Marckmann et al. conducted an 8-week randomized, double-blind parallel intervention study of 52 CKD patients, and revealed that serum levels of 25-(OH) D were almost 1000 times higher than 1, 25-dihydroxyvitamin D; in addition, serum 25-(OH) D was positively correlated with serum 1, 25-dihydroxyvitamin D, calcium, and FGF-23 levels, whereas it was inversely correlated with PTH levels. Here, PTH is linked to chronic kidney disease.