Introduction:
This study was designed to assess the influence of chosen aspects of anorexia nervosa eating disorders (AN ED) on 13C-methacetin breath test (MBT) results.

Material and methods:
We investigated a group of 81 girls, including 41 patients with confirmed diagnosis of AN ED. The study group was divided in two subgroups. According to the DSM-5 classification, 25 underweight patients met the criteria of anorexia nervosa (AN), and 16 girls with BMI value > 5th percentile for age and sex were diagnosed as atypical anorexia nervosa (AAN). Laboratory tests assessing the liver function and the MBT were performed in all the participants of the study.

Results:
In all healthy females the values of anthropometric parameters and laboratory results concerning the liver function were normal. Girls with AN ED achieved higher cumulative 13CO2 dose salvage during the MBT than healthy controls. Also, DOB (delta over baseline) values were significantly higher in the study group, as well as in both ANN and AN subgroups, compared to controls. Comparison of the patients’ subgroups revealed higher elimination of the cumulative 13CO2 dose in females with AN compared to those with AAN. Among the study group there was a negative correlation between the DOB values in all time points of the test and the weight, weight percentile, BMI, and BMI percentile.

Conclusions:
Girls with weight deficiency in the course of AN achieved higher cumulative 13CO2 dose recovery during the MBT than healthy controls and other AN patients. The recovery of 13CO2 during the MBT was negatively correlated with parameters describing patients’ weight status. The obtained results suggest a significantly faster 13C-methacetin metabolism in girls with AN ED, which is probably a consequence of the characteristics of the primary disease. The presented observations suggest that MBT is not useful in monitoring liver function in patients with AN ED.