Calcium Absorption

Ireland, P. & Fordtran, J.S. 1973. Effect of dietary calcium and age on jejunal calcium absorption in Humans studied by intestinal perfusion. J. Clin. Investig., 52: 2672-81.

Heaney, R.P., Saville, P.D. & Recker, R.R. 1975. Calcium absorption as a function of calcium intake. J. Lab. Clin. Med., 85: 881-890.

Wilkinson, R. 1976. Absorption of calcium, phosphorus and magnesium. Calcium, Phosphate and Magnesium Metabolism. Nordin, B.E.C. ed. p. 36-112. Edinburgh: Churchill Livingstone.

Marshall, D.H. 1976. Calcium and phosphate kinetics Calcium, Phosphate and Magnesium Metabolism. Nordin, B.E.C. ed. p. 257-297. Edinburgh: Churchill Livingstone.

Ingested calcium mixes with digestive juice calcium in the proximal small intestine from where it is absorbed by a process, which has an active saturable component and a diffusion component.

At low calcium intakes calcium is mainly absorbed by active(transcellular) transport, but at higher intakes an increasing proportion of calcium is absorbedby simple (paracellular) diffusion. The unabsorbed component appears in the faeces together with the unabsorbed component of digestive juice calcium known as endogenous faecal calcium. Thus, the faeces contain unabsorbed dietary calcium and unreabsorbed digestive juice calcium (Figure 14).

Heaney, R.P. & Skillman, T.G. 1964. Secretion and excretion of calcium by the Human gastrointestinal tract. J. Lab. Clin. Med., 64: 29-41.

Nordin, B.E.C., Horsman, A. & Aaron, J. 1976. Diagnostic procedures. Calcium, Phosphate and Magnesium Metabolism. Nordin, B.E.C. ed. p. 469-524. Edinburgh: Churchill Livingstone.

True absorbed calcium is the total 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).

Ireland, P. & Fordtran, J.S. 1973. Effect of dietary calcium and age on jejunal calcium absorption in Humans studied by intestinal perfusion. J. Clin. Investig., 52: 2672-81.

Heaney, R.P., Saville, P.D. & Recker, R.R. 1975. Calcium absorption as a function of calcium intake. J. Lab. Clin. Med., 85: 881-890.

Marshall, D.H., Nordin, B.E.C. & Speed, R. 1976. Calcium, phosphorus and magnesium requirement. Proc. Nutr. Soc., 35: 163-173.

Nordin, B.E.C. & Marshall, D.H. 1988. Dietary requirements for calcium. In: Calcium in Human Biology. Nordin, B.E.C., ed. p. 447-471. Berlin: Springer-Verlag,.

When the intake reaches about 200 mg (5 mmol), dietary and faecal calcium become 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 on ingested calcium approaches linearity with an ultimate gradient of about 5–10 percent.

Bogdonoff, M.D., Shock, N.W. & Nichols, M.P. Calcium, phosphorus, nitrogen, and potassium balance studies in the aged male. J. Gerontol. 1953;8:272-288.

Clarkson, E.M., Durrant, C. & Phillips, M.E., Gower, P.E., Jewkes, R.F., De Wardener, H.E. 1970. The effect of a high intake of calcium and phosphate in normal subjects and patients with chronic renal failure. Clin. Sci., 39: 693-704.

Johnston, F.A., McMillan, T.J. & Derby Falconer, G. 1952. Calcium retained by young women before and after adding spinach to the diet. J. Am. Diet. Assoc., 28: 933-938.

Malm, O.J. 1958. Calcium requirement and adaptation in adult men. Scand. J. Clin. Lab. Investig., 10(Suppl 36):1-289.

Owen, E.C., Irving, J.T. & Lyall, A. 1940. The calcium requirements of older male subjects with special reference to the genesis of senile osteoporosis. Acta Medica. Scand., 103: 235-250.

Steggerda, F.R. & Mitchell, H.H. 1939. The calcium requirement of adult man and the utilisation of the calcium in milk and in calcium gluconate. J. Nutr., 17: 253-262.

Steggerda, F.R. & Mitchell, H.H. Further experiments on the calcium requirement of adult man and the utilisation of the calcium in milk. J. Nutr., 21: 577-588.

Steggerda, F.R. & Mitchell, H.H. 1946. Variability in the calcium metabolism and calcium requirements of adult Human subjects. J. Nutr., 31: 407-422.

Calcium absorption and calcium intake, derived from 210 balance studies performed in 81 individuals collected from the literature, is shown in Figure 14. The relationships between calcium intake and calcium absorbed and excreted calcium calculated from 210 balance experiments in 81 subjects. 

Equilibrium is reached at an intake of 520 mg, which rises to 840 mg when skin losses of 60 mg are added and to 1100 mg when menopausal loss is included. The curvilinear relationship between intestinal calcium absorption and calcium intake can be made linear by using the logarithm of calcium intake toyield the equation: Caa = 174 loge Cai -909 ± 71 (SD) mg/day, where Cai represents ingested calcium and Caa net absorbed calcium. The relationship between urinary calcium excretion and calcium intake is given by the equation: Cau = 0.078 Cai + 137 ± 11.2 (SD) mg/day,where Cau is urinary calcium and Cai calcium intake.