Calcium Absorption

The fate of calcium in the gastrointestinal tract is indicated in Figure 9.

Daily ingested calcium Vi and that entering the gut with digestive juices Vd, both being only partially absorbed from the intestinal lumen, add to the unabsorbed element that appears in faeces VF. The difference between the rates of intake and faecal output, the net absorption rate Va(net), is given by the formula:

(1)

It may be obtained by simple determination of calcium in the food and faeces sampled on a time basis, that is, over an appropriate balance periode timed with time-markers (REIFENSTEIN et al., 1945).

By subtracting the excretion rate of endogenous faecal calcium Vf from the excretion rate of total faecal calcium, the excretion rate of exogenous faecal calcium β Vi is obtained:

(2)

The endogenous faecal calcium denotes calcium secreted into the gut with digestive juices and then excreted in the faeces, excluding that secreted into the gut and then reabsorbed. The excretion rate of endogenous faecal calcium is calculated from the amount of tracer excreted in faeces following its intravenous administration, since, being administered intravenously, all tracer in faeces is endogenous. Assuming the specific activity of calcium secreted into the gut and excreted in urine at the same time interval to be identical, the excretion rate of endogenous faecal calcium may be obtained by the formula of AUBERT and MILHAUD (1960):

(3)

where Vu = excretion rate of urinary calcium, ffaeces = fraction of tracer excreted in faeces, and furine = fraction excreted in urine, both accurately timed with a correction for faecal lag (cf. BRONNER et al., 1962):

(4)

Calcium secreted into the gut and then reabsorbed cannot be obtained in vivo, since its fractional reabsorption rate β Vd is not observable even with the aid of isotopes (MARSHALL, 1964).

The rate of entry of the radioisotope from the intestinal tract into the vascular system is usually given as the integrated steady state rate of absorption from the entire gut. Recently, HART and SPENCER (1967) studied the initial entry rate of the tracer, i. e. entry of the tracer from one compartment into another compartment of the system, without consideration of recirculation or feedback; this reflects the gradual passage of the remaining unabsorbed dose of tracer through different portions of the intestinal tract, exhibiting different transport activity (HART and SPENCER, 1967). The appearance of the tracer in the vascular space can be expressed by the Volterra integral equation:

(5)

in which G(t) is the plasma activity curve following an oral tracer dose at time t0 = 0; B(τ) is the rate of initial entry at time τ following an oral tracer dose, at which the tracer first appears in the vascular system; F(t) is the plasma activity curve following intravenous tracer administration (HART and SPENCER, 1967). Since G(t) and F(t) are determined experimentally, the integral equation can be solved by standard means, and B(τ) determined either analytically (BERKOWITZ et al., 1963; HART and SPENCER, 1967) or by a computer (HART and SPENCER, 1967).

Figure 10 illustrates a typical result for the initial entry functions from the intestine into the vascular system for 15 minutes intervals employed in the calculations (HART and SPENCER, 1967). The integrated area under the curve gives the cumulative absorption for tracer, identical with that obtained in above test on true absorption.

Detailed studies on the transport of calcium across the intestinal wall have led to the conclusion that calcium absorption is not due solely to passive diffusion, but that there is an...