Some gut microbiota adaptations to starvation are adaptive, but others are counterintuitive and damage the host by reinforcing complicating factors of anorexia nervosa like metabolic dysregulation and comorbidities. With rapid diet changes, starvation, and food scarcity, microorganisms that can thrive in harsh environments with limited calories tend to thrive, and these microorganisms may contribute to the disorder's maintenance by influencing weight regulation and behavior. While further study is needed, mouse studies transplanting germ-free mice with stool samples from healthy controls and from children with acute protein-energetic malnutrition have indicated potentially causal relationships between the gut microbiome and body weight regulation, as those transplanted with the ill children's stool faced metabolic abnormalities and lost significant weight. Methanobrevibacter smithii (M. smithii), methanogenic archaeon was found to be increased in anorexia nervosa patients, with greater abundance being inversely associated with BMI. While methanogens optimize bacterial fermentation and energy extractions, high levels of M. smithii also cause slowed intestinal transit, which can contribute to constipation and complicate weight restoration efforts.