The degree and rapidity of weight loss, as a 20-pound loss in 2 months with a resulting BMI of 17.5 exceeds the threshold for ARFID and indicates anorexia nervosa as the more likely diagnosis.

Both AN and ARFID can result in clinically significant weight loss. The degree of loss does not differentiate the diagnoses — both can produce dangerously low BMI.

The presence or absence of body image disturbance and fear of weight gain, as this is the core DSM-5 criterion distinguishing the motivation for restriction between the two diagnoses.

The key DSM-5 differentiator is the motivation for restriction. AN requires fear of weight gain or persistent behavior interfering with weight gain, with disturbance in body weight/shape experience. ARFID features restriction driven by other factors — sensory sensitivity, fear of choking or vomiting, or lack of interest in food — without body image concern. Clinically, this distinction can be difficult — many patients with AN deny body image concerns, and the "marathon training" explanation may be a rationalization for restrictive behavior. Denial of body image concerns does not rule out AN, and collateral history, observation of eating patterns, and longitudinal assessment help clarify.

The BMI of 17.5, as ARFID patients typically maintain a BMI above the underweight threshold and significant underweight status with restrictive eating is diagnostic of anorexia nervosa specifically.

ARFID patients can be significantly underweight. The diagnosis includes nutritional deficiency or dependence on supplementation as a criterion. BMI alone does not differentiate these conditions.

The presence of excessive exercise, as compulsive exercise behavior is a specific feature of anorexia nervosa that is not seen in ARFID and distinguishes the two diagnoses in this clinical scenario.

Excessive exercise occurs in both AN and in athletes without eating disorders. It is not a reliable differentiator between AN and ARFID, though the motivation behind the exercise may be informative.