The authors of this study suggest some possible explanations: (1) vitamin D and PTH receptor expression are downregulated by low 1,25(OH)2D and/or high PTH so that much higher 1,25(OH)2D and PTH concentrations are required to achieve calcium absorption than could be achieved by reasonable cholecalciferol supplementation and/or (2) there is inhibition of calcium absorption because of some condition specific to uraemia (e.g. metabolic acidosis) [66]. Here, PTH is linked to metabolic acidosis.