Mechanisms underlying the effects of inulin-type fructans on calcium absorption in the large intestine of rats

Inulin-type fructans (inulin, oligofructose, fructooligosaccharides) in the diet do increase intestinal calcium absorption in animals and humans, but the underlying mechanism has not been identified. We therefore assessed the effects of fermentation of inulin-type fructans on transepithelial calcium transport in rat large intestine. Transepithelial calcium fluxes in vitro (Ussing chamber), effects on gene expression, mucosal morphology, and composition of luminal contents were determined in rats fed a standard diet and/or a diet containing 10% (w/w) 1/1 inulin-oligofructose mixture (INOF). Net transepithelial calcium transport in large intestine of rats fed a standard diet was increased by high mucosal calcium concentrations, the presence of 100 mmol/L mucosal short-chain fatty acids (SCFAs), the presence of 10 g/L INOF at the mucosal side, but not by reducing mucosal pH. Tissues from rats fed INOF did not show altered calcium transport when compared to controls. However, when flux data were based on the total caecal surface area, INOF-fed rats nearly doubled absorption rate in caecum. INOF feeding altered transcript levels of several mucosal genes that can be linked to transcellular and paracellular calcium transport processes. In addition, a decreased luminal pH in caecum with markedly increased caecal pools of total, soluble, and ionized calcium resulted from INOF ingestion. Thus, inulin-type fructans increase the large intestinal calcium absorption by different mechanisms including enhanced pools of soluble and ionized calcium, an increase in the absorptive surface predominantly in caecum, the increased concentrations of SCFAs, and by direct interaction with the intestinal tissue.

Diet composition and age determine the effects of inulin-type fructans on intestinal calcium absorption in rat

BACKGROUND

Although several studies in animals and humans have established that inulintype fructans (inulin, oligofructose, fructooligosaccharides) enhance intestinal Ca absorption, there are also reports that failed to demonstrate any effects of added fructans on Ca absorption.

AIM OF THE STUDY

We investigated in a rat model what variables determine inulin actions on mineral absorption with special focus on the basic diet.

METHODS

To determine apparent intestinal mineral absorption, whole body retention and mineral accumulation in bone, we performed feeding experiments with a balance technique by which mineral intake, faecal and urinary output are determined.

RESULTS

In a first study we compared the effects of an inulin oligofructose mixture (0 and 10%,respectively) when added either to a standard diet or a semi-synthetic diet. Rats fed the semi-synthetic diet were younger (adolescent) than rats on standard diet (young adult). We observed that the apparent Ca absorption ratio was significantly increased by inulin and oligofructose only when provided in the semisynthetic diet and not in the standard diet that intrinsically already contained some fructans from wheat. In our second study with a semi-synthetic diet fed to growing (adolescent) rats, inulin and oligofructose increased not only Ca but also Mg and Zn absorption, whole body mineral retention and femur mineral content.

CONCLUSION

Inulin-type fructans at dietary levels of 10 % (w/w) do increase mineral absorption, retention and accumulation in bone in the case of Ca, Mg and Zn, but only when the basic diet for the control group contains no intrinsic fructans and when the mineral demand is particularly high as during growth.

A novel bifunctionality: PAT1 and PAT2 mediate electrogenic proton/amino acid and electroneutral proton/fatty acid symport

Recently, the PAT family of proton-dependent amino acid transporters has been identified as a novel class of mammalian amino acid symporters. PAT1 and PAT2 members mediate electrogenic uptake of small, neutral amino acids and derivatives by cotransport of protons. Analysis of the structural requirements for substrate recognition by PAT1 identified that a free amino group in a substrate is not essential for recognition. We therefore hypothesized that PAT1 and its ortholog PAT2 may also be able to recognize and transport the homologous short-chain fatty acids (SCFAs) such as acetate, propionate, and butyrate. We examined in Xenopus laevis oocytes whether the SCFAs interact with the transporter by employing flux studies, electrophysiology and intracellular pH recordings. SCFAs did not induce positive inward currents but inhibited glycine-induced transport currents. PAT-mediated uptake of radiolabeled proline was also dose-dependently reduced by SCFA and could be described by first order competition kinetics with apparent Ki-values for butyrate of 6.0 +/- 0.7 and 7.6 +/- 1.3 mM for PAT1 and PAT2, respectively. Acetate as well as propionate uptake was significantly enhanced in oocytes expressing PAT1 or PAT2. An electroneutral H+/SCFA symport mode was demonstrated by recording intracellular pH changes under voltage clamp conditions with rate constants for the initial intracellular acidification in the presence of SCFAs significantly increased in PAT-expressing oocytes. In conclusion, our data demonstrate that the PAT1 and PAT2 proteins are capable to transport selected SCFAs in an electroneutral and the homologous amino acids in an electrogenic mode and are therefore a paradigm for bifunctional solute carriers.

Calcium balance in young adults on a vegan and lactovegetarian diet

For people in Western countries, the vegan diet has the advantage of low energy intake, but the calcium status of this strictly plant-based diet is still unclear. The aim of this study was to determine the calcium balance of individuals on a vegan diet in comparison with a lactovegetarian diet in a short-term investigation. Seven women and one man, ranging in age from 19 to 24 years, received during the first 10 days a vegan diet based on plant foods and calcium-rich mineral water and a lactovegetarian diet during the following 10 days. Portion size was adapted to the subjects' individual energy requirements. Calcium status was assessed by means of calcium intake in food and calcium output in feces and urine as measured by flame atomic absorption spectrophotometry. In addition, deoxypyridinoline was measured in urine as a marker of bone resorption. The results show a significantly smaller daily calcium intake with an average of 843 +/- 140 mg in the vegan versus 1322 +/- 303 mg in the lactovegetarian diet. Apparent calcium absorption rates were calculated as 26% +/- 15% in the vegan and 24% +/- 8% in the lactovegetarian group (NS). The calcium balance was positive both in the vegan diet (119 +/- 113 mg/day) and in the lactovegetarian diet (211 +/- 136 mg/day) (NS). Deoxypyridinoline excretion showed no significant difference between the two diets (105 +/- 31 and 98 +/- 23 nmol/day). The present results indicate that calcium balance and a marker of bone turnover are not affected significantly when calcium is provided either solely by plant foods or by a diet including dairy products, despite the significantly different calcium intake levels in the diets. We conclude that a well-selected vegan diet maintains calcium status, at least for a short-term period.