Study of magnesium bioavailability using stable isotopes and the inductively-coupled plasma mass spectrometry technique in the rat: single and double labelling approaches

Segmental transport of Ca²⁺ and Mg²⁺ along the gastrointestinal tract

Calcium (Ca(2+)) and magnesium (Mg(2+)) ions are involved in many vital physiological functions. Since dietary intake is the only source of minerals for the body, intestinal absorption is essential for normal homeostatic levels. The aim of this study was to characterize the absorption of Ca(2+) as well as Mg(2+) along the gastrointestinal tract at a molecular and functional level. In both humans and mice the Ca(2+) channel transient receptor potential vanilloid subtype 6 (TRPV6) is expressed in the proximal intestinal segments, whereas Mg(2+) channel transient receptor potential melastatin subtype 6 (TRPM6) is expressed in the distal parts of the intestine. A method was established to measure the rate of Mg(2+) absorption from the intestine in a time-dependent manner by use of (25)Mg(2+). In addition, local absorption of Ca(2+) and Mg(2+) in different segments of the intestine of mice was determined by using surgically implanted intestinal cannulas. By these methods, it was demonstrated that intestinal absorption of Mg(2+) is regulated by dietary needs in a vitamin D-independent manner. Also, it was shown that at low luminal concentrations, favoring transcellular absorption, Ca(2+) transport mainly takes place in the proximal segments of the intestine, whereas Mg(2+) absorption predominantly occurs in the distal part of the gastrointestinal tract. Vitamin D treatment of mice increased serum Mg(2+) levels and 24-h urinary Mg(2+) excretion, but not intestinal absorption of (25)Mg(2+). Segmental cannulation of the intestine and time-dependent absorption studies using (25)Mg(2+) provide new ways to study intestinal Mg(2+) absorption.

Magnesium absorption from mineral water decreases with increasing quantities of magnesium per serving in rats

It is hypothesized that magnesium (Mg) absorption from mineral water is affected by the concentration of Mg in the water, the consumption pattern, and the volume consumed per serving. The present study examined the effect of serving volume and consumption pattern of artificial mineral water (AMW) and Mg concentration on Mg absorption in rats. Magnesium in AMW was labeled with magnesium-25 as a tracer. Each group consisted of 6 or 7 rats. In experiment 1, the rats received 1 mL of AMW containing 200 mg Mg/L at 4 times, 400 mg Mg/L twice, or 800 mg Mg/L at 1 time. In experiment 2, the rats received 1 mL of AMW containing 200 mg Mg/L or 0.25 mL of AMW containing 800 mg Mg/L at 4 times or 1 mL of AMW containing 800 mg Mg/L at 1 time. The absorption of Mg decreased with increasing Mg concentrations in the same serving volume of AMW with different serving frequencies. When the AMW containing 800 mg Mg/L was portioned into 4 servings, Mg absorption increased to the level of absorption in the group exposed to AMW containing 200 mg Mg/L served at the same frequency. These results suggest that the Mg concentration and the volume of AMW do not affect Mg absorption per se, but Mg absorption from AMW decreases when the amount of Mg in each serving is increased. Thus, frequent consumption is preferable for mineral water rich in Mg when the total consumption of mineral water is the same.

Dietary phytase increases the true absorption and endogenous fecal excretion of zinc in growing pigs given a corn-soybean meal based diet

We investigated the effect of dietary phytase on the true absorption and endogenous fecal excretion of zinc (Zn) using (67)Zn in growing pigs given a corn-soybean meal based diet. Ten crossbred barrows were fed the control diet containing 90-mg/kg Zn, 2.3-g/kg phytate-phosphorus and 3.7-g/kg non-phytate-phosphorus or the phytase diet containing similar amounts of Zn and phytate-phosphorus, and 1.4-g/kg non-phytate-phosphorus with 750-PU/kg phytase for 12 h/day. On day 6, the pigs were given 200 g of the corresponding diet labeled by (67)Zn for 2 h. We measured feed intake, fecal Zn concentration and (67)Zn abundance for the determination of apparent absorption, true absorption and endogenous fecal excretion of Zn. Although the apparent absorption of Zn did not significantly differ between the dietary groups, the phytase group had significantly more (P < 0.05) true absorption of Zn than the control group. The endogenous fecal excretion of Zn tended to be more (P = 0.07) in the phytase group than in the control group. These results suggest that dietary phytase improves Zn bioavailability through increasing the true absorption of Zn in growing pigs, which results in stimulating the endogenous fecal excretion of Zn when dietary Zn satisfies its requirement.

Comparison of urinary monitoring, faecal monitoring and erythrocyte analysis of stable isotope labels to determine magnesium absorption in human subjects

We have evaluated urinary monitoring and erythrocyte analysis to determine Mg absorption in human subjects as alternatives to the conventional technique of faecal monitoring by stable-isotope techniques. Ten healthy adults received 2·2 mmol 25Mg in water, together with wheat bread, followed 15 min later by intravenous injection of 0·6 mmol 26Mg (day 1). Brilliant blue and Yb (given on day 0 and day 1 respectively) served as qualitative and quantitative faecal markers. Urine was collected for 6 d after test meal intake. Complete collections of faeces were made until excretion of the second brilliant blue marker (given on day 7). Mg isotope ratios were determined by thermal ionisation-MS in urine and faeces and by inductively coupled plasma-MS in erythrocytes. Absorption was determined based on: (1) 6 d urine pools; (2) 24 h urine pools (collected 22–46 h after test meal intake); (3) erythrocytes from a blood sample drawn on day 14; (4) complete 6 d faecal pools; (5) faecal pools based on the first three consecutive stools after excretion of the first brilliant blue marker. Differences in mean Mg absorption (42 44 %) were statistically insignificant between techniques, except when based on 6 d urine pools for which the value was significantly lower (33 (sd 7) %, P=0·0003, ANOVA). The results indicate that Mg absorption can be determined from 24 h urine pools or erythrocytes obtained 14 d after test meal intake, an alternative method to the more time-consuming and labour-intense faecal monitoring. The choice of technique depends on practical and financial considerations.

SLC41A2 encodes a plasma-membrane Mg2+ transporter

The TRPM7 (transient receptor potential melastatin 7) ion channel has been implicated in the uptake of Mg2+ into vertebrate cells, as elimination of TRPM7 expression through gene targeting in DT40 B-lymphocytes renders them unable to grow in the absence of supplemental Mg2+. However, a residual capacity of TRPM7-deficient cells to accumulate Mg2+ and proliferate when provided with supplemental Mg2+ suggests the existence of Mg2+ uptake mechanism(s) other than TRPM7. Evaluation of the expression of several members of the SLC41 (solute carrier family 41) family, which exhibit homology with the MgtE class of prokaryotic putative bivalent-cation transporters, demonstrated that one, SLC41A2 (solute carrier family 41 member 2), is expressed in both wild-type and TRPM7-deficient DT40 cells. Characterization of heterologously expressed SLC41A2 protein indicated that it is a plasma-membrane protein with an N-terminus-outside/C-terminus-inside 11-TM (transmembrane)-span topology, consistent with its functioning as a trans-plasma-membrane transporter. In contrast with a previous report of ion-channel activity associated with SLC41A2 expression in oocytes, investigation of whole cell currents in SLC41A2-expressing DT40 cells revealed no novel currents of any type associated with SLC41A2 expression. However, expression of SLC41A2 in TRPM7-deficient cells under the control of a doxycycline-inducible promoter was able to conditionally enhance their net uptake of 26Mg2+ and conditionally and dose-dependently provide them with the capacity to grow in the absence of supplemental Mg2+, observations strongly supporting a model whereby SLC41A2 directly mediates trans-plasma-membrane Mg2+ transport. Overall, our results suggest that SLC41A2 functions as a plasma-membrane Mg2+ transporter in vertebrate cells.

Dietary inulin intake and age can affect intestinal absorption of zinc and copper in rats

Nondigestible inulin-type fructan intake can stimulate intestinal mineral absorption in both humans and animals. However, this stimulatory effect may depend on experimental conditions such as the duration of the experience, mineral levels in the diet, and the animal's physiological status. The aim of this study was to determine the effect of inulin intake on Zn and Cu absorption in rats at different ages. Male Wistar rats (n = 80) of 4 different ages (2,5, 10, and 20 mo) were randomly assigned to a control group or a group administered 3.75% inulin in their diet for 4 d followed by 7.5% inulin for 26 d. Absorption of Zn67 and Cu65 was determined on d 21 of the experiment by fecal monitoring using Zn67 and Cu65 isotopes. Zn and Cu status was also assessed. Absorption of Zn67 and Cu65 was significantly lower in 11- and 21-mo-old rats than in 3- and 6 mo-old-rats. Moreover, inulin intake significantly increased Zn67 and Cu65 absorption. In conclusion, age and dietary inulin intake can significantly affect intestinal absorption of zinc and copper in rats. Further studies are required to explore this effect over longer periods of inulin intake and to test the effects of inulin in humans.

Dietary inulin intake and age can significantly affect intestinal absorption of calcium and magnesium in rats: a stable isotope approach

BACKGROUND

previous studies have shown that non-digestible inulin-type fructan intake can increase intestinal mineral absorption in both humans and animals. However, this stimulatory effect on intestinal absorption may depend on experimental conditions such as duration of fermentable fiber intake, mineral diet levels and animals' physiological status, in particular their age.

OBJECTIVES

the aim of this study was to determine the effect of inulin intake on Ca and Mg absorption in rats at different age stages.

METHODS

eighty male Wistar rats of four different ages (2, 5, 10 and 20 months) were randomized into either a control group or a group receiving 3.75% inulin in their diet for 4 days and then 7.5% inulin for three weeks. The animals were fed fresh food and water ad libitum for the duration of the experiment. Intestinal absorption of Ca and Mg was determined by fecal monitoring using stable isotopic tracers. Ca and Mg status was also assessed.

RESULTS

absorption of Ca and Mg was significantly lower in the aged rats (10 and 20 mo) than in the young and adult rat groups. As expected, inulin intake increased Ca and Mg absorption in all four rat groups. However, inulin had a numerically greater effect on Ca absorption in aged rats than in younger rats whereas its effect on Mg absorption remained similar across all four rat age groups.

CONCLUSION

the extent of the stimulatory effect of inulin on absorption of Ca may differ according to animal ages. Further studies are required to explore this effect over longer inulin intake periods, and to confirm these results in humans.

Stable isotopes in studies of intestinal absorption, exchangeable pools and mineral status: the example of magnesium

Magnesium (Mg) is a biologically essential mineral and Mg deficiency is known to lead to severe biochemical and symptomatic disorders. Radioactive isotopes and, more recently, stable isotopes have been used as research tools to determine intestinal Mg absorption in humans and animals under different nutritional and physiological conditions. Mg isotopes are given orally or orally plus intravenously and analysed in faeces and/or in plasma and urine in order to calculate intestinal Mg absorption and possibly endogenous Mg excretion. Mg isotopes have been used to assess exchangeable pools of Mg under nutritional and physiopathological conditions. Mg isotopes are given intravenously and are analysed in plasma and urine to calculate the size and half-life of the various Mg exchangeable pools. More recently, in vitro isotopic tests have been developed to study the need of cells for Mg in different nutritional and genetic conditions. Whole blood is incubated with Mg isotopes and isotopic blood cell enrichment is measured, which reflects the avidity of cells for Mg and thus its initial status. This paper is a report on the use of stable Mg isotopes and their advantages in these different fields of Mg absorption and metabolism. The studies available have clearly demonstrated that stable isotopes provide a useful research tool for determining intestinal Mg absorption, and represent a precious research tool for the study of Mg metabolism and the assessment of Mg status.

Magnesium metabolism in mice selected for high and low erythrocyte magnesium levels

A genetic control of blood magnesium (Mg) levels has been suggested. To investigate the mechanisms and the biologic significance of this genetic regulation, a mouse model, ie, mice selected for low magnesium level (MGL) and high magnesium level (MGH), was developed. The purpose of this study was to explore the Mg status and Mg metabolism in female MGL and MGH mice. We observed that MGL mice had reduced total and ionized plasma Mg, lower erythrocyte Mg, lower tibia, and kidney Mg levels. In contrast, total urinary Mg and (25)Mg levels were significantly higher in MGL mice. MGL mice had smaller total Mg exchangeable pool masses compared with MGH, and fractional transport rates of Mg (exchange constant) were different. In vitro (25)Mg enrichments in erythrocytes from MGL mice were significantly lower. Moreover, Mg efflux from erythrocytes was significantly higher in MGL. In conclusion, this work demonstrates that MGL mice present lower body stores of Mg than MGH mice and lower body Mg retention. This is confirmed at a cellular level by a lower enrichment of (25)Mg in erythrocytes. The lower retention of Mg by MGL erythrocyte in comparison to MGH appears to be partly due to a higher Mg efflux in MGL erythrocyte. It can be hypothesized that a genetic factor that modulates Na(+)/Mg(2+) exchanger activity may be important in the regulation of Mg metabolism. Further investigations on the mechanisms responsible for differences in Mg retention between MGL and MGH mice could contribute to a better understanding of the genetic regulation of cellular Mg.

Exchangeable magnesium pool masses in spontaneously hypertensive rats

We explored magnesium (Mg) metabolism by determination of exchangeable Mg pool masses and Mg kinetic parameters using stable Mg isotopes in spontaneously hypertensive rats (SHRs). Classical intracellular and extracellular Mg status biomarkers were also measured. Male SHRs and their male Wistar Kyoto (WKY) controls were fed a semipurified diet containing Mg 550 mg /kg for 2 weeks. Each rat received then an intravenous injection of 1.37 mg (25)Mg. The plasma (25)Mg disappearance curve over the next 7 days was used to measure the mass and fractional transport rate of 3 rapidly exchanging Mg metabolic pools, M1, M2, and M3. In the SHRs, plasma and erythrocyte Mg levels and urinary Mg excretion were not modified compared with their control WKYs, but tibia Mg level was significantly lower in the SHRs. Pool M3, the deep tissue pool, was significantly lower in SHRs compared with WKYs, but pools M1 and M2, the extracellular Mg pools, were statistically similar. The fractional transport rate of Mg from M1 to M2 and from M2 to M1 in the SHRs was higher than in the controls. The half-life of M1 was significantly decreased in SHRs compared with WKYs. In conclusion, this work demonstrates a decrease in intracellular Mg stores in SHRs compared with WKYs and disturbance of Mg exchanges in extracellular Mg, confirming a Mg metabolism disturbance in spontaneously hypertensive rats. Further work is now needed to elucidate the origin of the Mg depletion in SHRs and to explore Mg pools in hypertensive patients.

Comparison of stable-isotope-tracer methods for the determination of magnesium absorption in humans

BACKGROUND

The double-labeling (DL) method for determining magnesium absorption is less cumbersome than is the fecal monitoring method, which has been used most often, but it has not been validated.

OBJECTIVE

The aim of this study was to compare methods and several sampling protocols for determining magnesium absorption to establish a simple and reliable alternative to the fecal monitoring approach. Fecal monitoring was used as the standard against which the DL methods based on urine data (DLU), plasma data (DLP), and plasma kinetics with the use of a deconvolution analysis (DP) were compared.

DESIGN

Six healthy adult men received 70 mg (26)Mg orally and 30 mg (25)Mg intravenously. Multiple blood samples and complete urine and fecal samples were collected over 12 d. Stable-isotope ratios were determined by inductively coupled plasma mass spectrometry.

RESULTS

Results from DLU were not significantly different from the fecal monitoring reference value (0.48 +/- 0.05; +/- SD) when based on 3-d urine pools from 72 to 144 h (0.54 +/- 0.04) and when based on the 24-h urine pools from 48 to 72 h (0.49 +/- 0.06), 72 to 96 h (0.51 +/- 0.11), and 96 to 120 h (0.50 +/- 0.06). Results with the DLP method 72 h after isotope administration also compared well with those with the fecal monitoring method (0.54 +/- 0.09). Magnesium absorption was 0.47 +/- 0.06 with the DP method, which also agreed with the fecal monitoring value.

CONCLUSIONS

The DL methods are an alternative to fecal monitoring when applied within the appropriate time intervals. Therefore, DLU-the simplest and least invasive approach-is recommended for determining magnesium absorption.

A new in vitro blood load test using a magnesium stable isotope for assessment of magnesium status

Magnesium (Mg) status is currently assessed by various biochemical biomarkers, most of which, however, have some limitations. We developed an in vitro blood load test as a new Mg biomarker using a Mg stable isotope. This test is based on the hypothesis that cellular Mg uptake is increased in Mg deficiency. For this purpose, Wister male rats were fed either Mg-deficient or Mg-adequate diets for 1 mo and blood was sampled and incubated with the (25)Mg isotope (10 mg/L) for 2 h at 37 degrees C. Erythrocytes, lymphocytes and platelets were isolated and (25)Mg concentrations were determined by inductively coupled plasma/mass spectrometry. The feasibility of this approach was then tested on human blood. (25)Mg enrichments in erythrocytes, lymphocytes and platelets from Mg-deficient rats were greater than those from controls. (25)Mg enrichment was low in human erythrocytes (3%) compared with rat erythrocytes (38%), whereas high (25)Mg enrichments were obtained in human lymphocytes and platelets, suggesting that lymphocytes and platelets may be more appropriate cells than erythrocytes for examining Mg status in humans with this approach. More studies are required to validate the utilization of this test as a Mg status biomarker in humans.

Effect of short-chain fructooligosaccharides on intestinal calcium absorption and calcium status in postmenopausal women: a stable-isotope study

BACKGROUND

An enhancing effect of short-chain fructooligosaccharides (scFOSs) on intestinal calcium absorption has been shown in animals and in some short-term human studies. However, the long-term effect of scFOSs on calcium absorption in humans is still unknown.

OBJECTIVE

We investigated the long-term effect of a moderate daily dose (10 g) of scFOSs on intestinal calcium absorption in postmenopausal women.

DESIGN

In a randomized, double-blind crossover protocol, 12 healthy, postmenopausal women received scFOSs or placebo for 5 wk. The treatments were separated by a 3-wk washout period. Subjects orally received (44)Ca (stable isotope) and a fecal marker. Feces were collected after the isotope intake for 5-7 d to measure unabsorbed isotope. Calcium-status indexes, calciotropic hormones, and bone turnover were also assessed.

RESULTS

Mean (+/-SD) intestinal calcium absorption with scFOS treatment was not significantly different from that with placebo treatment (35.63 +/- 9.40% and 36.55 +/- 8.48%, respectively). However, a tendency for calcium absorption to be higher with scFOS treatment than with placebo treatment was observed in women who had been going through menopause for >6 y.

CONCLUSIONS

scFOSs do not modify intestinal calcium absorption in postmenopausal women who do not receive hormonal replacement therapy. The results from a subgroup of women who had been going through menopause for >6 y (n = 6) suggest that scFOSs may influence calcium absorption in the late postmenopausal phase. The small number of subjects and the related P value warrant verification and further investigation with women in late menopause only.

Fractional intestinal absorption of magnesium is directly proportional to dietary magnesium intake in rats

The precise mechanisms of intestinal Mg absorption are still unclear and the possibility of an adaptative rise in the fraction of Mg absorbed as Mg intake is lowered is controversial. Mg deficiency has been studied extensively in rats where it is readily produced by dietary depletion. In this study, we investigated the effect of Mg intake on fractional absorption of Mg acutely and after adaptation to graded Mg intake in rats. For this purpose, male Wistar rats (n = 30) were fed a basal semipurified diet containing 600 mg Mg/kg (MgO) for 7 d. Three groups of 10 rats were then formed and fed the basal semipurified diet with 600, 300 or 150 mg Mg/kg, for 28 d. Apparent and true intestinal absorptions and fecal endogenous excretion of Mg were determined at the beginning and end of the experiment. As expected, plasma Mg levels were lower in the deficient groups than in the control by d 4 and differences were more marked at the end of the experiment. Erythrocyte Mg levels were significantly lower at the end of the experiment in the group fed the diet containing 150 mg Mg/kg. The amounts of Mg absorbed were directly proportional to the dietary Mg intakes in all three experimental groups at both testing periods. This indicated that the apparent and true intestinal absorption percentages of Mg were not different in rats fed the three different levels of dietary Mg. These results also show that fecal endogenous excretion of Mg was nearly directly proportional to the dietary Mg intakes. These results argue in favor of a passive diffusional process for intestinal Mg absorption.

Exchangeable magnesium pool masses in healthy women: effects of magnesium supplementation

BACKGROUND

Studying magnesium pools in the body with use of stable isotopes may be helpful for evaluating magnesium status. Data on the evaluation of magnesium pools in humans are scarce.

OBJECTIVE

We undertook this study to evaluate the effects of a magnesium supplementation program on the size of the exchangeable body pools of magnesium and on classic indexes of magnesium status in healthy women with normal magnesium status.

DESIGN

Ten healthy women participated in a kinetic study with magnesium stable isotopes before and after 8 wk of magnesium supplementation. Each woman received 3 supplements containing 5.08 mmol (122 mg) elemental Mg/d (366 mg/d). Before and at the end of the supplementation period, each woman received an intravenous injection of 1.67 mmol (40 mg) (25)Mg, and the plasma magnesium disappearance curve was followed for the next 7 d. Two methods were used to analyze the exchangeable pools of magnesium: 1) formal multicompartmental modeling and 2) a simplified estimation of the total mass of the rapidly exchangeable magnesium pool (EMgP).

RESULTS

In these healthy women, exchangeable magnesium pools represented 11-12% of total body magnesium on the basis of multicompartmental analysis. The simplified estimation of EMgP overestimated the size of the exchangeable magnesium pools by approximately 45-50%. Eight weeks of magnesium supplementation did not significantly modify the size of the exchangeable magnesium pools, whereas urinary magnesium excretion was significantly higher after 8 wk of supplementation.

CONCLUSION

Women with no clinical evidence of magnesium deficiency may not respond to short-term supplementation with increases in the mass of the exchangeable magnesium body pool or in magnesium turnover rates.

Five-week intake of short-chain fructo-oligosaccharides increases intestinal absorption and status of magnesium in postmenopausal women

Fermentable carbohydrates have been shown to be nondigestible by human enzymes in the small intestine but are fermented extensively in the large bowel to short-chain fatty acids (SCFAs), which can increase mineral absorption. It has been shown that feeding such carbohydrates including short-chain fructo-oligosaccharides (sc-FOSs) increases intestinal magnesium (Mg) absorption in animals, but their beneficial impact on Mg absorption in humans still remains to be established. Therefore, this work aimed to investigate the effect of moderate daily doses of sc-FOSs (10 g/day) on the intestinal absorption and status of Mg in postmenopausal women without hormone replacement therapy (HRT). Eleven healthy postmenopausal women aged 59 +/- 6 years (mean +/- SD) received for 5 weeks sc-FOS or sucrose (placebo) treatments according to a randomized, double-blind, crossover design separated by a washout period of at least 3 weeks. Subjects ingested 87.5 mg of stable isotope 25Mg together with a fecal marker. Subsequently, feces were collected for 5-7 days. An inductively coupled plasma mass spectrometer (ICP/MS) was used for 25Mg stable isotope measurements in feces, urine, and blood. Mg levels were assessed also at the beginning and at the end of each treatment in plasma, erythrocytes, and urine. These measurements allowed for the determination of net intestinal Mg absorption and Mg status. The results show that the addition of 10 g sc-FOS to the diet increased Mg absorption by 12.3%, from 30.2 +/- 5.0% (placebo treatment) to 33.9 +/- 7.2% (sc-FOS treatment; mean +/- SD; p < 0.02). This increase in intestinal Mg absorption was accompanied by an increase in plasma 25Mg level and led to a higher urinary 25Mg excretion. This is the first time that such an effect is shown in humans. The overall conclusion of this work is that the ingestion of moderate doses of sc-FOS did improve intestinal Mg absorption and status in postmenopausal women. Because of the important role of Mg in many cellular functions, such Mg absorption improvement may be particularly interesting when the dietary intake of Mg is limited.

Effects of two fermentable carbohydrates (inulin and resistant starch) and their combination on calcium and magnesium balance in rats

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.

Exchangeable magnesium pool masses reflect the magnesium status of rats

A sensitive and valid marker to assess magnesium (Mg) status in humans is not available. The kinetically determined exchangeable pool masses have been used for other minerals, such as zinc and selenium, as markers of whole-body mineral status. To evaluate the validity of this relationship for Mg, we measured the exchangeable pools of Mg in rats over a range of magnesium dietary intakes. Rats weighing approximately 170 g were fed a control diet (500 mg Mg/kg), a marginally Mg-deficient diet (200 mg/kg) or a severely Mg-deficient diet (60 mg Mg/kg) for 2 wk. Subsequently, rats were administered an intravenous injection of (25)Mg, and the plasma (25)Mg disappearance curve was followed for the next 7 d. The following two methods were employed to analyze the exchangeable pools of Mg: 1) formal compartmental modeling and 2) a simplified determination of the total mass of the rapidly exchangeable Mg pool (EMgP). The mass of the three exchangeable pools (two extracellular pools and one intracellular pool) determined by compartmental analysis decreased in proportion to dietary Mg intake. EMgP, the combined pools of Mg that exchange with the plasma Mg within 48 h, decreased significantly as dietary Mg was lowered. It was positively correlated with conventional markers of Mg status (total Mg in plasma, erythrocyte and tibia Mg levels). Compartmental analysis assesses Mg exchangeable pools more accurately, but determination of EMgP is simpler and faster. Our findings demonstrate that the exchangeable pools of Mg constitute a good marker of Mg status in rats.

Long-term consumption of red wine does not modify intestinal absorption or status of zinc and copper in rats

Red wines contain many components such as polyphenols and ethanol that may influence mineral absorption. We report on studies in a rat model that were designed to investigate the extent to which short- and long-term intake of red wine or ethanol may influence (67)Zn and (65)Cu absorption in rats. Rats (n = 96) were divided into three groups, a control group that received demineralized water, a group that received red wine diluted with water (v/v) and an ethanol group that received 6% ethanol. Half of each group was used for the short-term study; the others were used for the long-term study. After 3 d (short-term study) or 28 d (long-term study) of beverage consumption, the rats were gavaged with 2 mL of solution containing 2027 nmol (67)Zn and 902 nmol (65)Cu. Subsequently, 3-d urinary and fecal collections were performed and analyzed for total and isotopic Zn and Cu. In the long-term study, blood, tibia and liver were also sampled for mineral status assessment. Neither short- nor long-term intake of red wine altered (67)Zn or (65)Cu absorption. In contrast, long-term (but not short-term) ethanol consumption significantly increased both (67)Zn and (65)Cu absorption compared with the control and red wine groups. The long-term consumption of ethanol or red wine did not affect blood or tissue Zn or Cu levels. In conclusion, short- or long-term consumption of red wine did not have a negative effect on intestinal absorption or tissue levels of zinc and Cu in rats.

Determination of epithelial magnesium transport with stable isotopes

The inability to adequately determine Mg(2+) flux rates with radiotracer studies has stymied our efforts to understand how magnesium is transported by epithelial cells. To evaluate epithelial Mg(2+) transport, a stable 25Mg isotope was used to measure magnesium uptake into normal and Mg(2+)-depleted Madin-Darby canine kidney (MDCK) cells. 25Mg entry rates were significantly increased in Mg(2+)-depleted cells relative to those cultured in normal magnesium media, 0.5 mM. 25Mg uptake was inhibited by external La(3+) but not Ca(2+) in both normal and Mg(2+)-depleted cells suggesting a specific entry pathway. These results with 25Mg were the same as with microfluorescence determinations using mag-fura-2. We have shown that Mg(2+) entry into epithelial cells reflects transepithelial transport; accordingly, increased Mg(2+) uptake in Mg(2+)-depleted cells provides an important intrinsic control of renal magnesium absorption. Furthermore, these studies indicate that cellular Mg(2+) transport may be quantitated with the use of stable isotopes that may be successfully applied to cells other than epithelia.

Methodologies for using stable isotopes to assess magnesium absorption and secretion in children

OBJECTIVE

To provide methodological information regarding the absorption and excretion of dietary magnesium by children and adolescents.

METHODS

Recently, stable isotope techniques for assessing magnesium absorption and excretion have been developed which allow for these studies to be safely performed in subjects of all ages. In the report, we describe the dosing and sample requirements for such studies.

RESULTS

Our data demonstrate that, after oral and intravenous dosing of isotopes, a complete 72-hour urine collection will allow for determination of fractional magnesium absorption. In our study, urinary, but not endogenous fecal magnesium excretion, was closely correlated with magnesium intake (r = 0.47, p = 0.02 vs r = 0.08, p = 0.69). As endogenous fecal magnesium excretion is small relative to urinary magnesium excretion, measurement of endogenous fecal magnesium excretion is not needed to make a reasonable estimate of net magnesium retention for most studies. Using high-precision analytical techniques, an intravenous dose of 25Mg of approximately 0.2 to 0.3 mg/kg would be adequate for absorption measurements.

CONCLUSIONS

The cost and availability of isotopes and their analysis are such that it should be feasible for increasing numbers of investigators to make use of these techniques.

Short-term ingestion of chlorogenic or caffeic acids decreases zinc but not copper absorption in rats, utilization of stable isotopes and inductively-coupled plasma mass spectrometry technique

The amount of dietary trace elements absorbed from a meal depends, among other factors, on the quantities of certain minor plant constituents present in the meal. These substances can act as ligands and bind trace elements in the digestive tract in available or unavailable forms for absorption. The present study was designed to investigate the extent to which different polyphenols (PP) may influence Zn and Cu absorption in rats. Different PP of nutritional interest (chlorogenic acid, caffeic acid, catechin and rutin) were studied using meals extrinsically-labelled with stable isotopes 67Zn and 65Cu. Male Wistar rats were fed on a non-labelled semi-synthetic diet containing (mg/kg) 38 Fe, 35 Zn and 7.5 Cu for 8 d. PP were dissolved in dimethyl sulfoxide as the solvent and added to the meal at 1 g/kg during 3 d before isotope administration and until the end of the experiment (a further 3 d). The control group received the dimethyl sulfoxide only. After overnight food deprivation, rats were fed on the labelled test meals (4 g diet +0.1 mg 67Zn and 0.1 mg 65Cu) with 0.5 mg Dy as a faecal marker. Faeces and urine pools were collected for 3 d and analysed for 67Zn and 65Cu isotopic enrichment using the inductively-coupled plasma mass spectrometry technique. Zn absorption was significantly less in rats fed on chlorogenic acid or caffeic acid than in the control group. Catechin ingestion non-significantly inhibited 67Zn absorption. However, the PP studied were without effect on Cu absorption. The study illustrates the effect of metal-binding phenolic compounds on mineral nutrition in the rat, and the possible importance of the effects of different foods rich in these compounds on mineral absorption in man.