Plasma sodium level is associated with bone loss severity in women with anorexia nervosa: a cross-sectional study

Anorexia nervosa is a psychiatric disorder characterized by restrictive eating, low body weight, and severe bone loss. Recent data show a deleterious relationship between low circulating sodium levels and bone mass, and relative or absolute hyponatremia is a known complication of anorexia nervosa. Clinical studies of other medical conditions associated with hyponatremia suggest that detrimental effects of low sodium levels on health are seen even within the normal range. We hypothesized that women with anorexia nervosa and relatively low plasma sodium levels would have lower bone mineral density (BMD) than those with higher plasma sodium levels.

In a cross-sectional study (January 1, 1997-December 31, 2009) of 404 women aged 17 to 54 years (mean ± standard error of the mean [SEM] age = 25.6 ± 0.3 years) who met DSM-IV criteria for anorexia nervosa, we measured BMD using dual-energy x-ray absorptiometry. Bone mineral density was compared in women with plasma sodium levels < 140 mmol/L (midpoint of normal range) versus those with plasma sodium levels ≥ 140 mmol/L and in women with hyponatremia (plasma sodium < 135 mmol/L) versus those without. The study was conducted at the Neuroendocrine Unit of Massachusetts General Hospital, Boston.

Women with plasma sodium levels < 140 mmol/L had significantly lower BMD and t and z scores versus those with plasma sodium levels ≥ 140 mmol/L at the anterior-posterior (AP) spine (mean ± SEM z scores = -1.6 ± 0.1 vs -1.3 ± 0.1, P = .004) and total hip (mean ± SEM z scores = -1.2 ± 0.1 vs -0.9 ± 0.1, P = .029). In a model controlling for age, BMI, psychiatric drug use, and disease duration, differences in BMD and t and z scores remained significant at the AP spine. Women with hyponatremia had significantly lower BMD and t and z scores versus those without hyponatremia at the AP spine (mean ± SEM z scores = -2.2 ± 0.3 vs -1.3 ± 0.1, P = .009), lateral spine (mean ± SEM z scores = -2.4 ± 0.4 vs -1.5 ± 0.1, P = .031), and total hip (mean ± SEM z scores = -2.5 ± 0.5 vs -1.0 ± 0.1, P < .0001). In a model controlling for age, BMI, psychiatric drug use, and disease duration, differences in BMD and z and t scores remained significant at all sites.

These data suggest that relative plasma sodium deficiency may contribute to anorexia nervosa-related osteopenia.