Effects of Dietary Fibers on Magnesium Absorption in Animals and Humans

This study is aimed at investigating the effect of feeding a soluble or partly soluble fibre rich-diet on the apparent absorption and balance of calcium, magnesium, iron and zinc in healthy young men, by using a chemical balance technique. Nine healthy young men were given a control diet or the same diet complemented with either inulin (soluble) or sugar beet fibre (partly soluble) during 28 d periods according to a 3 x 3 latin square design with three repetitions. During the 20 d adaptation period to fibre ingestion, experimental fibres were incorporated into bread (60%) and liquid foods (40%) up to a maximum of 40 g/d. Ca, Mg, Fe and Zn were measured in diets and in a 8 d urine and faecal composites to assess mineral absorption and balance. The dietary mineral intake provided (mg/d) 859 +/- 196 of Ca; 311 +/- 43 of Mg; 11.6 +/- 1.7 of Fe; and 11.1 +/- 1.6 of Zn from the control diet. The apparent absorption of minerals from the control diet was (%) Ca: 21.3 +/- 12.5; Mg: 46.3 +/- 10.9; Fe: 21.8 +/- 12.3 and Zn: 14.0 +/- 14.5 (mean +/- s.d.). Ingestion of inulin significantly increased the apparent absorption and the balance of Ca. Sugar beet fibre ingestion resulted in a significant increase in Ca intake and balance, without modification its apparent absorption. Apparent absorption and balance of Mg, Fe and Zn were not significantly altered by the ingestion of either experimental fibre. Addition of the two experimental fibres (inulin or sugar beet fibre) to normal mixed diets can improve Ca balance without adverse effects on other mineral retention. This project was supported by the French Ministry of Agriculture, Fisheries and Foods (programme Aliment #2002-Aliment Demain; No. 906335). The authors acknowledge the société Agro Industries, Recherche et Developpement (Mr R. De Baynast) who supplied them with the experimental fibres.

The digestive and metabolic effects of inulin (from chicory) were studied in rats adapted to semipurified diets containing 0, 5, 10 or 20% inulin (wt/wt). Moderate levels of inulin (5-10%) did not significantly affect food intake or body weight gain. Dietary inulin resulted in considerably greater cecal fermentation and a significantly greater intraluminal concentration of propionate (peaking at 58.4 mmol/L). A lower concentration of acetate (42.6 mmol/L) was observed in rats fed 20% inulin. Lactic fermentations were observed in rats fed the 10 or 20% inulin diets. The cecal pool of volatile fatty acids tended to reach a plateau in rats fed diets containing more than 10% inulin (up to 600-700 mumol), but volatile fatty acid absorption was a slightly hyperbolic function of the dietary inulin level. Butyrate absorption was proportionally lower than that of propionate. Inulin-containing diets induced an enlargement of the cecal pool of calcium, phosphate and (to a lesser extent) magnesium. There was also an enhanced absorption of these divalent cations. The cecal pool of bile acids was greater in rats fed inulin, and this oligosaccharide displayed a slight hypocholesterolemic effect, even in rats fed the 5% inulin diet. However, plasma triglycerides were depressed only in rats fed the 20% inulin diet. In conclusion, inulin seems very effective in promoting propionic fermentation and in enhancing the calcium content of the large intestine. However, high levels of inulin (greater than 10%) may affect growth in rats and lead to acidic (pH 5.65) cecal fermentation.

Resistant starch and inulin are complex carbohydrates that are fermented by the microflora and known to increase colonic absorption of minerals in animals. The fermentation of these substrates in the large bowel to short-chain fatty acids is the main reason for this increase in mineral absorption. The purpose of the present study was to examine the potential synergistic effect of a combination of these two fermentable carbohydrates. For this purpose, thirty-two adult male Wistar rats weighing 200 g were used in the present study. The rats were distributed into four groups, and fed for 21 d a fibre-free basal purified diet or diet containing 100 g inulin, or 150 g resistant starch (raw potato starch)/kg diet or a blend of 50 g inulin and 75 g resistant starch/kg diet. After an adaptation period of 14 d, the rats were then transferred to metabolic cages and dietary intake, faeces and urine were monitored for 5 d. The animals were then anaesthetized and caecal Ca and Mg absorption were measured. Finally, the rats were killed and blood, caecum and tissues were sampled. Ca and Mg levels were assessed in diets, faeces, urine, caecum and plasma by atomic absorption spectrometry. Our results confirmed that inulin and resistant starch ingestion led to considerable caecal fermentation in the three experimental groups compared with the control group diet. Moreover, both carbohydrates significantly increased the intestinal absorption and balance of Ca and Mg, without altering the plasma level of these two minerals. Interestingly, the combination of the studied carbohydrates increased significantly the caecal soluble Ca and Mg concentrations, the apparent intestinal absorption and balance of Ca, and non-significantly the plasma Mg level. In conclusion, a combination of different carbohydrates showed synergistic effects on intestinal Ca absorption and balance in rats. Further studies with other types of carbohydrate combinations should be carried out to extend these findings.