In the rat, Mg absorption occurs in the large bowel, namely the cecum and the colon, as well as in the small intestine.68,101-106 The absorption of Mg in the large intestine is associated with a low intraluminal pH due to the presence of lactic acid and SCFAs, the products of fermentation of poorly or nondigestible carbohydrates.102-105 Inulin-type fructans increase Mg absorption as shown by a large number of studies with different protocols using both inulin and oligofructose. These studies are summarized in Table 10.2.
The vast majority of these studies used the metabolic balance method to measure fractional or apparent absorption (FA%) calculated as:
Intake (IN) □ Excretion (OUT, feces and urine)/Intake (IN) □ 100
One study, however, used a more sophisticated kinetic approach to measure true absorption.80 The concentration of Mg in diet, feces, or urine was measured using atomic absorption spectrometry, plasma atomic absorption spectrometry, or inductive coupled plasma emission spectrometry (ICPS). During the metabolic balance study, rats were kept in metabolic cages and dietary intake, fecal, and urinary excretions were quantified for periods of 4 d. The concentration of Mg in diet was ±0.05% (the recommended value for rats) in most studies, but in some cases it was lower (0.02%) or higher (0.09 or 0.1%). Rat strains used were Sprague-Dawley or Wistar, and all animals were males. Usually the rats were rather young (i.e., 4-6 weeks old) at the start of experiments that lasted for a few weeks (10 to 31 d). The concentration of inulin-type fructans in the diet was 5 and 10% (w/w) in 2 of the protocols, respectively. In addition, some experiments did compare the effect of different concentrations e.g., 1, 2.5, 3, or even 15% but the database is too limited to draw any conclusion concerning a dose-effect relationship. As with Ca absorption, the relative increase (in %) in apparent Mg absorption, calculated as:
FA in the rats fed inulin - FA in the control rats/FA in the control rats correlates (r = 0.859 p < 0.01) with the absorption in the control rats (Figure 10.3).
Similar to what has been observed with Ca, in two studies that measured the FA percentage four times during the oligofructose feeding at 1 week intervals, data seem to indicate that, in rats, the increase in Mg absorption progressively diminishes but it is still statistically significant after 4 weeks of feeding.107,106 In a protocol comparing the absorption of Mg (0.025% w/w in diet) from different sources (i.e., MgO, cocoa, defatted rice bran, or acid extract of defatted rice bran), it was shown that oligofructose (5% w/w in diet for 21 d) significantly increased the absorption ratio of Mg in all cases.130 Finally, it has been reported that preventing coprophagy, a common behavior in many rodent species, did not modify the basal absorption of Mg, but it did enhance the absorption of Mg in the rats fed with oligofructose.110
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