Ultraviolet B activated 1,25(OH)₂D affects the level of fibroblast growth factor-23 in human

Fibroblast growth factor 23 (FGF-23) is known as a phosphaturic factor regulating phosphate homeostasis. Several studies suggest that dietary phosphate, serum phosphate and 1,25-dihydroxyvitamin D [1,25(OH)₂D] are candidate regulators of FGF-23. While the human studies, which modulated the dietary or serum phosphate showed in rather controversial results, manipulation of the active vitamin D definitely affected FGF-23 in animals. This study was conducted to elucidate the relationship between active vitamin D directly stimulated by ultraviolet B (UVB) exposure and FGF-23 level in human. Ten healthy young adults were recruited to get the UVB exposure thrice a week at sub-minimal erythemal dose with gradual increment by 10% only for 4 weeks. Serum calcium, phosphate, mineral-related hormones and bone turnover markers were analyzed before and after the UVB exposure every 4 week for 12 whole weeks. Twenty fivehydroxyvitamin D [25(OH)D] increased by 115% (19.8 ng/mL to 40.5 ng/mL, p < 0.001) after 4 weeks of UVB exposure. While 1,25(OH)₂D increased by 75% (49.9 pg/mL to 64.4 pg/mL, p < 0.001) then both level decreased after 4 weeks of withdrawal. C-telopeptide peaked at 2nd week then decreased, while osteocalcin increased gradually. FGF-23 started to increase from the 4th week of UVB exposure then significantly at the 4th week after withdrawal of UVB (27.8 pg/mL to 41.4 pg/mL, p < 0.05). UVB exposure effectively increased 1,25(OH) ₂D with delayed stimulatory effect on FGF-23. This result could support the regulatory loop of 1,25(OH)₂D and FGF-23 in human, FGF-23 regulation by 1,25(OH)₂D.