Determination of whether magnesium (Mg) is a nutrient of public health concern has been hindered by questionable Dietary Recommended Intakes (DRIs) and problematic status indicators that make Mg deficiency assessment formidable. Balance data obtained since 1997 indicate that the EAR and RDA for 70-kg healthy individuals are about 175 and 250 mg/day, respectively, and these DRIs decrease or increase based on body weight. These DRIs are less than those established for the USA and Canada. Urinary excretion data from tightly controlled metabolic unit balance studies indicate that urinary Mg excretion is 40 to 80 mg (1.65 to 3.29 mmol)/day when Mg intakes are250 mg (10.28 mmol)/day. However, changing from low to high urinary excretion with an increase in dietary intake occurs within a few days and vice versa. Thus, urinary Mg as a stand-alone status indicator would be most useful for population studies and not useful for individual status assessment. Tightly controlled metabolic unit depletion/repletion experiments indicate that serum Mg concentrations decrease only after a prolonged depletion if an individual has good Mg reserves. These experiments also found that, although individuals had serum Mg concentrations approaching 0.85 mmol/L (2.06 mg/dL), they had physiological changes that respond to Mg supplementation. Thus, metabolic unit findings suggest that individuals with serum Mg concentrations >0.75 mmol/L (1.82 mg/L), or as high as 0.85 mmol/L (2.06 mg/dL), could have a deficit in Mg such that they respond to Mg supplementation, especially if they have a dietary intake history showing <250 mg (10.28 mmol)/day and a urinary excretion of <80 mg (3.29 mmol)/day.