Dietary fibre is composed mainly of the indigestible components of plant cell walls (e.g. cellulose, pectin and lignin); but also includes the oligosaccharides (e.g. oligofructose), resistant starches (RS1, RS2, RS3 and RS4), hydrocolloids and alcohol sugars. These various forms of dietary fibre have been shown to be beneficial to human health because they have a number of biological effects including serving as a source of nutrition for colonic microorganisms, increasing gastric transit time and providing bulk to the gastric contents. Some research has suggested that dietary fibre is able to decrease the absorption of certain minerals (e.g. calcium, zinc and iron) because fibre and its associates phytate content have in vitro mineral binding properties. However, recent evidence suggests that magnesium absorption may actually be increased by certain dietary fibres because of its unique mechanisms and sites.
Magnesium is poorly soluble and its absorption is not fully understood. Magnesium is most likely absorbed in the jejunum and ileum of the small intestine. At low concentrations, magnesium is absorbed by a carrier mediated process that is saturable, but at higher doses magnesium is probably absorbed via solvent drag and simple diffusion. Fibre has been shown to have a favourable effect on magnesium absorption probably because it is fermented by intestinal microorganisms to short chain fatty acids (SCFA) which decreases the pH of the luminal contents. Acidification of the luminal content by increased SCFA production may improve the solubility of magnesium and increase its absorption. Alternatively, short chain fatty acids may also improve the absorption of magnesium because they may stimulate the flux of magnesium across the enterocytes towards the basolateral membrane by activation of an apical Mg2+/2H+ antiporter.
Resistant starch from uncooked potato has been shown to increase the absorption of magnesium in rats, as well as increase the production of SCFA, acidify the caecum, enlarge the caecum, as well as increase magnesium solubilisation. This would support the evidence that the presence of SCFA are necessary to cause improvements in magnesium absorption. In other studies, rats fed inulin or oligosaccharides showed caecal hypertrophy and an increase in SCFA production, increased caecal blood flow as well as increase magnesium absorption. Research suggests that oligofructose can increase magnesium absorption by 46 % to 68 % in rats, with absorption being improved further by addition of inulin. Addition of SCFA to the gut also increase magnesium absorption. Amongst the SCFA tested, butyrate was more effective at increasing magnesium absorption that acetate, propionate or isobutyrate.
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