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Purpose: Severe malnourishment may reduce cognitive performance in anorexia nervosa (AN). We studied cognitive functioning during intensive nutritional and medical stabilization in patients with severe or extreme AN and investigated associations between weight gain and cognitive improvement.

Methods: A few days after admission to a specialized hospital unit, 33 patients with severe or extreme AN, aged 16–42 years, completed assessments of memory, cognitive flexibility, processing speed, and attention. Mean hospitalization was 6 weeks. Patients completed the same assessments at discharge (n = 22) following somatic stabilization and follow-up up to 6 months after discharge (n = 18).

Results: The patients displayed normal cognitive performance at admission compared to normative data. During nutritional stabilization, body weight increased (mean: 11.3%; range 2.6–22.2%) and memory, attention, and processing speed improved (p values: ≤ 0.0002). No relationship between weight gain and cognitive improvement was observed at discharge or follow-up.

Conclusions: Cognitive performance at hospital admission was normal in patients with severe or extreme AN and improved during treatment although without association to weight gain. Based on these results, which are in line with previous studies, patients with severe or extreme AN need not be excluded from cognitively demanding tasks, possibly including psychotherapy. As patients may have other symptoms that interfere with psychotherapy, future research could investigate cognitive functioning in everyday life in patients with severe AN.

Trial registration number: The study is registered at clinicaltrials.gov (NCT02502617). Level of evidence: Level III, cohort study.