Low bone mineral density (BMD) is one of the most frequent complications of anorexia nervosa (AN). We report the clinical outcomes of a female patient with severe AN, whose chest had become deformed due to thoracic fracture. Lumbar BMD was 0.358 g/cm2 (T-score = -6.3), and total hip BMD was 0.411 g/cm2 (T-score = -4.4). Active vitamin D increased these parameters by 81.0% and 57.4%, respectively, but a drop in her nutrition status afterward resulted in a sharp decrease in BMD values. These findings suggest that adequate nutrient intake is essential for effective osteoporosis treatment in patients with AN.