Diagnosis of magnesium-induced diarrhea BACKGROUND. There is no specific method of diagnosing magnesium-induced diarrhea. Therefore, the frequency and clinical importance of diarrhea caused by magnesium are unknown. The purposes of this study were to establish a method for diagnosing magnesium-induced diarrhea and to apply it to patients with chronic diarrhea. METHODS. We measured fecal output of soluble magnesium and fecal magnesium concentration in 19 normal subjects with formed stools (15 collection periods), with non-magnesium-induced diarrhea (36 collection periods), and with diarrhea induced by magnesium hydroxide alone (11 collection periods) or in combination with phenolphthalein (3 collection periods), and in 359 patients with chronic diarrhea. RESULTS. The upper limits of fecal output of soluble magnesium and fecal magnesium concentration in normal subjects were 14.6 mmol per day and 45.2 mmol per liter, respectively. When normal subjects had diarrhea due to the ingestion of magnesium hydroxide alone or in combination with phenolphthalein, fecal magnesium output was always abnormally high. For each millimole increase in fecal magnesium output, fecal weight increased by approximately 7.3 g. The fecal magnesium concentration was very high when magnesium was the only cause of diarrhea but only moderately elevated when diarrhea was induced by magnesium hydroxide plus phenolphthalein. Biochemical and clinical evidence indicated that excessive ingestion of magnesium was an important cause of chronic diarrhea in 15 of the 359 patients with chronic diarrhea (4.2 percent), if not the only cause. CONCLUSIONS. Quantitative fecal analysis for soluble magnesium is an accurate method of diagnosing magnesium-induced diarrhea. Some patients with chronic diarrhea ingest excessive amounts of magnesium (in antacids or food supplements), and physicians may fail to discover this before embarking on an expensive and invasive diagnostic evaluation. 