Calcium Absorption...Continued

Nordin, B.E.C. 1976. Nutritional considerations. In: Calcium, Phosphate and Magnesium Metabolism. Nordin, B.E.C., ed. p. 1-35. Edinburgh: Churchill Livingstone.

True absorption is an inverse function of calcium intake, falling from some 70 percent at very low intakes to about 35 percent at high intakes (Figure 15). Percent net absorption is negative at low intakes, becomes positive as intake increases, reaches a peak of about 30 percent at an intake of about 400 mg, and then falls off as the intake increases. The two lines converge as intake rises because the endogenous faecal component (which separates them)becomes proportionately smaller.

Many factors influence the availability of calcium for absorption and the absorptive mechanism itself. The former includes substances, which form insoluble complexes with calcium, such as the phosphate ion. The relatively high calcium-phosphate ratio of 2.2 inhuman milk compared with 0.77 in cow milk may be a factor in the higher absorption ofcalcium from human milk than cow milk.

Jones, G., Strugnell, S.A. & DeLuca, H.F. 1998. Current understanding of the molecular actions of vitamin D. Physiol. Revs., 78: 1193-1231. 

Intestinal calcium absorption is mainly controlled by the serum concentration of 1,25(OH)2D (see Chapter 8). The activity of the 1-α-hydroxylase, which catalyses 1,25(OH)2D production from 25-hydroxycolecalciferol (25OHD) in the kidneys, is negatively related to the plasma calcium and phosphate concentrations and positively to plasmaparathyroid hormone. 

Thus the inverse relationship between calcium intake and fractional absorption described above is enhanced by the inverse relationship between dietary calcium and serum 1,25(OH)2D (21,40,41).

Phytates, present in the husks of many cereals as well as in nuts, seeds, and legumes,can form insoluble calcium phytate salts in the gastrointestinal tract. Excess oxalates canprecipitate calcium in the bowel but are not an important factor in most diets.