ARTICLE
Auteur(s) : C
Coudray, M Rambeau, C Feillet-Coudray, E Gueux, JC Tressol, A
Mazur, Y Rayssiguier
Centre de Recherche en Nutrition Humaine d’Auvergne, Laboratoire
des Maladies Métaboliques et Micronutriments, INRA de
Theix/Clermont-Ferrand, 63122 Saint-Genès-Champanelle, France
Magnesium deficit may occur because of a decrease in intake or
absorption, internal redistribution, or increased loss of this
element through either renal or non-renal routes. Magnesium
deficiency is not uncommon among the general population: its intake
has decreased over the years especially in the western world [1,
2]. As Mg plays an essential role in a wide range of fundamental
biological processes, it is not surprising that Mg deficiency may
lead to serious biochemical and symptomatic changes [3-5].
Increasing the consumption of vegetables and cereal products
contributes to improved Mg intake but Mg supplementation may be
indicated when a specific health problem or condition causes an
excessive loss of Mg or limits Mg absorption [6, 7].Net Mg
absorption results from dietary Mg absorption and Mg secretion into
the intestinal tract via bile, gastric and pancreatic juices. In
the healthy adult, 30 to 50% of dietary Mg is absorbed [8-10]. The
secreted Mg is efficiently reabsorbed and endogenous faecal losses
are only 20 to 50 mg/d. Mg absorption occurs along the entire
intestinal tract but the distal small intestine (jejunum and ileum)
are the primary sites. It is essentially a passive intercellular
process mediated by electrochemical gradients and solvent drag, and
active transport occurs only for extremely low dietary Mg intake
and its regulation is unknown [11]. Mg uptake in the brush border
may be mediated by a Mg/anion complex and Mg efflux across the
basolateral membrane may involve Na/Mg antiport systems [12]. A
gene implicated in Mg deficit in humans has been identified. It is
expressed in the intestine and kidneys and appears to encode for a
protein that combines Ca- and Mg-permeable channel properties with
protein kinase activity [13]. This gene might be implicated in Mg
absorption. Because of the importance of the passive process, the
quantity of Mg in the digestive tract is the major factor
controlling the amount of Mg absorbed.Literature data on the
bioavailability of various Mg forms provide scarce information on
the best Mg salt to be used in animal and human Mg supplementation.
Some human studies have attempted to investigate the
bioavailability of different Mg salts, but these studies are based
on two or three Mg preparations and were therefore unable to
clearly establish which Mg salt possesses the best bioavailability.
Moreover, these studies did not directly measure Mg absorption but
often simply measured 24-hour urinary Mg excretion or plasma Mg as
a marker of Mg bioavailability [14-16]. Measurement of intestinal
Mg absorption is the only direct approach for comparing
bioavailability between different Mg salts. The conventional
balance study suffers from the methodological problems inherent to
all balance studies [17], such as imprecision in Mg intake,
environmental contamination and endogenous Mg excretion. The
development of inductively coupled plasma mass spectrometry
(ICP-MS) has provided the possibility of a new and reliable method
for conducting Mg bioavailability studies [18-21]. This technique
can be usefully applied to compare intestinal Mg absorption between
a large number of Mg salts. Accordingly, we investigated the
bioavailability of Mg from different organic and inorganic Mg salts
using a single-labelling technique with Mg stable isotope in rats.
Materials and methods
Materials and reagents
Enriched Mg isotopes (26Mg) as MgO were obtained from
Chemgas, (Boulogne, France). HNO3 (ultrapure), Mg and
beryllium standard solutions (1 g/L) were obtained from Merck
(Darmstadt, Germany). All other chemicals were of the highest
quality available. Distilled water was used throughout. A
Perkin-Elmer 6100DRC system (Perkin-Elmer Instruments, Courteboeuf,
France) with a Meinhard nebulizer was used for isotopic
measurement, and a Perkin Elmer 560 (Perkin Elmer Instruments,
Courteboeuf, France) was used for total Mg measurement.
Animals and diet
Eighty male Wistar rats aged about 6 weeks and weighing 150 g were
used in the present study. They were derived from the colony of
laboratory animals of the National Institute of Agronomic Research
(INRA) of Clermont-Ferrand/Theix, France. The rats were housed
under constant temperature conditions (20-22° C), constant
humidity (45-50%) and a standard dark cycle (20:00 to 08:00). They
received human care in compliance with European Community
guidelines for the use of experimental animals (L358-86/609/EEC).
At the beginning of the experiment (d0), blood samples were
obtained from 20 animals by orbital sinus puncture to evaluate
baseline Mg status in these rats. The animals received ad libitum a
semi-purified diet that contained (g/kg) [22]: casein, 200; starch,
650; corn oil, 50; fibre, 50; AIN-93 mineral mixture (1993) 35;
including magnesium oxide (MgO) as necessary, AIN-93A vitamin
mixture (1993), 10; DL-methionine, 3; and choline bitartrate, 2. In
a first three-week stage, all 80 animals received this diet
containing only 150 mg Mg/kg, to induce Mg deficiency. Then,
blood samples were again taken from 20 animals by orbital vein
puncture to evaluate Mg status after the period of insufficient Mg
intake. The rats were then randomised into 10 groups (8 rats/group)
and for two weeks received a semi-purified diet containing
sufficient Mg levels but in different salt forms: group 1 received
Mg oxide, group 2 received Mg chloride; group 3 received
Mg sulphate, group 4 received Mg carbonate, group 5 received
Mg acetate, group 6 received Mg pidolate, group
7 received Mg citrate, group 8 received Mg gluconate,
group 9 received Mg lactate, and group 10 received Mg
aspartate. The target Mg level in these diets was 600 mg Mg/Kg
diet. Powder diet (100 g) was made up with 100 mL of distilled
water to form a kind of semi-liquid food prepared on site.
Stable isotope administration and biological sample
collection
Two hundred mg of the enriched 26Mg (in oxide form =
323 mg) were first moistened with two mL of distilled water,
and then two mL of 12 N HCl (ultrapure) were added to transform the
oxide into the soluble chloride of Mg. The solution was then
diluted with 120 mL of distilled water and adjusted to pH6
with powdered sodium bicarbonate. The isotopic analysis of
26Mg solution yielded the following atomic percentages:
24Mg = 2.33%, 25Mg = 1.28%, 26Mg =
96.38%, and gave 26Mg concentration as 1.51 mg/ml.
Animals received by gavage 1.2 mL of this solution one week
after the consumption of the different Mg salts (about 1.8 mg
of 26Mg per rat). The urine and faeces of each rat were
quantitatively collected for four consecutive days. Urine volume
was determined and 5 mL urine were sampled, acidified with 50
μl of concentrated HNO3 (12 N) and frozen until
analysis. The faeces were freeze-dried, powdered and kept at room
temperature until analysis. The rats were sacrificed at the end of
the experiment and the blood and femur were sampled for Mg
analysis. The gastrointestinal tract was also sampled and divided
into three parts corresponding to the small, large intestine and to
the cecum. Red blood cells were separated from plasma by
centrifugation, washed twice with saline solution and then lysed
into ten volumes of distilled water.
Total Mg and isotopic 26Mg analysis
Lysed red blood, faeces, femur and intestine contents were dried
and ashed at 500°C for 10 hours and the ash was dissolved in
0.2 mL of concentrated HNO3 and 9.2 mL of
distilled water. For total Mg determination, samples were diluted
appropriately with 0.1% lanthanum chloride and Mg level was
determined by flame atomic absorption spectrometry (Perkin Elmer
560, Courteboeuf, France). For isotopic Mg determination, samples
were appropriately diluted before analysis using 1%
HNO3. Mg concentration and isotope ratios were
determined by ICP-MS (Perkin-Elmer 6100DRC system) using Mg and
beryllium as external and internal standards, respectively. The
following instrument operating conditions were set after
optimization with a solution of 1μg indium/l (RF Power: 1050 W,
Nebulizer Ar flow rate: 0.79 L/min, Auxiliary Ar flow rate:
1.2 L/min, Outer Ar flow rate: 15 L/min). Data
acquisition conditions were as follows (Sweeps/reading: 50,
Readings/replicate: 1, Number of replicates: 3, Dwell time: 100ms,
Scanning mode: peak hopping).
Calculations
Mg has three stable isotopes having the following percentage
natural abundance: 24Mg (78.90%), 25Mg
(10.00%) and 26Mg (11.10%) [23]. Isotopic enrichment
percentages were obtained from the following equation = 100 x
((26Mg/24Mg measured ratio -
26Mg/24Mg baseline ratio)/
(26Mg/24Mg baseline ratio)), where isotopic
natural baseline ratio was calculated as follows:
26Mg/24Mg = 0.1407.
Non-absorbed 26Mg isotope present in the faecal
sample (coming only from the 26Mg isotope label) was
calculated as follows = (total faecal Mg X
(IR26Mg/24Mg sample -
IR26Mg/24Mg baseline)) / (1.267 +
(IR26Mg/24Mg sample -
IR26Mg/24Mg baseline)), where IR was isotopic
ratio, total faecal Mg (mg) was determined by atomic absorption
spectrometry, and 1.267 is the sum of 1/0.789 converting
24Mg faecal quantity to total faecal Mg. The calculation
could also be made directly from ICP-MS data. The two modes of
calculation give the same results when the ICP-MS quantitative
procedure is used [24].
Statistical analysis
Results were expressed as means (SD). Statistical analysis were
based on one-way ANOVA followed by a Student-Newman-Keuls test for
parametric variables and a Kruskal-Wallis test for non-parametric
variables. The limit of statistical significance was set at p <
0.05. Statistical analyses were performed using GraphPad software
(V3.00, GraphPad Software, San Diego, CA).
Results
Effect of Mg-deficient diet on Mg status
Mg status was evaluated at the beginning of the experiment and
after 3 weeks on the Mg-deficient diet. The results showed normal
Mg status at the beginning of the experiment with plasma Mg level
at 16.3 ± 1.0 mg/L and Mg red blood cell level at
45.9 ± 3.5 mg/L. Blood Mg levels (n = 20) after three weeks of
Mg-deficient diet decreased significantly to 7.8 ± 1.2 mg/L in
plasma (p < 0.0001) and to 42.5 ± 3.2 mg/L in red blood
cells (p < 0.05). The inter- and intra-variability of total Mg
and Mg isotope measurements have been previously reported [24].
Effect of intake of different Mg salts on rat body weight
Rat body weight was monitored twice per week throughout the
duration of the study. There was no significant difference in the
body weight evolution between the ten different groups at any point
in the experiment (table 1( Table 1
)).
Table 1 Effects of a two-week intake of organic or
inorganic Mg salts on rat growth rate.
|
Inorganic Mg salts
|
Organic Mg salts
|
p
|
|
MgO
|
MgCl2
|
MgSO4
|
MgCO3
|
Acetate
|
Pidolate
|
Citrate
|
Gluconate
|
Lactate
|
Aspartate
|
|
Body weight (g)
|
|
|
|
|
|
|
|
|
|
|
|
At D 0
|
146 ± 5
|
148 ± 7
|
150 ± 5
|
152 ± 6
|
150 ± 11
|
149 ± 7
|
149 ± 11
|
146 ± 8
|
147 ± 9
|
147 ± 7
|
NS
|
|
At D 4
|
164 ± 6
|
167 ± 10
|
170 ± 6
|
169 ± 8
|
169 ± 11
|
165 ± 8
|
167 ± 10
|
167 ± 9
|
168 ± 11
|
165 ± 7
|
NS
|
|
At D 11
|
221 ± 11
|
219 ± 14
|
222 ± 9
|
221 ± 13
|
228 ± 15
|
219 ± 12
|
219 ± 14
|
217 ± 14
|
225 ± 14
|
214 ± 11
|
NS
|
|
At D 18
|
271 ± 10
|
268 ± 19
|
271 ± 12
|
270 ± 18
|
278 ± 15
|
268 ± 17
|
267 ± 22
|
265 ± 18
|
276 ± 18
|
259 ± 14
|
NS
|
|
At D 25
|
311 ± 12
|
312 ± 23
|
313 ± 14
|
308 ± 21
|
320 ± 19
|
308 ± 21
|
310 ± 29
|
308 ± 22
|
321 ± 21
|
297 ± 18
|
NS
|
|
At D 28
|
320 ± 14
|
317 ± 29
|
323 ± 13
|
319 ± 24
|
331 ± 22
|
321 ± 22
|
319 ± 28
|
318 ± 25
|
332 ± 20
|
310 ± 19
|
NS
|
|
At D 32
|
334 ± 18
|
335 ± 28
|
340 ± 15
|
333 ± 26
|
345 ± 23
|
334 ± 23
|
334 ± 29
|
338 ± 28
|
350 ± 22
|
324 ± 21
|
NS
|
|
At D 36
|
355 ± 21
|
360 ± 32
|
365 ± 18
|
359 ± 30
|
371 ± 26
|
360 ± 28
|
357 ± 32
|
357 ± 30
|
377 ± 26
|
345 ± 23
|
NS
|
Effect of intake of different Mg salts on intestinal Mg
solubility
The pH content of different fragments of intestine and caecum
increased gradually from the proximal intestine (around pH 5.8) to
the distal intestine (around pH 6.5), and reached around 6.8 in the
caecum in the different groups. There was no significant difference
in intestine and caecum pH between the different groups (table 2(
Table 2 )). In contrast, Mg solubility
decreased gradually from the proximal intestine (85%) to the distal
intestine (50%) down to about 40% in the caecum in all the groups.
There was no significant between-group difference in intestinal or
caecal Mg solubility (table 2).
Table 2 Effects of two-week intake of organic or
inorganic Mg salts on intestinal pH and intestinal Mg solubility in
rats.
|
Inorganic Mg salts
|
Organic Mg salts
|
p
|
|
MgO
|
MgCl2
|
MgSO4
|
MgCO3
|
Acetate
|
Pidolate
|
Citrate
|
Gluconate
|
Lactate
|
Aspartate
|
|
Small intestine
|
|
pH
|
5.78 ± 0.25
|
5.72 ± 0.11
|
6.00 ± 0.26
|
5.68 ± 0.23
|
5.81 ± 0.24
|
5.70 ± 0.14
|
5.82 ± 0.27
|
6.00 ± 0.38
|
5.70 ± 0.24
|
5.92 ± 0.44
|
NS
|
|
Mg solubility, %
|
98.9 ± 6.1
|
86.8 ± 10.6
|
97.9 ± 19.1
|
87.5 ± 12.7
|
85.8 ± 11.8
|
89.9 ± 10.1
|
80.6 ± 10.5
|
89.4 ± 20.1
|
90.6 ± 10.7
|
83.0 ± 10.3
|
NS
|
|
Large intestine
|
|
pH
|
6.60 ± 0.19
|
6.48 ± 0.19
|
6.58 ± 0.15
|
6.37 ± 0.51
|
6.44 ± 0.89
|
6.68 ± 0.17
|
6.72 ± 0.40
|
6.54 ± 0.32
|
6.54 ± 0.18
|
6.56 ± 0.34
|
NS
|
|
Mg solubility, %
|
61.5 ± 14.2
|
53.0 ± 17.9
|
46.7 ± 8.6
|
52.1 ± 15.5
|
56.2 ± 21.4
|
43.8 ± 7.2
|
48.7 ± 11.2
|
48.7 ± 17.9
|
52.2 ± 14.1
|
56.1 ± 12.3
|
NS
|
|
Caecum
|
|
pH
|
6.89 ± 0.55
|
6.67 ± 0.54
|
6.99 ± 0.30
|
6.45 ± 0.16
|
6.55 ± 0.20
|
7.01 ± 0.27
|
6.72 ± 0.55
|
6.78 ± 0.54
|
6.80 ± 0.41
|
6.61 ± 0.27
|
NS
|
|
Mg solubility, %
|
42.3 ± 7.5
|
43.2 ± 9.8
|
39.2 ± 4.7
|
46.1 ± 13.2
|
52.0 ± 11.0
|
50.1 ± 13.0
|
47.5 ± 12.6
|
49.9 ± 13.1
|
43.9 ± 4.0
|
43.3 ± 9.9
|
NS
|
Intake effects of different Mg salts on intestinal Mg
absorption (tables 3 and 4)
( Table 3 )( Table
4 )With both classical and isotopic approaches, the
intestinal Mg absorption from the different tested salts seemed
adequate and exceeded 35% of ingested Mg. However, the results from
the classical approach showed that Mg absorption seemed better from
the organic Mg salts (+ 13%) than from the inorganic Mg salts
and in particular from gluconate salt. This last salt exhibited the
best bioavailability with an intestinal Mg absorption of 56%
accompanied with very high urinary Mg excretion, proving the higher
intestinal absorption of Mg from this organic salt.
With regard to the isotopic approach, faecal excretion of
26Mg varied between 0.62 and 0.89 mg, i.e. between
35 and 50% of 26Mg given per group. The highest
excretion of 26Mg was observed in the rats receiving
MgSO4 and MgCO3, whereas the lowest excretion
of 26Mg was observed in the rats receiving Mg gluconate
and Mg aspartate. There was a significant difference in
26Mg excretion between rats receiving the Mg gluconate
and rats receiving MgO, MgCl2, MgSO4,
MgCO3 and Mg acetate. Thus, 26Mg absorption
varied between 0.92 and 1.22 mg. The lowest absorption of
26Mg was observed in the rats receiving MgSO4 and MgCO3,
whereas the highest absorption of 26Mg was observed in
the rats receiving the organic Mg salts, in particular Mg
gluconate. There was a significant difference in 26Mg
absorption between rats receiving the Mg gluconate and rats
receiving MgO, MgCl2, MgSO4, MgCO3
and Mg acetate. Urinary 26Mg excretion values confirmed
the absorption data. Urinary 26Mg varied between 0.20
and 0.33 mg. The lowest urinary 26Mg excretion was
observed in the rats receiving the four inorganic Mg salts, Mg
aspartate and Mg lactate, whereas the highest urinary
26Mg excretion was observed in the rats receiving the
organic Mg salts, in particular Mg gluconate and Mg pidolate. There
was a significant difference in urinary 26Mg excretion
in the rats receiving Mg gluconate compared to all the other rats.
Finally, 26Mg retention varied between 0.71 and
0.90 mg. The lowest 26Mg retention was observed in
the rats receiving MgSO4 and MgCO3, whereas
the highest 26Mg retention was observed in the rats
receiving the organic Mg salts, in particular Mg gluconate and Mg
lactate. 26Mg retention in the rats receiving Mg
gluconate and Mg lactate only differed significantly from
26Mg excretion in the rats receiving
MgSO4.
Table 3 Effects of a two-week intake of organic or
inorganic Mg salts on intestinal absorption, urinary excretion, and
retention of Mg in rats.
|
Inorganic Mg salts
|
Organic Mg salts
|
p
|
|
MgO
|
MgCl2
|
MgSO4
|
MgCO3
|
Acetate
|
Pidolate
|
Citrate
|
Gluconate
|
Lactate
|
Aspartate
|
|
Food intake, g/d
|
19.8 ± 2.1
|
21.1 ± 1.5
|
20.4 ± 1.8
|
20.8 ± 2.0
|
20.6 ± 2.5
|
20.7 ± 1.5
|
20.5 ± 1.8
|
21.4 ± 3.0
|
21.9 ± 1.6
|
20.4 ± 1.5
|
NS
|
|
Mg intake, mg/d
|
12.6 ± 1.3abc
|
14.1 ± 1.0c
|
12.0 ± 1.1ab
|
12.9 ± 1.3abc
|
13.1 ± 1.7bc
|
13.1 ± 0.9abc
|
12.7 ± 1.1abc
|
12.1 ± 1.7ab
|
11.5 ± 0.8a
|
12.9 ± 0.9abc
|
0.0015
|
|
Fecal Mg, mg/d
|
6.53 ± 1.11b
|
7.23 ± 1.28b
|
7.91 ± 1.88ab
|
7.25 ± 0.76b
|
6.92 ± 0.94b
|
6.80 ± 1.31ab
|
6.35 ± 1.15ab
|
5.27 ± 1.44a
|
6.02 ± 0.72ab
|
6.86 ± 0.65ab
|
0.0024
|
|
Mg absorption, mg/d
|
6.07 ± 0.65b
|
6.90 ± 1.31b
|
4.14 ± 1.20a
|
5.66 ± 0.73b
|
6.18 ± 1.09b
|
6.28 ± 1.25b
|
6.39 ± 1.20b
|
6.83 ± 1.19b
|
5.49 ± 0.35b
|
6.05 ± 0.95b
|
< 0.0001
|
|
Mg absorption,%
|
48.4 ± 4.5bc
|
48.8 ± 8.8bc
|
34.8 ± 10.8a
|
43.8 ± 3.2b
|
47.2 ± 4.3cb
|
48.0 ± 9.4bc
|
50.1 ± 8.2bc
|
56.8 ± 8.9c
|
47.9 ± 3.3bc
|
46.8 ± 5.3bc
|
< 0.0001
|
|
urinary Mg, mg/d
|
1.80 ± 0.86a
|
1.90 ± 0.33ab
|
1.92 ± 0.44ab
|
2.80 ± 0.53c
|
2.38 ± 0.24abc
|
2.09 ± 0.46abc
|
2.02 ± 0.68ab
|
2.48 ± 0.27abc
|
2.39 ± 0.27abc
|
2.64 ± 0.53bc
|
0.0007
|
|
Mg retention, mg/d
|
4.27 ± 0.85bc
|
5.00 ± 1.26c
|
2.22 ± 1.02a
|
2.86 ± 0.93a
|
4.17 ± 1.01bc
|
4.19 ± 1.49bc
|
4.37 ± 1.56bc
|
4.35 ± 1.30c
|
3.11 ± 0.46ab
|
3.41 ± 0.93ab
|
< 0.0001
|
|
Mg retention,%
|
33.8 ± 5.6bc
|
35.3 ± 8.1c
|
18.6 ± 8.6a
|
21.9 ± 6.2ba
|
29.8 ± 5.1b
|
32.0 ± 11.7bc
|
33.9 ± 11.4bc
|
36.0 ± 9.6c
|
26.9 ± 3.0ab
|
26.2 ± 5.7ab
|
0.0001
|
Table 4 Effects of a two-week intake of organic or
inorganic Mg salts on intestinal absorption, urinary excretion, and
retention of 26Mg in rats.
|
Inorganic Mg salts
|
Organic Mg salts
|
p
|
|
MgO
|
MgCl2
|
MgSO4
|
MgCO3
|
Acetate
|
Pidolate
|
Citrate
|
Gluconate
|
Lactate
|
Aspartate
|
|
26Mg gavaged, mg
|
1.81 ± 0.04
|
1.83 ± 0.03
|
1.81 ± 0.05
|
1.83 ± 0.02
|
1.81 ± 0.04
|
1.85 ± 0.10
|
1.82 ± 0.02
|
1.84 ± 0.03
|
1.86 ± 0.11
|
1.81 ± 0.04
|
NS
|
|
26Mg faecal mg
|
0.78 ± 0.13ab
|
0.83 ± 0.13b
|
0.89 ± 0.14b
|
0.88 ± 0.08b
|
0.81 ± 0.06b
|
0.74 ± 0.10ab
|
0.77 ± 0.14ab
|
0.62 ± 0.13a
|
0.76 ± 0.12ab
|
0.72 ± 0.13ab
|
0.0009
|
|
26Mg absorbed, mg
|
1.03 ± 0.12a
|
1.00 ± 0.13a
|
0.92 ± 0.14a
|
0.95 ± 0.09a
|
1.00 ± 0.09a
|
1.10 ± 0.12ab
|
1.05 ± 0.15ab
|
1.22 ± 0.13b
|
1.10 ± 0.13ab
|
1.09 ± 0.14ab
|
0.0007
|
|
26Mg absorbed,%
|
57.0 ± 7.0a
|
54.8 ± 7.1a
|
51.0 ± 7.5a
|
52.0 ± 4.6a
|
55.1 ± 4.1a
|
59.7 ± 7.7ab
|
57.6 ± 7.7ab
|
66.5 ± 7.1b
|
59.0 ± 6.1ab
|
60.4 ± 7.5ab
|
0.0006
|
|
26Mg urine, mg
|
0.22 ± 0.05a
|
0.22 ± 0.06a
|
0.21 ± 0.05a
|
0.20 ± 0.03a
|
0.22 ± 0.04a
|
0.27 ± 0.06a
|
0.24 ± 0.07a
|
0.33 ± 0.05b
|
0.20 ± 0.04a
|
0.25 ± 0.06a
|
< 0.0001
|
|
26Mg retention, mg
|
0.81 ± 0.10ab
|
0.78 ± 0.09ab
|
0.71 ± 0.12a
|
0.75 ± 0.09ab
|
0.78 ± 0.09ab
|
0.83 ± 0.14ab
|
0.81 ± 0.11ab
|
0.89 ± 0.10b
|
0.90 ± 0.13b
|
0.85 ± 0.11ab
|
0.0239
|
|
26Mg retention,%
|
44.8 ± 5.9ab
|
42.8 ± 0.51ab
|
39.5 ± 6.8a
|
41.0 ± 4.6ab
|
42.9 ± 4.0ab
|
45.0 ± 5.8ab
|
44.7 ± 5.6ab
|
48.7 ± 5.4b
|
48.0 ± 5.5b
|
46.9 ± 5.8ab
|
0.0240
|
Effect of intake of different Mg salts on Mg status
Mg status was monitored at the end of the study by measuring Mg
levels in plasma, red blood cells and bone (femur). There was no
significant difference in the three Mg status parameters between
the different groups studied in this experiment (table 5( Table 5 )).
Table 5 Effects of a two-week intake of organic or
inorganic Mg salts on Mg status parameters.
|
Inorganic Mg salts
|
Organic Mg salts
|
p
|
|
MgO
|
MgCl2
|
MgSO4
|
MgCO3
|
Acetate
|
Pidolate
|
Citrate
|
Gluconate
|
Lactate
|
Aspartate
|
|
Plasma Mg, mg/L
|
18.0 ± 1.2
|
18.3 ± 0.7
|
17.9 ± 0.7
|
17.4 ± 1.2
|
17.2 ± 0.7
|
17.9 ± 0.4
|
17.2 ± 0.9
|
17.5 ± 0.8
|
17.1 ± 1.46
|
17.3 ± 0.9
|
NS
|
|
RBC Mg, mg/L
|
45.3 ± 4.0
|
45.0 ± 5.5
|
44.9 ± 4.0
|
45.5 ± 3.2
|
46.1 ± 5.8
|
44.3 ± 4.1
|
44.6 ± 3.0
|
46.7 ± 2.4
|
45.1 ± 6.7
|
43.4 ± 4.4
|
NS
|
|
Bone Mg, mg/g
|
3.11 ± 0.13
|
3.13 ± 0.31
|
3.14 ± 0.23
|
3.17 ± 0.24
|
3.04 ± 0.14
|
3.08 ± 0.14
|
3.06 ± 0.17
|
2.98 ± 0.13
|
2.96 ± 0.25
|
3.00 ± 0.28
|
NS
|
Discussion
Mg absorption and bioavailability depend on a variety of factors,
including the Mg salt form. Literature data on the bioavailability
of various Mg forms provide scarce information on the best Mg salt
to be used in animal and human supplementation. Indeed, in the few
human studies published, only two or three Mg salts were compared
in terms of their bioavailability. None of these studies directly
measured intestinal Mg absorption of these salts, but simply
measured 24-hour urinary Mg excretion or plasma Mg levels [14-16,
25]. However, given the variety of Mg preparations
examined, the available studies do not clearly identify the best Mg
preparation to be used for supplementation. This is understandable
because it is very difficult to examine all these salts in humans
in one single study. Moreover, surprisingly, there are very few, if
any, animal studies investigating the bioavailability of different
Mg salts. Cook [26] investigated in 1973 the availability of
various inorganic magnesium salts in rats using balance studies.
These included carbonate, chloride, oxide, phosphate, sulfate and
silicate. They concluded that Mg from chloride and carbonate was
slightly more available than from the other four salts. We,
therefore, conducted the present study to investigate the
bioavailability of ten different forms of organic and inorganic
salts of Mg in rats using Mg stable isotope.
We applied a robust technique using Mg stable isotope to
determine intestinal Mg absorption and urinary excretion. This
approach allows much greater precision than in the conventional
balance technique where many methodological problems have been
reported [17]. In the isotopic approach, the amount of isotope
administered in the whole animal is well controlled, with minimal
repercussions resulting from any environmental contamination during
sample collection and preparation or Mg measurement. Moreover,
using stable isotope allowed us to work with “true” intestinal
absorption because endogenous isotope excretion is considerably low
in comparison to total Mg endogenous excretion in the faeces [27].
Our results clearly showed that Mg organic salts are better sources
of Mg than Mg inorganic salts, and suggest that Mg gluconate is the
best source of Mg because it exhibited the highest Mg absorption
and Mg retention values in the ten studied groups (66.5% and 48.7%,
respectively). It is important to stress that Mg inorganic salts
remain a good source of Mg because the absorption and the retention
of Mg observed with these salts were perfectly acceptable (more
than 50% and 39%, respectively). Although human and rat have some
differences in intestinal physiology, these results may be
extrapolated to human Mg nutrition with the necessary precautions.
These results are in agreement with a study in adult humans by
Walker’s team [14], who reported that Mg is more bioavailable from
Mg citrate than from Mg oxide. Firoz & Graber [25] have
determined Mg bioavailability in four commercial Mg preparations -
Mg oxide, Mg chloride, Mg lactate and Mg aspartate - in human
subjects using urinary Mg excretion. They concluded a relatively
poor bioavailability of Mg oxide but greater and equivalent
bioavailability of the other three Mg salts. Lindberg [28] compared
Mg citrate and Mg oxide with respect to in vitro solubility and in
vivo gastrointestinal absorbability in normal healthy volunteers.
He concluded that Mg citrate had better solubility and
bioavailability than Mg oxide. In an old study, based on 24-h
urinary Mg excretion, Morris [15] reported that Mg was absorbed to
a limited extent in healthy adults following administration of Mg
sulfate. More recently, the influence of three different salts at
different concentrations on Mg absorption in the rat small
intestine using the area under the curve as end-point of Mg
bioavailability has been studied [29], where Mg absorption was
shown to be most efficient from gluconate compared to fumarate or
chloride forms.
The solubility of minerals in the intestinal tract is an
essential factor in their absorption. In previous studies we showed
that administration of fermentable fibers which decreased cecal pH
content increase considerably Mg absorption in human and animals
[30]. That is why we examined Mg solubility in the different
segments of rat intestine. The values of pH and Mg solubility of
intestinal contents confirmed increasing pH and decreasing Mg
solubility throughout the intestine from the proximal part to the
distal part of intestine and then to the caecum. However, the
administration of different organic and inorganic Mg salts did not
result in any significant differences in these intestinal and
caecum parameters. This may explain why there is no major
difference in intestinal Mg absorption between the different Mg
salts investigated in this study. Given that the main intestinal
site of Mg absorption is the ileum in humans and in particular the
caecum in the rat, pH content and Mg solubility in the caecum are
more significant indicators than in the proximal and the distal
parts of intestine. Again, we observed higher caecal Mg solubility,
even if not significantly so, in the rats receiving principally the
Mg organic salts, in particular acetate, pidolate, citrate and
gluconate. This may also partially explain why intestinal Mg
absorption was slightly higher with the organic rather than the
inorganic Mg salts in this study.
Mg levels in biological samples such as plasma, erythrocyte or
urine have been used as markers of Mg bioavailability in several
studies, although biological variations in Mg levels in these media
in apparently healthy humans are very large [31]. So, we evaluated
the efficiency of the ten Mg organic and inorganic salts in
restoring Mg status. The rats presented a low Mg status at the
beginning of Mg salt administration. Two weeks later, Mg status was
efficiently restored in the different groups. Globally, mean plasma
Mg levels in the rats receiving the four inorganic salts and the
six organic salts were 17.9 mg/L and 17.4 mg/L,
respectively, and mean erythrocyte Mg levels were 45.2 mg/L
and 45.0 mg/L, respectively, whereas mean bone Mg levels were
3.14 mg/g dry weight and 3.02 mg/g dry weight of
inorganic and organic Mg salts, respectively. Indeed, the
statistical analysis failed to show any trend or significant
difference between the ten groups for the three Mg status
parameters measured in this study, i.e. plasma Mg, erythrocyte Mg
and bone Mg levels. From many previous studies, it has been shown
that intestinal Mg absorption was generally proportional to the
dietary Mg intake [32]. This may explain why the various Mg forms
studied all showed the same efficacy in overcoming marginal Mg
deficiency in our model. In agreement with our results, Cook [26]
showed that all of the inorganic Mg salts he studied were nearly
equivalent in their ability to support growth, plasma Mg levels and
kidney Mg concentrations. However, it was reported that Mg
L-asparate was more bioavailable than Mg oxide in healthy
volunteers [33]. White et al. [34] determined Mg bioavailability
from Mg chloride solution, slow-release Mg chloride tablets and Mg
gluconate tablets in humans. Urinary Mg excretion and Mg serum
levels were not different between the three supplement forms.
Borella et al. [35] reported that erythrocyte Mg level was a
suitable marker for evaluating Mg retention after oral
physiological supplementation in marginally Mg-deficient subjects.
Wilimzig et al. [36] studied the bioavailability of different
dosages of trimagnesium dicitrate in humans by measuring plasma Mg
levels between 0 and 12 h of Mg salt administration.
In conclusion, the present study demonstrated that all ten
organic and inorganic Mg salts were equally efficient in restoring
blood Mg levels in plasma and red blood cells in rats. Because of
the importance of the passive process, the quantity of Mg in the
digestive tract is the major factor controlling the amount of Mg
absorbed. However, the organic forms of Mg, in particular Mg
gluconate, seem more absorbable than inorganic salts as assessed by
intestinal absorption and urinary excretion.
Acknowledgements
The authors are gratefully to Claudine Lab, Séverine Thien and
Lydia Jaffrelo for their enthusiastic cooperation.
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