Magnesium loading effect on magnesium deficiency in endurance-trained subjects during prolonged restriction of muscular activity.

The aim of this study was to evaluate the effect of magnesium (Mg) loading (10.0 mg Mg/kg body wt) and daily Mg supplements (5.0 mg Mg/kg body wt) on Mg deficiency shown by increased and not by decreased serum Mg concentration during hypokinesia (decreased km number/d). The studies were done during 30 d of prehypokinesia and 364 d of hypokinesia (HK) periods. Forty endurance-trained volunteers aged 22-26 yr with a peak VO2 max of 66.3 mL.kg-1 min-1 and with an average 15.0 km/d running distance were chosen as subjects. They were equally divided into four groups: 1. Unsupplemented ambulatory control subjects (UACS). 2. Unsupplemented hypokinetic subjects (UHKS). 3. Supplemented hypokinetic subjects (SHKS). 4. Supplemented ambulatory control subjects (SACS). The SHKS and SACS groups took daily 5.0 mg elemental Mg/kg body wt and subjected to Mg loading (10.0 mg Mg/kg body wt). Both the SHKS and UHKS groups were maintained under an average running distance of 4.7 km/d, whereas the SACS and UACS groups did not experience any modifications to their normal training routines and diets. During the prehypokinetic and hypokinetic periods, excretion of Mg in feces and urine, concentration of Mg in serum, and Mg balance were measured. Urinary and serum sodium (Na), potassium (K), and calcium (Ca) were also determined. In both SHKS and UHKS groups, fecal Mg loss, urinary excretion of electrolytes, and serum concentrations of electrolytes increased significantly (p < or = 0.05) when compared with the SACS and UACS groups. During Mg loading tests, urinary and fecal Mg excretion was also greater in the SHKS and UHKS groups than in the SACS and UACS groups. Throughout the study, Mg balance was negative in the SHKS and UHKS groups, whereas in the SACS and UACS groups, Mg balance was positive. It was concluded that significant losses of Mg occurred in the presence of negative Mg balance and Mg deficiency in endurance-trained subjects during prolonged exposure to HK, daily mg supplements, and Mg loading tests. This suggests that Mg is not entering or being retaining by the bones and cells of many tissues where most Mg is deposited normally, resulting in Mg deficiency as was shown by the increased serum Mg concentration.