Ironically, though most of the severe headache sufferers initially had low concentrations of ionized magnesium in their blood, their bodies’ total magnesium levels fell within the normal range. Additional studies have shown a similar pattern among other types of patients. For instance, Altura notes, stroke victims with a history of high blood pressure, including diabetic hypertension, "had the lowest level of ionized magnesium when they stroked out." Yet in some cases, their total magnesium concentrations had been within the normal range.
Diet also fails to predict who will tend to have low ionized magnesium concentrations in blood and tissues, Altura has found. In a study 4 years ago, he and his colleagues recruited 18 healthy young men to take different magnesium supplements for 6 days at a time. Although the volunteers had started the trial with normal total-magnesium concentrations—suggesting their diet had contained the RDA of this mineral—a number of the men still had below normal concentrations of the mineral’s ionized form. However, once these men were placed on diets containing four to five times the RDA of the mineral, the proportion of ionized magnesium in their blood climbed into the healthy range—despite no change in total magnesium concentrations.
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To enrich the diet naturally with this mineral, look to nuts, legumes, and unmilled grains. Removing the germ of cereals and the outer seed husks eliminates 80 percent of any magnesium present in the whole grain. Green vegetables and bananas are also relatively good sources of the mineral. You won’t find much, however, in other fruits or fish, meat, and milk.
Can you get too much? Except in persons with kidney disease, "there is no evidence that large oral intakes of magnesium are harmful," according to the RDA handbook, published by the National Research Council. However, people can develop diarrhea when they take large quantities of these synthetic forms of the mineral; many supplements provide the mineral in the same oxide form that serves as the basis of common laxatives.