Background <p>Most people with anorexia nervosa (pAN) have digestive symptoms, which affect tolerance to nutritional treatment. Oligomeric oral nutritional supplements may be better absorbed and tolerated than polymeric ones. Our aim was to compare an oligomeric versus a polymeric nutritional supplement in pAN in terms of worsening of digestive symptoms after ingestion (primary objective) and in terms of sensory perception of the supplement (secondary objective).</p> Methods <p>A randomised, crossover, double-blind, controlled clinical trial was conducted. Inclusion criteria were: male or female aged 12–50 years with anorexia nervosa treated at the Eating Disorders Unit of the Hospital Regional Universitario de Málaga, Spain. Participants were randomised to first receive one container of the oligomeric or polymeric supplement. A digestive symptom questionnaire was administered, which they completed again after taking the supplement, together with a questionnaire for sensory evaluation of the supplement. One week later, they received a container of the other type of supplement, and the process was repeated. Analysis was performed on a per protocol basis.</p> Results <p>Ten people were randomised to the oligomeric-polymeric sequence and 12 to the polymeric-oligomeric sequence, of whom 9 and 11, respectively, finally received the intervention and were analysed. There were no statistically significant differences in digestive symptoms between the two supplements. The oligomeric one scored lower on taste and higher on acidity. The median (interquartile range) for oligomeric and polymeric were respectively: taste 6.5 (4), 8(3), <i>p</i> 0.03; acidity 0 (1), 0 (0), <i>p</i> 0.01.</p> Conclusions <p>In pAN, there is no difference in the worsening of digestive symptoms after ingestion of oligomeric versus polymeric supplements. Polymerics are perceived as having better taste and less acidity.</p> Trial registration <p>The trial is registered at ClinicalTrials.gov, number NCT05184738.</p>