What are biological causes of anorexia?
There is evidence for biological, psychological, developmental, and sociocultural risk factors, but the exact cause of eating disorders is unknown.
Biological causes may include:
Genetics: anorexia nervosa is highly heritable that even when twins are raised apart when one is shown to be anorexic, the other is as well.
Obstetric complications (during pregnancy until 6 weeks old): complications may factor into the development of anorexia nervosa, such as maternal anemia, diabetes mellitus, preeclampsia, placental infarction, and neonatal cardiac abnormalities.
Neuroendocrine dysregulation: the problem between the nervous stimulation and endocrine secretion may contribute to the anorexia nervosa by disrupting the regulation of hunger and satiety.
Infections: Some people are hypothesized to be anorexic as a reaction to a streptococcus or mycoplasma infection, especially in children.
Gastrointestinal diseases: people with gastrointestinal disorders may be more risk of developing disorders eating practices than the general population.
Those reasons stated above are only for biological causes. You may read more here: https://en.wikipedia.org/wiki/Anorexia_nervosa#Causes
Hope this helps! :-)