Intestinal absorption and renal reabsorption of calcium throughout postnatal development

Abstract

Calcium is vital for many physiological functions including bone mineralization. Postnatal deposition of calcium into bone is greatest in infancy and continues through childhood and adolescence until peek mineral density is reached in early adulthood. Thereafter, bone mineral density remains static until it eventually declines in later life. A positive calcium balance, i.e. more calcium absorbed than excreted, is crucial to bone deposition during growth and thus to peek bone mineral density. Dietary calcium is absorbed from the intestine into the blood. It is then filtered by the renal glomerulus and either reabsorbed by the tubule or excreted in the urine. Calcium can be (re)absorbed across intestinal and renal epithelia via both transcellular and paracellular pathways. Current evidence suggests that significant intestinal and renal calcium transport changes occur throughout development. However, the molecular details of these alterations are incompletely delineated. Here we first briefly review the current model of calcium transport in the intestine and renal tubule in the adult. Then, we describe what is known with regard to calcium handling through postnatal development, and how alterations may aid in mediating a positive calcium balance. The role of transcellular and paracellular calcium transport pathways and the contribution of specific intestinal and tubular segments vary with age. However, the current literature highlights knowledge gaps in how specifically intestinal and renal calcium (re)absorption occurs early in postnatal development. Future research should clarify the specific changes in calcium transport throughout early postnatal development including mediators of these alterations enabling appropriate bone mineralization. Impact statement This mini review outlines the current state of knowledge pertaining to the molecules and mechanisms maintaining a positive calcium balance throughout postnatal development. This process is essential to achieving optimal bone mineral density in early adulthood, thereby lowering the lifetime risk of osteoporosis.

Keywords: Calcium transport; bone and development; calcium homeostasis; intestine; kidney.

Figure 1

Net calcium absorption and secretion by intestinal section during suckling and after weaning. A pictorial representation of current evidence. Absorption is represented by a downward arrow indicating net movement of calcium from lumen to serosa. When both transcellular and paracellular pathways are present in a segment, a larger arrow indicates the predominant pathway. Note that the transcellular pathway in the duodenum pre-weaning occurs after 1–2 weeks of age in rodents. The dashed line indicates that evidence supporting the pathway is limited. Evidence for calcium absorption in the cecum is from animal models only. Weaning is at 21 days in rodents. ND: no data. (A color version of this figure is available in the online journal.)

Figure 2

Renal calcium reabsorption during suckling and after weaning. An illustrative representation of the current knowledge. The leftward arrow indicates net movement of calcium from lumen to serosa represents reabsorption. The thickness of the arrow represents relative reabsorption between tubule segments and ages. The dashed line indicates that reabsorption is inferred from expression studies or limited functional studies. Weaning is at 21 days in rodents. PT: proximal tubule; TAL: thick ascending limb; DCT: distal convoluted tubule; CNT: connecting tubule. (A color version of this figure is available in the online journal.)