Eating Disorder and Fasting
A woman with a history of anorexia says that fasting on Yom Kippur can bring on a set of consecutive days of fasting that is very detrimental to her health. What should she do for fast days?
She needs to be professionally assessed by her doctor and then in consultation with a Rav determine if fasting poses a risk of relapse as you describe. If there is a fair chance it could cause something similar to the above it would classify as pikuach nefesh and she would be permitted to eat.
The option of eating “shiurim” should be examined and pursued if possible.