Study of calcium absorption in man: a kinetic analysis and physiologic model

Abstract

A B S T R A C T A physical model of calcium absorption was developed from analysis of data obtained on 23 subjects, including 13 patients having a variety of abnormalities of calcium metabolism. The model was tested and found consistent in all subjects studied. This technique provides a quantitative description of the rate of entry of oral dose of 'TCa into the circulation as a function of time by analysis of serum or forearm radioactivity in response to intravenous and oral administration of 'TCa. The kinetics of the absorption process as proposed by the model are characterized by an initial delay phase of 15-20 min, by a maximal rate of absorption at 40-60 min after ingestion, and by 95% completion of the absorption within 2i hr. Partial identification of the physiological counterparts of the model was possible by introduction of the isotope at various levels of the gut Although the region of the duodenum was found to have the greatest rate of absorption per unit length in normal subjects, it was least responsive to stimulation by parathyroid hormone and suppression by calcium loading. Furthermore, the response of the gut to parathyroid hormone was delayed, whereas the suppression of absorption by intravenous or oral calcium loading was rapid and dramatic. The implications of these observations are discussed. (14). True net calcium absorption has been obtained by correcting for endogenous fecal excretion determined by the fractional dose recovered in the stool after intravenous administration of isotope. More recently this time-consuming and cumbersome approach has been avoided by analysis of the fraction of radioactivity excreted in urine (5, 6) or appearing in the forearm as determined by external monitoring (7) after oral and intravenous administrations of the isotope. This technique is based on the assumption (8) that fractional loss of the absorbed portion of the oral dose is identical with that of the intravenous dose and also permits quantification of the fraction of dose absorbed. The relative rate of appearance of the isotope in the circulation by serial blood samples (4, 9-11) or by external monitoring of the forearm (12) METHODSThe population studied consisted of 10 normal volunteers between the ages of 21 and 55 yr and 13 patients with a variety of disorders of calcium metabolism including hypoparathyroidism, hyperparathyroidism, idiopathic hypercalcuria, and nontropical sprue. Except for two patients with hypoparathyroidism and one patient with idiopathic hypercalcuria, all subjects were studied on a metabolic ward and consumed diets constant in calcium, phosphorous, and total calories. All subjects were maintained on a constant diet and activity regimen for a minimum of 2 wk before the initiation of the absorption studies. No patient or volunteer was studied while receiving a daily dietary intake that differed by more than 150 mg of calcium from his customary intake as estimated by careful history.On the day of isotope administration, a standard 12 mg calcium meal consisting of cream of wheat, ...

The acute effect of chlorothiazide on serum-ionized calcium. Evidence for a parathyroid hormone-dependent mechanism.

“…Both possibilities were considered in previous studies (3)(4)(5). The hypercalcemic action of PTH is a result of its skeletal mobilization of calcium and increased gastrointestinal absorption of calcium (16)(17)(18)(19). The results of our study confirmed the previously reported hypocalciuric effect of PTH in hypoparathyroid patients (17) and demonstrated it also in normal subjects.…”

Postprandial Mineral Metabolism and Secondary Hyperparathyroidism in Early CKD

“…20,21 It is unlikely that we missed transient increases in serum mineral levels before the first postprandial time point when we sampled blood and urine, because the maximal gastrointestinal absorption of calcium and phosphorus occurs 40 to 60 min after a meal. 22 Furthermore, if efficient gastrointestinal absorption occurred earlier, then we should have observed a different response between the meals. An alternative hypothesis is that neurohormonal changes associated with eating led to renal vasodilation with an increase in glomerular filtration of calcium and phosphorus and a decrease in their tubular reabsorption that was independent of serum levels and intake.…”

Biodisponibilidade do cálcio dietético

“…O local onde a maior parte da absorção do elemento vai ocorrer depende da capacidade absortiva, do comprimento do segmento intestinal, do tempo de trânsito, da biodisponibilidade e da concentração intraluminal de Ca (3). O duodeno tem a maior capacidade absortiva por unidade de comprimento, mas a maioria do Ca é absorvida no jejuno devido ao seu maior comprimento total (4,5).…”

“…Contudo, com exceção das gorduras e algumas drogas, estes compostos têm pouca importância prática na vida diária (3,4). O Ca e o fósforo (P) absorvidos participam da manutenção da absorção do Ca devido aos efeitos do Ca sérico sobre a secreção do PTH e dos efeitos do P e PTH séricos sobre a produção renal de 1,25(OH) 2 D (9).…”