Seasonal BMI differences between restrictive and purging anorexia nervosa subtypes.

Objective: Seasonal differences in anorexia nervosa is an area of research which has received scarce attention in the literature. The aim of this study was to explore patterns of seasonal variation in body weight in anorexia nervosa patients admitted to an eating disorders unit from January 2007 to December 2011.

Method: Anorexia nervosa restrictive (ANR) and binge eating/purging (ANBP) subtypes were compared with respect to BMI on admission during the Warm and Cold Semester, and in terms of lenght of hospital stay.

Results: The main result was that BMI differences between ANBP and ANR patients were due to bodyweight differences during the colder months of the year (p < 0.01) whereas no difference in bodyweight on admission was observed between both subtypes during the Warm Semester months. Furthermore, ANR patients' bodyweight was lower when admitted during the Cold compared to the Warm Semester (p < 0.05). Length of stay was significantly associated with reduced BMI in ANR patients (p < 0.01), but not so for ANBP patients.

Discussion: To our knowledge, this is the first study in the literature providing data on seasonal variations in the BMI on admission of anorexia nervosa patients, and in the length of hospitalization.