Metabolic balance studies can be performed both in experimental animals and in humans. Metabolic balance is based on the calculation of net Ca or Mg retention as intake (IN) □ excretion (OUT)
over a specific period of time. It is used to determine apparent net absorption but not true absorption. Indeed, disappearance of a mineral during mouth-to-anus transit time does not equate true absorption because minerals, including Ca or Mg, can be retained in the intestine (due to, e.g., adsorption on insoluble particles, adsorption, or chelation in mucins). Moreover, endogenous mineral ions can be secreted in the gastrointestinal lumen and subsequently excreted in feces via the bile, the pancreatic, or mucosal secretions, or because of mucosal cell loss.
Most studies on Ca/Mg absorption in animals (essentially rats) used the metabolic balance methodology. Animals are kept in metabolic cages for a few days (usually 3-5), food intake is strictly controlled or food is restricted (±90% of daily intake) so as to force the rats to eat all the food and urine plus feces are quantitatively collected. Ca/Mg in food (IN) and excreta (OUT) is measured by atomic absorption spectrometry or inductive coupled plasma emission spectrometry (ICPS). A nonabsorbable marker (e.g., chromium-mordanted cellulose75) can be added in food to calculate apparent absorption. In one protocol Ca pool was labeled with a mixture of the radioactive isotopes 47Ca and 47Sc, the latter being used as a nonabsorbable marker, and the radioactivity is measured in feces and urine.63,76 In order to distinguish between absorption in the whole gastrointestinal tract and in the large bowel, some authors calculated Ca/Mg absorption after oral administration or infusion in the stomach and infusion directly in the cecum of CaCO3, MgO, or MgCl2.76,77 In the last condition, Ca/Mg absorption taking place in the large bowel only can be calculated. Using a more sophisticated protocol that includes the measurement of cecal blood flow and arteriovenous differences across the cecum, together with the cecal concentration and cecal pool of Ca/Mg, it is also possible to assess the kinetic of Ca/Mg absorption expressed as □mol/min.78,79
In humans, even rigorously controlled balance studies will only distinguish large differences (e.g., 200 mg retention per day) unless large numbers of volunteers are included in the protocol. The greatest difficulties when performing such studies are in assessing, with precision and compliance, consumption of the diet and precise Ca or Mg intake, in collecting the totality of urine and feces, and in failing to measure other losses including dermal losses that might account for ±60 mg/d. The best balance studies are those that are conducted in metabolic wards where subjects' activities are monitored, especially food consumption and excreta collection. In experimental studies, the animals are kept in metabolic cages during the whole period of test usually after a pretest adaptation.
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