Stable nitrogen and carbon isotopic ratios of hair strands of six patients suffering from anorexia nervosa were measured to monitor a dietary change from near starvation to recovery. This paper presents the results of a first‐time study of nitrogen and carbon balance of the patients prior to and after admittance to a hospital and therapy. Sequential analysis of the isotopic ratios of hair strands of all patients could be related to the respective body mass index (BMI) of each patient. Our hypothesis concerning the diachronic change in δ15N and δ13C during therapy was met: The δ15N values were inversely related to the BMI, indicating a slow‐down in catabolism of bodily protein due to the process of gluconeogenesis during the starvation phase. In contrast, the δ13C values and BMI were in phase: an increase in BMI resulted in an increase in the δ13C values. This rise in δ13C ratios is best interpreted by an increased supply of protein in the diet. Furthermore, δ15N and δ13C were inversely related. We conclude that hair, which is easily and non‐traumatically sampled, is an adequate monitor that reflects dietary change and nitrogen balance within days. This isotopic method may also be applied in forensic studies with regard to cases of deprivation, and starvation, and may be a method for investigating starvation in historic populations. Copyright © 2006 John Wiley & Sons, Ltd.