Orthorexia nervosa (ON) is characterized by the pursuit of extreme dietary purity due to an exaggerated focus on food quality that could ultimately lead to a new kind of eating disorder. Even though researchers have tried to reach a univocal description of ON, to this date, there is no consensus on its diagnostic criteria, making it considerably more difficult to develop a valid questionnaire for assessing the symptoms of ON and to assess its actual prevalence. The aim of this review was to evaluate and gather scientific evidence about the prevalence of ON in both clinical and non-clinical adult populations, using the main validated scale for ON evaluation.

Electronic databases (PubMed, Scopus, and Web of Science) were reviewed to identify studies in accordance with PRISMA guidelines; at the end of the selection process, 62 studies were included.

Prevalence rates of ON vary greatly due to the differences in psychometric qualities of the tools used and the socio-cultural norms of the countries, with the lowest being obtained with the Dusseldorf orthorexic scale (DOS) (2.6% up to 36.7% in cancer survivor women) and the BOS-T (12.8% up to 34.7%), the greatest variability concerning the two thresholds of the ORTO-15 (14.6% with the >35 threshold and up to 86% with the >40 threshold) and the higher score being reported with the ORTO-11 in post-partum women (87.7%).

Additional research is necessary to support the development of a thorough, sensitive, and valid questionnaire for assessing the symptoms of ON.