Eating behavior in general is controlled by a plethora of factors: from genetic predisposition to environmental triggers and from psychological vulnerability to adverse life conditions, particularly during childhood. All these factors are interconnected and have been receiving attention as they presumably contribute to the development an eating disorders (EDs). In 2022 a report with alarming data was drawn up: 5.5–17.9% of young women and 0.6–2.4% of young men have experienced an ED during their lifetime. The challenge of this chapter, which describes genetic architectures together with biological data, will be to re-evaluate and refine diagnostic approaches of eating disorders, in particular anorexia nervosa (AN), the most studied one, which can lead to malnutrition and a series of physical and psychological complications. Indeed, despite the severe malnutrition associated with fasting and physical exercise abuse, AN patients show normal serum parameters used to assess the nutritional status (protein, albumin, prealbumin, and triacylglycerides). Understanding how the long-term starvation may influence not only anthropometric but also novel biochemical parameters is extremely important to develop a personalized treatment for AN based also on genetic risk.