Define about the Paracellular Route - Calcium?
It involves passive Ca transport through the tight junctions between mucosal cells. The salient features include:
- It is independent of nutritional and physiological regulation.
- It is concentration-dependent and occurs when there is an increased intake or a person is taking supplements.
- It occurs throughout the small intestine, ileum being the important site.
Thus, we can say that most Ca absorption takes place in the small intestine. There is some evidence, which suggests that not more than 4% (8 mg) of dietary Ca is absorbed by the colon per day. The unabsorbed component which appears in the faeces together with the unabsorbed component of digestive juice calcium is known as endogenous faecal calcium. The faeces, therefore, contain unabsorbed dietary calcium and digestive juice calcium that was not reabsorbed. True absorbed calcium is the total amount of calcium absorbed from the calcium pool in the intestines and therefore contains both dietary and digestive juice components.
Net absorbed calcium is the difference between dietary calcium and, faecal calcium and is numerically the same as true absorbed calcium minus endogenous faecal calcium. At zero calcium intake, all the faecal calcium is endogenous and represents the digestive juice calcium which has not been reabsorbed; net absorbed calcium at this intake is therefore negative to the extent of about 200 mg (5 mmol). When the intake reaches about 200mg (5 mmol), dietary and faecal calcium becomes equal and net absorbed calcium is zero. As calcium intake increases, net absorbed calcium also increases, steeply at first but then, as the active transport becomes saturated, more slowly until the slope of absorbed or ingested calcium approaches linearity with an ultimate gradient of about 5 -10%. True absorption is an inverse function of calcium intake, falling from some 70 % at very low intakes to about 35% at high intakes. There are several factors which influence the amount of calcium absorbed through the intestine. These factors can thus be related to the bioavailability of calcium. The subsequent discussion will look at these factors in detail. But first we shall look at the excretion of unabsorbed calcium.