older men, higher serum phosphorus concentrations are associated with all-cause and CVD death. In contrast, urine phosphorus-creatinine ratio and FE_Pi are not. These findings do not support using urine phosphorus-creatinine ratio or FE_Pi as adjuvant measures to predict risk of mortality or CVD in the general population.