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Magnesium bioavailability from mineral water. A study in adult men
Magnesium plays an essential role in a wide variety of fundamental cellular reactions. Neuromuscular hyperexcitability is the initial problem cited in individuals who have or are developing magnesium deficiency. Many recent findings have augmented the significance of magnesium in human health and disease. There is mounting evidence that alterations in magnesium metabolism have a negative impact on cardiovascular biology. The recommended dietary intake of magnesium for the French population has been recently set at 6 mg/kg/body weight/day for both men and women. The American recommendation for Mg intake has recently been augmented and it is now close to the French recommendation. In industrialised countries reduction in energy intake and increase in consumption of foods containing only energy without minerals have contributed to decreasing Mg intake. In France in 1997, more than 70% of subjects had dietary Mg intake lower than the RDA and 20% consumed less than two-thirds of the RDA. The Mg intake is lower in the USA. From a public health viewpoint it is important not only to identify the possible sources of Mg, but also to assess their respective bioavailabilities. Mineral water may represent a substantial source of dietary magnesium, with levels of up to 4 mmol/l in some mineral waters. It could thus provide one-quarter to one-third of the magnesium RDA per litre intake. Epidemiological studies underlie the importance of mineral water as a source of dietary Mg and suggest that Mg in drinking water is associated with lower mortality from cardiovascular diseases.
In the present study, we measured the magnesium absorption rate in humans of a single product, magnesium-rich water (3.9 mmol/l). Magnesium has two stable isotopes, Mg and Mg. Because their high natural abundance (10.00 and 11.01%, respectively) reduces the sensitivity with which they can be measured in body fluids, we used the radioactive isotope Mg instead, which is more suited to assessing the bioavailability of a 1.2 mmol Mg load in a 300 ml sample of water, as already described.
Ten male volunteers (mean age 29.4 y; range 25/42 y), deemed healthy on the basis of a complete medical history and examination, normal blood count, kidney function, electrolytes and plasma magnesium (0.65-1 mmol/l), without any treatment or particular diet, were recruited after they had given informed consent to the protocol.
The study of Mg bioavailability allows us to measure digestive absorption of Mg from the liquid (or any nutrient) in which this isotope has been mixed because the isotope acts as a tracer. It has been demonstrated that solubility of magnesium chloride in water is high (solubility product 55 g MgCl2 anhydrous/100 g water at 20° C) and thus one can assume that Mg in the form of MgCl2 is a reliable tracer. In two different sessions (minimum washout period of 1 week), each volunteer received respectively Mg orally in 300 ml water and Mg intravenously. The counting rates of both these procedures were compared, their ratio giving the magnesium digestive absorption. The counting rates measured on the forearm were expressed as the percentage of the orally or intravenously administered dose. The actual size of the administered dose parameter in counts per minute was measured by the counting rate of the whole body volunteer 1 h after isotope administration and before any urinary or faecal excretion. All counting rates were corrected for radioactive decay.
On their first session day, volunteers came to the laboratory at 8:00 after a 12 h fast and were allocated at random to an intravenous or an oral administration of Mg. On their second session day, the treatments were reversed. In the meantime, subjects were instructed not to modify their dietary habits. Before tracer administration, background activity was determined by whole body counting and forearm measurements. For the intravenous administration, a 20-gauge catheter (Critikon 4220) with stylet was inserted in the antecubital vein of the right forearm, the left forearm being used for the measurement of radioactivity.
After recording background activity, one volunteer received an oral load of approximately 185 kBq (5 µCi) of Mg in 300 ml of mineral water, and the other volunteer was injected i.v. with 74 kBq (2 µCi) Mg in NaCl 9 . The radionuclide Mg (T11/22=21.2 h, 400 keV) was obtained by irradiation of Al and from the reaction Al (a.3p) Mg. The irradiation was carried out at the Cyclotron Unit of the Vrije Universiteit Brussel (VUB). Approximately 370 kBq (10 µCi) of Mg were produced in each run and were devoid of contaminants after chemical purification.
One hour after administration of Mg and in the absence of faecal and urinary loss, a whole body count was taken as 100% of the administered dose for expression of whole body retention. Forearm counts (five measurements of 1 min) with identical geometry were carried out every hour for 6 h and 8 and 24 h after administration of Mg. A decay correction was applied after periodic standard measurements of an aliquot containing 37 kBq in a volume of 12 l.
Daily dietary intake was assessed in the month preceding the first test day from quantitative 1 week food records. The subjects were instructed by a dietician to note in detail, over 7 consecutive days, their daily food and drink intake and the portion size, either weighed or expressed in usual household measures. The record was then reviewed and validated by the dietician who translated the quantities expressed as household measures into grams using a computerised dietary system. The software converted the collected data into daily energy and nutrient intakes using a food composition database containing the energy (kJ) and 41 food constituents of more than 1500 food items (compilation of SOUCI, Southgate and CIQUAL databases). Calcium and magnesium in mineral water were determined by atomic absorption spectrophotometry. Serum magnesium was determined using the calamagite method.
For the forearm activity measurements, the subject was positioned outside the whole body counter chamber in a standardised manner; the left arm was stretched out through a window into the wall of the chamber with the forearm lying on a splint inside the chamber. Two detectors (D1 up, D3 down) were used in opposition. Forearm background activity was recorded in the same way. To obtain an identical counting geometry for the measurement of the administered dose and the activity of the forearm, the administered dose was measured with the disposable syringe lying on the same forearm splint inside the whole body chamber. Magnesium-rich mineral water is a reliable source of magnesium. Our observation of decreased magnesium absorption with age deserves further investigations.
Source: www.nature.com
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