study that measured the FA percentage four times during the oligofructose feeding at one week intervals, data seem to indicate that, in rats, the increase in Ca absorption progressively diminishes and finally levels off to become statistically nonsignificant after 4 weeks of feeding.108 This observation is not confirmed in another study of the same group.109 Finally, it has been reported that preventing coprophagy, a common behavior in many rodent species, modified neither the basal absorption of Ca nor the increase induced by oligofructose.110
However, increasing gastrointestinal Ca absorption by inulin-type fructans is not accompanied by an increase in Ca serum concentration.76,80 111-114 In a high Ca (1.2% w/w)-high P (1.1% w/w) diet, adding oligofructose (5% w/w) has been reported even to lower serum Ca concentration ( 6%, p < 0.05) after 25 d.111
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 adding oligofructose (5% w/w in diet for 21 d) significantly increased the absorption ratio of Ca in all cases.77
One crossover protocol applied to Beagle dogs (n = 5; age 3-11 years) confirmed the enhancing effect of an oligofructose-supplemented (1% w/w) diet fed for 3 weeks on Ca absorption (8.6% to 16% or +86%).115
Using a more complex protocol to measure the arteriovenous differences across the cecum in anaesthetized rats, Levrat et al. have demonstrated a dose-dependent increase in Ca concentration in the cecum and in the cecal pool as well as in cecal absorption of Ca by inulin (0, 5, 10, and 20% w/w in diet).79 In that protocol, the 3 doses of inulin increased cecal blood flow (1.5D, 1.8D, and 3D, respectively) similarly to the increase in cecal wall weight (1.3D, 1.6D, 2.6D, respectively). However, and at each dose, the stimulation of Ca absorption was higher (1.9D, 4.3D, and 8D, respectively), indicating that the effect of inulin was not only due to an increased blood flow or cecal absorptive surface.79 The same group has confirmed that effect by showing that, at two different concentrations of Ca in the diet (0.3 and 0.8% w/w), inulin (15% w/w in the diet) increased Ca cecal absorption 4-6 fold.107 Using the same protocol, Lopez et al. similarly showed a significant increase in cecal absorption of Ca (+40%) by inulin (10% w/w in diet) even in the presence of phytic acid (0.7% w/w in diet).113 In that experiment the addition of phytic acid in the diet did not modify basal cecal Ca absorption but in the presence of inulin Ca absorption was more than twice as high as in the absence of phytates (+100%).113
Additional studies were performed that used cecectomized, gastrectomized, or iron-deficient rats. In cecectomized rats, Ohta et al. have reported that oligofructose had no effect on Ca absorption, concluding that the cecum was the site of action of oligofructose, even though its fermentation was still observed in the colon.108 However, using a different protocol to search for the site of the enhancing effect of oligofructose on Ca absorption, the same authors compared FA(%) after administration of CaCO3 by gastric intubation or cecal infusion. Results showed that oligofructose increased Ca absorption in the former but not in the later condition. According to these data, the cecum and the colon do not appear to be the major sites of action of oligofructose.76
The gastrectomized rats (4-week-old male Sprague-Dawley) were used to test for the preventing effect of oligofructose (10% w/w in diet for 10 d) on osteopenia know to occur in gastrectomized patients. In that protocol, oligofructose highly significantly increased Ca absorption when comparing the gastrectomized rats fed the standard diet and the oligofructose-containing diet (17% and 62%, respectively, or a 3.65-fold increase). In that case the absorption of Ca was similar to that in the control sham-operated rats (61%).116
Iron-deficient rats have a lower Ca absorption capacity than normal rats.117 That experimental model was thus used to test the effect of oligofructose and results showed that, like in normal rats, Ca absorption was significantly increased (from 66.8 to 75.2% or +12%) in oligofructose-fed iron-deficient rats.118 The positive effect on mineral availability was demonstrated by prevention of anemia, i.e., haemato-logical parameters or by prevention of mineral deficiency.
Aged virgin ovariectomized female rats is an accepted experimental model for human postmenopausal osteoporosis.119 Experiments have thus been performed using that protocol (5 month-old ovariectomized virgin female rats) to test for the effect (at weeks 4, 8, and 16) of low (2.5%), medium (5%), and high (10%) doses of inulin-type fructans in the presence of the recommended dietary intake of Ca (0.5%) and P (0.5%), and of medium dose (5%) of oligofructose in the presence of high-dietary Ca (1%) on Ca absorption and bone health (see Section 10.5.3).120 In the group of rats receiving the recommended Ca intake (0.5% w/w in the diet), Ca apparent absorption was not increased by oligofructose feeding except at week 8 in rats fed the 10% oligofructose-containing diet. But in the group of rats fed the diet containing the high Ca concentration (1% w/w in the diet), supplementing the diet with 5% oligofructose significantly increased Ca absorption by 45, 30, and 21% at week 4, 8, and 16, respectively (p < 0.05) due to persistently reduced fecal calcium loss. Increased absorption was associated with increased urinary calcium, reflecting metabolic reaction on higher calcium influx from the gut and not calcium mobilization from the skeleton because bone mineral content was higher on oligofruc-tose.120 In a similar experiment in which 6-month-old ovariectomized virgin female rats were fed a diet containing 0.5% Ca and 5% oligofructose-enriched inulin Synergy 1, Ca absorption was increased 4-fold (p < 0.005).121
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