We were not able to measure levels of parathyroid hormone
(PTH) and calcium in blood. The first one is indirect biomarker
for vitamin D deficiency and rise when levels of 25(OH)D fall
below 30 ng/ml [1]. The other, as proposed in earlier studies, it
was suggests that active calcium absorption is optimal when the
level of 25(OH)D is 30 ng/ml [39]. When calcium intake is insuffi-
cient, production of 1,25-dihydroxyvitamin D increases enhancing
25-hydroxyvitamin D-24-hydroxylase activity and increasing the
catabolism of 25(OH)D