Seasonal variation of BMI at admission in German adolescents with anorexia nervosa

Abstract

ObjectiveRecent preliminary studies indicated a seasonal association of BMI at admission to inpatient treatment for anorexia nervosa (AN), indicating lower BMI in the cold season for restrictive AN. An impaired thermoregulation was proposed as the causal factor, based on findings in animal models of AN. However, findings regarding seasonality of BMI and physical activity levels in the general population indicate lower BMI and higher physical activity in summer than in winter. Therefore, we aimed to thoroughly replicate the findings regarding seasonality of BMI at admission in patients with AN in this study.MethodAN subtype, age- and gender-standardized BMI scores (BMI-SDS) at admission, mean daily sunshine duration and ambient temperature at the residency of 304 adolescent inpatients with AN of the multi-center German AN registry were analyzed.ResultsA main effect of DSM-5 AN subtype was found (F(2,298) = 6.630, p = .002), indicating differences in BMI-SDS at admission between restrictive, binge/purge and subclinical AN. No main effect of season on BMI-SDS at admission was found (F(1,298) = 4.723, p = .025), but an interaction effect of DSM-5 subtype and season was obtained (F(2,298) = 6.625, p = .001). Post-hoc group analyses revealed a lower BMI-SDS in the warm season for restrictive AN with a non-significant small effect size (t(203.16) = 2.140, p = .033; Hedges′g = 0.28). Small correlations of mean ambient temperature (r = −.16) and daily sunshine duration (r = −.22) with BMI-SDS in restrictive AN were found. However, the data were widely scattered.ConclusionsOur findings are contrary to previous studies and question the thermoregulatory hypothesis, indicating that seasonality in AN is more complex and might be subject to other biological or psychological factors, for example physical activity or body dissatisfaction. Our results indicate only a small clinical relevance of seasonal associations of BMI-SDS merely at admission. Longitudinal studies investigating within-subject seasonal changes might be more promising to assess seasonality in AN and of higher clinical relevance.

Seasonal and subtype differences in body mass index at admission in inpatients with anorexia nervosa

“…In the current study, this differentiation was based on the ICD‐10‐GM, whereas other studies referred to the criteria in the fourth (Carrera et al, ; Fraga et al, ) or fifth (Kolar et al, ) version of the Diagnostic and Statistical Manual of Mental Disorders (DSM) or did not specify the definition criteria (Born et al, ). Similarly, prevalence of patients being classified as binge/purge‐type AN differed across studies between 4% (Kolar et al, ) and 53% (Born et al, ), which might be due to different subtype definitions. A crucial aspect may be the presence of excessive exercise, which would not represent a purging behavior according to the DSM but which is not explicitly excluded in the definition of binge/purge‐subtype AN in the ICD‐10‐GM.…”

“…Another aspect refers to the definition of the warm and cold seasons. In the current study, we differentiated between the four meteorological seasons (as the large sample size allowed us to do so), whereas previous studies combined autumn/winter and spring/summer (Born et al, ) or differentiated between warm and cold season based on outside temperature, but with different definitions (Carrera et al, ; Fraga et al, ; Kolar et al, ). Thus, these methodological differences may partly explain inconsistent findings about seasonal effects on body weight in AN.…”

“…Hypothetically, this process may be mediated by brown adipose tissue, which is activated by hypothermia, dissipates energy, and generates heat (Freemark & Collins, ). Finally, it has also been suggested that reduced exposure to sunlight in the winter and lower vitamin D concentrations may also influence body weight, for example, through increased depressiveness (Kolar et al, ).…”

“…Previous studies applied different categorizations of warm versus cold season (e.g., differing in the assignment of October and April; Carrera et al, ; Fraga et al, ; Kolar et al, ). Thus, we decided to follow the classification of meteorological seasons (similar to Born et al, ): spring (March to May), summer (June to August), autumn (September to November), and winter (December to February) based on the patients' admission date.…”

“…To overcome these limitations, Kolar et al () examined clinical records of 304 adolescent inpatients from a multi‐centric database in Germany. Although they indeed found an interactive effect between season and AN subtype on BMI at admission, the nature of this interaction indicated higher body weight during the cold than the warm season in patients with restrictive type AN.…”