Hypomagnesemia in adolescents with eating disorders hospitalized for medical instability.

BACKGROUND Hypomagnesemia in patients with eating disorders is poorly characterized, particularly among adolescents. METHODS To determine the prevalence of hypomagnesemia (Mg ≤ 1.7 mg/dL) and clinical characteristics of adolescents hospitalized with a DSM-IV-diagnosed eating disorder who developed hypomagnesemia, a retrospective chart review was conducted on all adolescents aged 10-21 years with an eating disorder were hospitalized at a tertiary care children's hospital from 2007 to 2010. Patients were refed orally with standard nutrition and high-energy liquid supplements. Serum magnesium and phosphorus were obtained on admission, every 24-48 hours for the first week, and thereafter as clinically indicated. Clinical characteristics of patients with hypomagnesemia were compared with those of individuals with normal magnesium levels and those with hypophosphatemia. RESULTS Eighty-six of 541 eligible participants (15.9%) developed hypomagnesemia. Forty (47%) with hypomagnesemia admitted to purging in the year before admission, with 88% purging during the prior month. Compared with those with normal serum magnesium levels, patients with hypomagnesemia were older (P = .0001), ill longer (P = .001), more likely to be purging (P = .04), and more likely to have an alkaline urine (P = .01). They did not differ in eating disorder diagnosis, BMI, or other electrolyte disturbances. Hypomagnesemia developed 4.9 ± 5.5 days after refeeding was initiated, significantly later than the onset of hypophosphatemia, 0.95 ± 2.6 days (P < .001). CONCLUSIONS Hypomagnesemia is prevalent in adolescents hospitalized for an eating disorder and is associated with purging and alkaline urine. Hypomagnesemia develops later in the course of refeeding than hypophosphatemia. Magnesium levels should continue to be monitored after the more immediate risk of hypophosphatemia has passed, especially in those with alkaline urine.