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Over the past twenty years a number of studies of acute bronchial asthma have shown that i.v. or nebulized MgS04 may improve symptoms over a course of hours. With respect to dietary supplementation, short term (3 wk) oral Mg has been associated with a significant decrease in symptoms but no significant effect on measurements like FEV1 or bronchial hyperreactivity by methacholine challenge. Although a large number of studies have attempted to address this issue, we believe that major gaps still exist. One of the gaps is in the comparison of large numbers of asthmatics and non-asthmatics, with regard to dietary intake, and a variety of measures of Mg status. We will evaluate baseline Mg intake (diet, tap and bottled drinking water, vitamin-mineral supplements, laxatives, and antacids), and multiple measures of Mg status, such as total and free serum Mg, total erythrocyte Mg, and Mg retention after an IV Mg load in subjects with and without asthma. Furthermore there are no large-scale studies evaluating the effects of Mg supplementation on asthma control and clinical markers, and markers of inflammation. We propose to assess the effects of 6 1/2 months of oral Mg on clinical markers of asthma control (asthma symptom diary, monthly spirometry, asthma quality of life questionnaire (QOL)), indirect biomarkers of inflammation (exhaled nitric oxide and serum eosinophil cationic protein) and bronchial hyperresponsiveness (methacholine challenge)in subjects with mild to moderate persistent asthma. Dietary Mg will be assessed using the 24 hr recall. Our hypotheses are that 1.) subjects with mild to moderate persistent asthma, as defined by National Institutes of Health National Asthma Education and Prevention Program (NIH NAEPP) clinical guidelines will have poorer Mg status than nonasthmatics, and 2.) that marginal Mg intake and status may modulate the severity of asthma. Thus, subjects with asthma who have marginal intake/status and thus relatively lower total and free plasma Mg, lower erythrocyte total Mg, and higher Mg retention will show improvement in the aforementioned clinical and indirect biomarkers. In contrast, Mg supplements will have little effect in subjects with highest intakes and Mg status. We do not anticipate that Mg supplementation will replace conventional treatment, but may complement and decrease the need for conventional medication.