Hyponatremia and decreased bone density in adolescent inpatients diagnosed with anorexia nervosa.

OBJECTIVE: Recent studies demonstrated an association between low serum sodium levels and reduced bone density. Patients with anorexia nervosa (AN) are at greater risk for osteoporosis as well as for hyponatremia. The aim of the present study was to assess the association between hyponatremia and bone mineral density (BMD) in a large cohort of adolescent inpatients with AN.

METHODS: A historic cohort study of 174 adolescent females (mean age 15.7 ± 1.8 y) hospitalized because of AN between 2003 and 2013. Demographic and clinical data, including age, psychiatric comorbidity, anthropometric measurements, laboratory tests, and BMD scores were obtained from the patients' medical charts.

RESULTS: Mean lumbar spine BMD z-score of the patients was lower than expected in the normal population (mean -1.5 ± 1.2) and positively correlated with body mass index standard deviation score (r = 0.42, P < 0.0001). Sixty-four participants (36.8%) had at least one episode of hyponatremia during the year preceding the BMD measurement. These participants had a significantly lower lumbar spine BMD z-score (-1.8 ± 1.2 versus -1.3 ± 1.2, P = 0.01) compared with participants with no hyponatremia. Lumbar spine BMD z-score was also positively correlated with the levels of free triiodothyronine (r = 0.16, P = 0.038), 17 b-estradiol (r = 0.23, P = 0.005), and luteinizing hormone (r = 0.25, P = 0.001), and negatively correlated with cortisol levels (r = 0.33, P < 0.0001). Having at least one episode of hyponatremia, BMI z-score and cortisol levels were identified as independent predictors of BMD z-score (P < 0.001, P < 0.001, and P = 0.034, respectively).

CONCLUSIONS: Hyponatremia may be associated with decreased bone density in adolescent females with AN. Additional studies are required to evaluate whether the correction of hyponatremia will improve BMD.