Renaud Viellevoye

Calcium and Phosphorus Homeostasis: Pathophysiology

Ninety-eight percent of the calcium and eighty percent of the phosphorus in the body are in the skeleton; these elements are also constituents of the intracellular and extracellular spaces. The metabolic homeostasis of calcium, phosphorus, and magnesium and mineralization of the skeleton are complex functions that require the intervention of various parameters; an adequate supply of nutrients; the development of the intestinal absorption process; and the effects of several hormones, such as parathyroid hormone, vitamin D, and calcitonin, as well as optimum renal and skeletal controls [1]. Bone formation requires protein and energy for collagen matrix synthesis, and an adequate intake of calcium and phosphorus is necessary for correct mineralization. During development, nutrients are transferred mainly across the placenta. It has been calculated that during the last trimester of gestation the daily accretion per kilogram of body weight represents around 120 mg of calcium and 70 mg of phosphorus. Therefore, at birth the whole-body content of a term infant represents approximately 30 grams of calcium and 16 grams of phosphorus. After birth, the use of the gastrointestinal tract to provide nutrients for growth causes a reduction in calcium availability for bone accretion promoting the occurrence of relative osteopenia in preterm infants and to a lesser extent in term infants during the first weeks of life. In addition to their roles in bone formation, calcium and phosphorus play important roles in many physiologic processes, such as transport across membranes, activation and inhibition of enzymes, intracellular regulation of metabolic pathways, secretion and action of hormones, blood coagulation, muscle contractility, and nerve conduction. The 20% of phosphorus not complexed within bone is present mainly as adenosine triphosphate, nucleic acids, and cell and organelle membranes.