Even though inulin-type fructans might also affect the gastrointestinal absorption of other minerals like Fe or Zn (see Section 10.6), the present discussion on mechanisms will focus on Ca and Mg absorption, the two, up to now, most extensively studied processes.
In the gastrointestinal tract, Ca absorption is either active, being vitamin D dependent, or passive, being vitamin D-independent. The first mechanism is already saturated by low Ca intake whereas the second, which may involve both a para- and a transcellular mechanism, is not saturable. This implies that all of the increment in Ca absorption that occurs when diet is supplemented with extra Ca or when it provides a food component that increases Ca absorption is likely to be mediated by the vitamin D-independent mechanism. The absorption of Mg is considered to be exclusively passive (mostly paracellular) since no active mechanism has yet been described.
To explain the increased absorption of Ca and Mg after inulin-type fructans feeding, several hypotheses have been proposed (Figure 10.4):
1. Higher water content in the large bowel due to osmotic effect
2. Lower pH due to higher production of SCFAs by fermentation
3. Increased solubility in the intestinal lumen due to formation of SCFA salts
4. Increased absorptive surface due to hypertrophy of the mucosa
5. Increased production of calbindin, a Ca transport protein, especially in the colonic epithelium
These hypotheses are supported by data showing a selective stimulation of bacterial (mainly bifidobacteria and lactobacilli) growth in the intestinal lumen, (see Chapter 5, Section 5.4.1 and Chapter 6, Section 6.2.4) as a result of fermentation of inulin-type fructans by the microflora with, as a consequence, a reduction of pH due to increased production of SCFAs, mainly propionate and acetate and, at a lower level but at a higher rate, butyrate and lactic acid. At this more acidic pH, more Ca and Mg ions are solubilized in the gut lumen and, thus, are more readily absorbed.75,79 110 Apart from stimulating the passive absorption indirectly by increasing solubility via lowering the pH, SCFAs might also directly stimulate mineral disappearance across the colon, propionate being more effective than acetate. The higher efficacy of propionate compared to acetate might be due to a greater lipid-solubility, which is associated to chain length.154 Butyrate, the third major SCFA produced by fermentation of inulin-type fructans, is also a potent candidate for enhancing mineral absorption because it is a substrate for colonic epithelial cell growth and proliferation, leading to an enlargement of the gut's absorptive area,
another way to contribute to the enhanced mineral absorption.155 Moreover, increased butyrate production might stimulate the active transcellular transport of Ca via an increased concentration of calbindin and the activity of the 1,25(OH)2 vitamin D3 receptor, as shown in chick primary cell culture.156 In rats, calbindin D9k is an intestine-specific, high-affinity, Ca-binding cytosolic protein, and its expression is regulated by 1,25(OH) 2 vitamin D3 and Ca intake (up-regulation if low Ca diet and down-regulated if high Ca diet).157 It has been hypothesized that calbindin contributes directly to transcellular and transepithelial Ca transport,152 and a correlation has been observed between calbindin D9k expression and intestinal Ca absorption.158 159 But calbindin also contributes indirectly to miscellaneous essential enzyme and protein-dependent cellular and metabolic processes that are controlled by the "free" intracellular Ca concentration, a key parameter in eukaryotic cells.160 The expression of calbindin is highest in the duodenum but it exists also in other parts of the intestine and especially in the large bowel, even though at much lower basal level.161162 The concentration of calbindin D9k is low in suckling and adult rats, and it is maximum (3-fold higher) during the 3-4 weeks following weaning.
In looking for a mechanism of the observed increase in Ca absorption, Ohta et al. have reported a higher relative expression of the calbindin D9k in the large intestine over that in the small intestine in rats fed oligofructose.163 At the lowest dose (5% w/w in the diet), the increase in calbindin expression was higher (2.5fold) in the colon than in the cecum (1.25-fold), and the increase in Ca absorption positively correlated with the increase in calbindin expression. Similarly, in gastrec-tomized rats, an experimental model of reduced Ca absorption and osteopenia (see Section 10.5.1.2), in which Ohta et al. had previously demonstrated a significant increase in Ca absorption, adding oligofructose to the diet (10% w/w) at the age of 4 weeks, stimulated calbindin expression both in the cecum (3.7-fold) and in the colon (8.7-fold), far above the increase induced by gastrectomy alone (2-fold).131 136 137 It should be kept in mind that the experiments performed so far have used young weaned rats (4-5 weeks of age), i.e., the age at which the basal level of calbindin D9k in the intestine is maximum.131163 Thus, the increase in calbindin might be one of the mechanisms explaining the effect of inulin-type fructans in adolescents (see Section 10.5.2.2) but, at the present state of knowledge, that cannot be used to explain the improved Ca absorption seen in adults, postmenopausal women, or the elderly.
Other candidates for Ca and Mg absorption enhancers are the polyamines, e.g., spermine, spermidine, and putrescine, that are generated by several strains of microbes as well as by higher organisms163 and that are potent agents to stimulate proliferation and induce enlargement of the intestinal tissues because they are readily taken up by enterocytes or can pass into the circulation to stimulate gene expression of calcium binding proteins in the gut.155 165-167 Inulin-type fructans have been shown to stimulate ornithine decarboxylase activity, the rate-limiting step in polyamines synthesis, and when rats were fed oligofructose for 4 weeks, putrescine concentration was higher and spermidine concentration lower, whereas total polyamine concentration was unchanged in the cecal contents compared to a standard diet.107 In addition and in the cecal tissue putrescine, spermidine and total polyamine concentrations were increased.168
To further test whether the oligofructose-induced stimulation of Ca absorption was mediated by polyamines, a study with adult female ovariectomized Fisher 344 rats did compare the effect on femur weight and ash, an indirect measure of Ca absorption, of feeding either oligofructose or polyamines. A significant loss of femur weight occurred following ovariectomy but only oligofructose and not exogenous polyamines alone did prevent loss of femur weight and femur ash. Thus, the potential of oligofructose to prevent ovariectomy-induced loss of bone weight and bone mineral content may not be mediated via polyamines, since exogenous polyamines were not protective, possibly because endogenous polyamine synthesis compensated for it.
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