The purpose of this research study is to analyze the microorganisms residing in the gut of patients with anorexia nervosa. Research has begun to link changes in the intestinal microbiota with diseases such as inflammatory bowel disease (IBS), asthma, and obesity, but although some studies have investigated the intestinal microbiota in overweight/obese individuals, very little is known about the intestinal microbiota in underweight individuals. The investigators aim to identify the enteric bacterial groups associated with adiposity, BMI, anxiety, and stress in patients with anorexia nervosa.