| HOME | HELP | FEEDBACK | SUBSCRIPTIONS | ARCHIVE | SEARCH | TABLE OF CONTENTS |
|
|
|
|||||||
|
|||||||||
| ||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||
THIRST AND VOLUME, ELECTROLYTE HOMEOSTASIS
1Nestle Water Institute, 88804 Vittel; 2Mass Spectrometry Unit, Institut National de la Santé et de la Recherche Médicale, 31024, Toulouse, France; and 3Western Human Nutrition Research Center, United States Department of Agriculture, University of California, Davis, California 95616
Submitted 9 December 2002 ; accepted in final form 22 May 2003
 |
ABSTRACT |
|---|
 TOP ABSTRACT SUBJECTS AND METHODSRESULTSDISCUSSIONDISCLOSURESREFERENCES |
|---|
magnesium absorption; magnesium fecal endogenous excretion; inductively coupled plasma-mass spectrometry
However, for the reasons described above and despite the advantages of such a tool, there are only two compartmental models of Mg in humans using isotopic tracers and plasma, urine, and fecal kinetics (5, 32). The study of Avioli and Berman (5) was carried out in men with the use of 28Mg, a radioisotope. On the basis of this model, the second was developed in adolescent girls with the use of stable isotopes. To our knowledge, no Mg compartmental model has been built using stable isotopes in healthy men.
The aim of this study was to develop a model using data collected in healthy men. Plasma, urine, and fecal kinetics data were obtained in a study also designed to compare isotopic methods for the determination of Mg absorption (29). The rate of Mg absorption and that of fecal endogenous excretion were deduced from the model and are presented hereafter.
|
SUBJECTS AND METHODS |
|---|
|
TOPABSTRACTSUBJECTS AND METHODSRESULTSDISCUSSIONDISCLOSURESREFERENCES |
|---|
|
Protocol. All participants were studied in the metabolic research unit (MRU) of the USDA/Agricultural Research Service Western Human Nutrition Research Center (WHNRC) in San Francisco, CA. The study was carried out over an 18-day period of time, and stable isotopes were administered after 6 days of adjustment to the diet. The diet was a 3-day rotating menu with an energy level set to meet the energy requirement of the smallest subject. A liquid formula drink with additional minerals to meet the RDAs of all nutrients and energy to maintain the body weight of each subject was added to the diet, divided into three portions, and given with each meal. The average Mg content in the diet was 257 ± 27 mg/day. After inclusion of the isotopes administered, the average daily Mg intake was 265 ± 27 mg/day.
Tracer administration. On the morning of the first day of the kinetic study, subjects were given an oral dose of 70 mg of 26Mg (98.82 atom% 26Mg) in 200 ml of water (Arrowhead) with breakfast. Fifteen minutes later, 30 mg of 25Mg (99.58 atom% 25Mg) were infused into the arm vein. The isotopically enriched Mg was obtained as enriched magnesium oxide powder from the Oak Ridge National Laboratory (Oak Ridge, TN).
Sampling. Blood was drawn at the following times relative to the intravenous dose: 0, 5, 15, and 30 min and 1, 2, 4, 6, 11, 16, 24, 48, 72, 96, 120, 144, and 168 h. Complete urine and fecal collections were obtained throughout the study. Urine was pooled by 8-h periods for 3 days after isotope administration and by 24-h periods. Fecal samples were pooled by 3-day periods.
Analysis. The plasma volume for each subject was calculated on the basis of age and body mass by use of a nomogram (20a). Mg concentrations in plasma, urine, and digested feces and diet composites were determined by flame atomic absorption spectrophotometry (18) (model 5100; Perkin-Elmer, Norwalk, CT). Isotopic ratios were determined with an inductively coupled plasma-mass spectrometer (ICP-MS; ELAN 6000, Perkin-Elmer Instruments, Norwalk, CT) equipped with an ultrasonic nebulizer (U-6000AT+; Cetac Technologies, Omaha, NE). Internal precision for 25Mg/24Mg and 26Mg/24Mg measurements was below 1%. External precision for both ratios was below 0.5% in urine, plasma, and fecal samples. Isotopic ratios were converted into milligrams of tracers excreted or appearing in matrices (35, 36).
Kinetic modeling. Subjects were assumed to be in a steady state because they were consuming a diet with a constant Mg intake. Urinary, plasma, and fecal 25Mg and 26Mg kinetics were analyzed with the compartmental module of the SAAMII software (Simulation Analysis and Modeling; SAAM Institute, Seattle, WA). In the tracer model, plasma data were expressed as tracer-to-tracee ratios for oral and intravenous tracers. Urinary and fecal data were expressed as excreted cumulative dose (mg). Individual values of dietary unlabeled Mg intake were directed into compartment 6 in the corresponding tracee model. A data-based weighting structure was used for parameter estimation. A constant standard deviation of 0.1 was applied to the data because manipulation and measurement errors were the same for all samples. It resulted in the best data fit, contrary to a fractional standard deviation that privileged the points with a low value and did not improve the fit. The tracee model was built from a tracer model (21) derived from that of Avioli and Berman (5). The circles indicate compartments representing kinetically distinct pools of Mg. The arrows represent transfers between compartments. Fractional transfer coefficients are symbolized by k(i,j) (fraction/h) and are defined as the fraction of compartment j mass moving into compartment i per unit time. Transport rate K(i,j) (mg/h) is the mass of tracee transfer per unit time. K(i,j) was calculated as the product of the fractional transfer k(i,j) and the compartment mass of tracee Qj (mg): K(i,j) = k(i,j) x Qj.
The model (Fig. 1) consists of a unidirectional chain of three compartments describing the gastrointestinal tract, in which a delay (compartment 9) was inserted, represented by a rectangle. Compartment 6 corresponds to the stomach; compartment 7 symbolizes the part of the digestive tract from which Mg is absorbed. Compartment 1 corresponds to plasma, which exchanges in a bidirectional manner with compartment 7 (absorption and endogenous excretion) and with two other extraplasma compartments (compartments 2 and 3). A unidirectional transfer is described between the plasma and urine compartment (compartment 4), which represents the urinary excretion.
|
Tracers were introduced orally and intravenously into compartments 6 and 1. Tracer-to-tracee ratios were measured into plasma (compartment 1), urine (compartment 4), and fecal (compartment 8) compartments.
Fractional absorption was calculated as the fraction of Mg entering the circulation from the intestinal site of absorption (compartment 7; Fig. 1) (14, 38) as follows
 |
Fecal endogenous excretion was calculated as the plasma Mg fraction entering the intestine without being reabsorbed as follows (38)
 |
Statistics. Individual parameter estimate values are reported for each subject as well as the mean ± SD. The Mg absorption rate derived from the model was compared with the values calculated with the fecal monitoring method reported elsewhere for the same subjects (29). Student's paired t-test was used for statistical comparison of the data. Means were considered significantly different when P < 0.05.
|
RESULTS |
|---|
|
TOPABSTRACTSUBJECTS AND METHODSRESULTSDISCUSSIONDISCLOSURESREFERENCES |
|---|
Kinetics analysis. The fitting kinetics and the simultaneous elaboration of the model (shown in Fig. 1) were done following the chronology proposed by Avioli and Berman (5). First, we tried to fit the data with constant fractional transfer coefficients. A good fit of urinary data could not be obtained under these conditions. Because the urinary Mg is directly derived from the plasma Mg, urinary and plasma Mg kinetics relationships were studied separately from the rest of the model by use of a forcing function. The forcing function associated with the plasma kinetics "forces" the system to predict the urinary curve from the plasma data set (13, 24). Thus the shape of the urinary curve of the injected label was calculated from the measured plasma curve. This function and the adjustment of the mathematical prediction to the urinary observed values are reported in Fig. 2. The simulation revealed a systematic error appearing in the early part of the cumulative urinary excretion of the tracer that was administered intravenously when the fractional transfer coefficient between both compartments was constant [k(4,1) = constant]. In other words, predicted values for the urinary excretion of the tracer were lower than the data, as illustrated in Fig. 2A. Changes introduced in parameters could not eliminate this discrepancy.
|
It was then concluded that the experiments did not solve a very rapid phase of excretion of the intravenous tracer during the first 15 h after the isotope injection. It has been suggested that after the intravenous tracer administration, a small fraction was rapidly excreted from the plasma compartment into the urine.
In the model of Avioli and Berman (5), this discrepancy was also observed. An operational unit was thus introduced into the model to account for the early appearance of small amounts of the intravenous tracer in the urine. This operational unit did not correspond to a physiological entity but was required for the model to be solved.
In the present study, instead of the operational unit, the fractional transfer coefficient between both compartments was transformed into a nonconstant function as follows
 |
where 
and 
are constant, and qplasma is the
mass of plasma compartment. When leaving the
k(4,1) value and thus K(4,1)
(the urinary excretion rate) to modulate in relation to the amount of Mg
appearing in the plasma compartment, observed kinetics could be fitted without
any further changes in the model. As shown in
Fig. 2B, this solution
gave a very good fit of the urinary data. The forcing function was then turned
off, and the data were fitted with all the k(i,j)
adjustable plus the and values of
k(4,1). The final result was a very good fit of
the urinary curves. An example of the adjustment achieved between observed and
calculated values for one subject is presented in
Fig. 3. A lower standard
deviation for most parameters and lower statistical indexes associated with
the models indicated unambiguously that the improvement was statistically
significant compared with the constant k(4,1)
model. The important point is that the forcing function was only used as a
study and simulation tool; it was not used in the final model.
|
The relationship between time and the variations of the fractional transfer
coefficient k(4,1) between urine and plasma is
reported in Fig. 2C.
It is shown that k(4,1) was increased temporarily
after the injection and returned to its basal steady state in a few hours. The
k(4,1) steady-state value was used to
calculate the urinary excretion rate.
Model and parameters. The multicompartmental Mg model in healthy men is illustrated in Fig. 1. It was tested in six men. All subjects had one central plasma compartment (compartment 1); the mean mass of this pool was 203 ± 54 mg (range 131-273 mg). This compartment exchanges with two extraplasma compartments (compartments 2 and 3). Their mean masses were 1,068 ± 545 mg (range 512-2,063 mg) and 5,350 ± 1,420 mg (range 3,542-7,133 mg), respectively. Their transport rates were 179 ± 88 mg/h (range 113-320 mg/h) and 48 ± 13 mg/h (range 34-67 mg/h), respectively. Individual parameters are reported in Table 2.
|
The combined mass of the three compartments (compartments 1-3) is 6,620 ± 1,285 mg (range 4,706-7,946 mg). The Mg content of an adult man (70 kg) is 24 g, i.e., 340 mg/kg body wt. Thus this technique allows the exploration of 25 ± 3% of total body Mg, meaning that 25% of the total body Mg exchanges in 12 days. The pool masses differ considerably between and among themselves. Compartment 3 contains 80 ± 9% of rapidly exchangeable Mg; 3 ± 1% of the rapidly exchangeable Mg is contained in the plasma pool, which is <1% of total body Mg.
For three subjects (subjects 3-5), a better fit was obtained with an additional extraplasma pool (pool 5),
which is not represented in Fig. 1. This compartment exchanged in a bidirectional manner with the plasma pool. It had an average mass of 132 ± 12 mg (range 120-145 mg). This corresponds to only 2% of the combined mass of pools 1-3 and 5. Its transport rate was extremely rapid: 351 ± 35 mg/h (range 318-388 mg/h). The addition of this pool for the three subjects changed the mean value of all parameters. Values are reported in Table 3. Those changes were not significantly different from the mean value obtained with the same number of compartments for all six subjects. However, for most of the parameters related to compartments 2 and 3, the coefficient of variation was lower when compartment 5 was added. In this case, the mean sizes of compartments 2 and 3 were 1,080 ± 513 mg (range 642-2,063 mg) and 5,171 ± 1,219 mg (range 3,542-6,624 mg), respectively. Their transport rates were 118 ± 21 mg/h (range 92-149 mg/h) and 45 ± 9 mg/h (range 34-55 mg/h), respectively. No changes were observed in the intestinal delay, the absorption and excretion rate, or the fecal endogenous excretion. Because the compartment was small and not detectable in all subjects, it was not used in further analysis.
|
Absorption and fecal endogenous excretion. Individual values of the true Mg absorption (MgA) and fecal endogenous excretion for the simpler model are reported in Table 2. The mean fractional absorption deduced from the model was 0.44 ± 0.07 (range 0.35-0.55). Fractional absorption, using the fecal monitoring method, was calculated previously (29) and was 0.46 ± 0.05 (range 0.39-0.52) for the apparent Mg absorption (MgAA) and 0.48 ± 0.05 (range 0.41-0.55) when corrected for the fecal endogenous excretion, i.e., MgA. Statistical comparison of means showed no difference between MgAA and values predicted using the model. However, MgA calculated with the fecal monitoring was significantly higher than the value predicted by the model (P = 0.0,232 < 0.05). The total fecal endogenous excretion calculated from MgA, total unabsorbed dietary Mg, and Mg excreted in feces (29) was estimated to be 31 ± 24 mg Mg/day (range 2-63 mg/day). The mean value for endogenous excretion into the gastrointestinal tract predicted by the model was 49 ± 11 mg Mg/day (range 38-64 mg/day) and was not significantly different from the calculated value. The mean value for the urinary excretion rate is 5 ± 2 mg/h (range 2-7 mg/h). The time for gastrointestinal transit after the end of Mg absorption represented by a delay component was on average 53 ± 19 h (range 25-81 h).
|
DISCUSSION |
|---|
|
TOPABSTRACTSUBJECTS AND METHODSRESULTSDISCUSSIONDISCLOSURESREFERENCES |
|---|
The problem of the adjustment of mathematical curves to the experimental values for the intravenous tracer excreted in urine, emphasized during the preliminary study of kinetics, was observed in two other Mg models described in literature (5, 32). Avioli and Berman (5) tackled this difficulty and proposed a solution to solve the model. In the work of Sojka et al. (32), the disagreement between model-predicted curves and experimental points can be seen (see Fig. 2 in Ref. 32, whereby the fitting of the mean values obtained for urinary excretion is shown). Mg excretion in the urine is underestimated in the early part of the curve. The effect of such approximation on the final results of whole parameters is not known. However, a correction applied by Avioli and Berman or that proposed herein allows an appreciable amelioration of the fitting. The discrepancy between calculated and observed values could be explained by the intravenous amount of administered isotope. Because of the high natural abundance of Mg isotopes, it is necessary to administer large amounts of tracer to get measurable enrichment in samples. In our study, 30 mg 25Mg was injected into the Mg plasma compartment (Q1 = 203 mg), i.e., 15% of the total amount of Mg. In the study of adolescent girls aged 12-14 yr, Sojka et al. injected 20 mg into a pool of 275 mg. Avioli and Berman injected 14-16 mg 28Mg into a pool of 196.9 mg in adult men, i.e., 7.2 and 7.6% of the total amount of the compartment, respectively. Those amounts could be responsible for a transitory hypermagnesemia, resulting in an increase in Mg urinary excretion in the early part of the experiment. It has been shown that Mg homeostasis is regulated by the kidney (26). Thus the fractional transfer coefficient from the plasma pool into the urinary compartment or the urinary excretion rate depends on the plasma Mg concentration. Consequently, it can be defined as a nonconstant function. This solution has been chosen in the present study to fit the kinetics. Inspection of the time course of the fractional transfer coefficient between plasma and urine showed that this state is transitory and lasted only a few hours. The steady state among different components of the model was then reached and maintained. The plasma kinetics of the oral tracer and the preliminary study of kinetics demonstrated that it was unlikely that the oral tracer was responsible for this perturbation, although the oral dose increased intake on the day of administration. This is probably because the oral tracer was given with breakfast, only 0.44 was absorbed, and it was absorbed over a period of time as it passed through the gastrointestinal tract.
The utilization of a multicompartmental model enabled the study of 25% of total body Mg after 12 days of sampling. The combined mass of the defined compartments corresponds to 86 ± 12 mg Mg/kg body wt in adult men. Because of the short half-life of the radioisotope used in the experiment of Avioli and Berman (5), the study lasted only 6 days. Consequently, only 15% of total body Mg could be explored, and the size of the rapidly exchangeable Mg pool (expressed per kg body wt) is lower (43 mg/kg) than the estimated value in the present study. Sojka et al. (32) reported values similar to ours after a 14-day study using stable isotopes in adolescent girls (90 mg/kg). No individual values or standard deviations were reported in that study. A close correlation has been observed between the size of the exchangeable pool of Mg and the rate at which this pool exchanges with the longer-term storage pool and the fat-free mass (1), making a comparison of data difficult when expressed per kilogram of total body weight.
Because only 25% of total body Mg can be explored in 12 days, 
75%
either exchanges more slowly or is not exchangeable. The study of this aspect
of Mg kinetics would require a longer study. Avioli and Berman
(5) estimated the half-life of
Mg in the body to be 1,000 h.
Mg distribution is equally divided between the skeleton and soft tissues. About 60% of Mg are present in bone tissues, 20% in muscles, and 20% in other soft tissues (3). A fraction of bone Mg exchanges extremely slowly and could account for this unexplored compartment. However, it has been suggested that a part of bone Mg is in a readily exchangeable labile form (23). Consequently, it is difficult to make an association between the described compartment and anatomic entities.
Eighty percent of rapidly exchangeable Mg is found in one compartment. Its rate of turnover is two times less important than the second extraplasma compartment present in all six subjects. The plasma compartment contains <1% of total body Mg. These data are in agreement with values found in the literature (5, 9). In the present experiment, the Mg in the plasma compartment had a mass of 203 ± 54 mg in adult men (mean ± SD). Sojka et al. (32) found the same compartment had a mass of 275 mg but gave no individual values or standard deviations, making comparison hazardous. However, as already indicated, the combined mass of the defined compartments was similar in adult men and adolescent women. This could suggest a difference in Mg distribution between the compartments due to age. Some authors suggested that variability of Mg metabolism according to age could be due to the rapid growth of bone and muscle tissue during early adolescence (11).
The results showed that for three subjects the plasma pool could exchange with a third extraplasma compartment (5). This compartment was suppressed because data from three of the six subjects did not support a supplementary pool. This could be due to lack of accuracy in the measurements. When it exists, the pool size is small, and its transport rate is extremely rapid. A firm justification of its presence is difficult because of the absence of data enabling the evaluation of Mg status, stress, or pathological state. Although plasma Mg concentration did not indicate Mg deficiency, the lack of reliability of this marker for the evaluation of Mg status does not make it possible to establish a link between this supplementary pool and deficiency (10, 12, 25). However, the fit and associated statistics were better in 50% of our subjects when this pool was introduced. Its presence is due to the sharp shape of the curve at the early time points and could be also linked to the hypermagnesemia after isotope administration.
The overestimation of the true Mg absorption calculated with the fecal monitoring compared with the predicted value by a model has also been observed for Zn (22, 40). In the case of Mg, the absolute disagreement of the fraction absorbed between both values is low (0.04). The fecal monitoring method was corrected with the tracer fecal endogenous excretion (tFEE) to get the true absorption. On average, the tFEE was 2.4% of tracer/day. Average total endogenous excretion into the gastrointestinal tract derived from the model was 49 ± 11 mg/day, with a Mg dietary intake of 265.3 mg, and was in accordance with an estimate of the total fecal endogenous excretion based on fecal and dietary Mg and calculated true absorption. Similar values have been reported (20, 28). However, in the Avioli and Berman study (5), fecal endogenous excretion was 14.3 ± 8.6 mg/day. The use of various calculations and a different period of time to estimate this value could explain this difference. In adolescent girls, fecal endogenous excretion was also lower (13.3 ± 5.2 mg/day) (32). However, no close relationship between Mg intake and endogenous fecal excretion was found (2).
Perspectives
This study has established parameters for a Mg multicompartmental model in healthy men. The model was based on plasma, urinary, and fecal kinetics after oral and intravenous tracer administration. An approach was proposed to solve the discrepancy between model-predicted curves and experimental points. Therefore, the model can be used to evaluate parameters under other circumstances or in parallel with other tests to establish a relationship between the rapidly exchanging Mg pool and Mg status.
|
DISCLOSURES |
|---|
|
TOPABSTRACTSUBJECTS AND METHODSRESULTSDISCUSSIONDISCLOSURESREFERENCES |
|---|
|
ACKNOWLEDGMENTS |
|---|
Part of the results was presented as a poster at the 11th International Symposium on Trace Elements in Man and Animals, Berkeley, CA, June 2-6, 2002.
|
FOOTNOTES |
|---|
The costs of publication of this article were defrayed in part by the payment of page charges. The article must therefore be hereby marked "advertisement" in accordance with 18 U.S.C. Section 1734 solely to indicate this fact.
|
REFERENCES |
|---|
|
TOPABSTRACTSUBJECTS AND METHODSRESULTSDISCUSSIONDISCLOSURESREFERENCES |
|---|
| ||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||
| HOME | HELP | FEEDBACK | SUBSCRIPTIONS | ARCHIVE | SEARCH | TABLE OF CONTENTS |
| Visit Other APS Journals Online |
and 
, Measurements of both
tracers; circles with nos., compartments; arrows, Mg movement between
compartments. Constant k(i,j) (fraction/h) is the
fractional transfer coefficient into compartment i from
compartment j. Compartments 2 and 3 are extraplasmatic pools
exchanging rapidly with plasma compartment 1. The unidirectional
chain of compartments 6-8, in which a delay (rectangle;
compartment 9) is included, represents the gastrointestinal tract.
k(1,7) illustrates the absorption, and
k(7,1) is the fecal endogenous excretion.
Irreversible losses are represented by compartments 8 and 4,
which are feces and urine, respectively.
) and
fitted (solid line) plasma kinetics. 