Part II presents the definition of orthorexia nervosa and a proposal of its new definition (‘Salussitomania’) due to the inaccurate etymology of the term Orthorexia Nervosa. It includes diagnostic criteria sets for orthorexia nervosa (proposed by Setnick, 2013; Moroze et al., 2015; Barthels et al., 2015; Dunn and Bratman, 2016) to depict key features of orthorexia nervosa, global study distribution on orthorexia nervosa as well as the divergence and overlap of orthorexia nervosa and other mental disorders, namely anorexia nervosa, obsessive-compulsive disorder and avoidant/restrictive food intake disorder, to enable a differential diagnosis. A summation of the highlights is included at the end of this chapter. The commentaries of the invited international experts (Dr Caterina Novara, University of Padova, Italy and Dr Hana Zickgraf, Rogers Behavioral Health, USA) provide valuable insights on orthorexia nervosa.

Self-esteem is a global, one-dimensional and relatively stable construct regarding a positive or negative judgment towards oneself (Rosenberg, 1965). There is evidence that low self-esteem is associated with unhealthy eating behaviours. The recent meta-analysis of longitudinal studies (Krauss et al., 2023), includingdata from more than 19,000 participants, has found that self-esteem negatively predicted total eating pathology over time and total eating pathology negativelypredicted self-esteem over time. It has been suggested that increased self-esteem regarding adherence to the diet characterises individuals with orthorexianervosa (Bratman & Knight, 2000). Therefore, it is essential to know if this factor might influence the development of orthorexia nervosa. Thus far, few studies have focused on the relationship between orthorexia nervosa and self-esteem. Moreover, there are contradictory results (Table 8.1). Self-esteem was found generally to be unrelated to orthorexia nervosa among university students of health education in Turkey (Özenoğlu & Ünal, 2015), adults in Australia (Barnes & Caltabiano, 2017), adults in Turkey (Yılmaz & Dundar, 2022), and college students in the United States of America (Oberle, Samaghabadi & Hughes, 2017). However, two studies have found that both higher trait self-esteem (Haddad et al., 2019) and higher state self-esteem (Sfeir et al., 2022) were linked to lower levels of orthorexia nervosa in a representative sample of the Lebanese population. Another study has also demonstrated that having high self-esteem reduces the tendency to healthy eating obsession (Yılmaz & Dundar, 2022).

Over the past decades, there has been an upsurge of interest in health and health behaviours. More precisely, this interest and efforts to change health behaviours emerged from the mid twentieth century onwards (Armstrong, 2009). Health behaviours (or health-related behaviour) have been defined as ‘personal attributes such as beliefs, expectations, motives, values, perceptions, and other cognitive elements; personality characteristics including affective and emotional states and traits and overt behaviour patterns, actions, and habits that relate to health maintenance, to health restoration, and health improvement’ (Gochman, 1997, p. 3). This means that not only observable, overt actions but also mental events and emotional states can be reported and measured. Health behaviours include a set of actions related to nutrition, addic-tive substances, movement and physical exercises (Havigerová, Dosedlová & Burešová, 2018).

Diet is one of the ‘big three’ modifiable health behaviours (together with sleep and physical activity) (Wickham et al., 2020). The World Health Organization (WHO, 2020) defines a healthy diet as achieving energy balance, limiting energy intake from total fats, free sugars and salt and increasing consumption of fruits and vegetables, legumes, whole grains and nuts. Regular consumption of a wide variety of foods from key food groups in the right proportions and consuming the right amount of food and drink are conducive to achieving, improving, enhancing and maintaining a healthy body weight (National Health Service (NHS), 2022) and health by reduction of the risk of chronic illness (WHO, 2020).

Pathogenesis of orthorexia nervosa is hitherto an under-researched field; thus, it urgently needs exploration. Unfortunately, no research indicates which factors cause orthorexia nervosa. Therefore, the identification of potential risk factors of orthorexia seems to be warranted (Bundros et al., 2016). The socio-cultural context related to social media’s influence on orthorexia nervosa has been explored for several years, mainly using qualitative research methods (e.g. Fixsen, Cheshire & Berry, 2020; Scheiber, Diehl & Karmasin, 2023; Santarossa et al., 2019; Turner & Lefevre, 2017; Valente et al., 2022a, 2022b; White et al., 2021). The popularity of orthorexia nervosa on social media platforms, such as Instagram and Twitter, is perceptible (Valente et al., 2022b). To date, there are over 194,000 posts with the hashtag (#) orthorexia on Instagram (as of 6 December 2023).

Several dietary patterns (e.g. the ‘prudent diet’ being high in fruit and vegetables, legumes, fish and whole grains, the ‘Western diet’ being higher in processed meats, red meats, French fries, sweets and desserts and refined grains) have been identified and described in the scientific literature up to now (Green, 2015). Previous research has established that individuals with orthorexia nervosa are less likely to consume saturated fats or animal fat products (Grammatikopoulou et al., 2018), products high in sugar, snacks, dressings and refined bread (Plichta & Jeżewska-Zychowicz, 2019; Plichta, Jeżewska-Zychowicz & Gębski, 2019). Daily bread consumption amount and carbohydrate intake in individuals with orthorexia nervosa were found to be significantly lower than in those without orthorexia nervosa (Yassıbaş & Gençer Bingöl, 2023). Moreover, the percentage of individuals who do not consume bread was approximately three times higher among individuals with orthorexia nervosa compared to those without orthorexia nervosa. In contrast, individuals with orthorexia nervosa are more likely to consume vegetables, fruits, nuts, legumes, seeds and meat (Plichta & Jeżewska-Zychowicz, 2019; Plichta, Jeżewska-Zychowicz & Gębski, 2019).

Part V focuses on research findings on orthorexia nervosa across two decades. It demonstrates the results of the studies conducted in non-clinical and clinical samples to provide the current knowledge on orthorexia nervosa. It focuses on college and university students and the general population as well as on patients with eating disorders, obsessive-compulsive disorder, somatic symptom disorder and depression. Research on orthorexia nervosa in non-Western societies, with the example of East Asian countries (Mainland China and South Korea), is also presented. A summation of the highlights is included at the end of this chapter. The commentary of the invited international expert (Dr Jinbo He, the Chinese University of Hong Kong, China) provide valuable insights on orthorexia nervosa.

• The definition and concept of healthy eating varies and changes, shaped by socio-cultural context (McDonald & Braun, 2022). In recent years, efforts have been made to define healthy and sustainable diets. A healthy diet should optimise health, defined broadly by the World Health Organization as complete physical, mental and social well-being, not just the absence of disease (Willett et al., 2019). A healthy diet emphasises the importance of having an appropriate calorie intake which consists of a variety of nutrient-dense foods and beve-rages including increasing intake of plant-based foods, consuming unsaturated rather than saturated or trans-fats and limiting the intake of animal source foods, refined grains, highly processed foods, added sugars, salt and foods of minimal nutritional value (Global Nutrition Report, 2020).

Part VI outlines several future directions for orthorexia nervosa, including a health-promotion perspective, a holistic approach to orthorexia nervosa treatment and indications of how specialists may assess orthorexia nervosa and the future way forward (conceptual, methodological and research issues). The commentaries of the invited international experts (Professor Phillipa Hay, Western Sydney University, Australia and Professor Thom Dunn, the University of Northern Colorado, USA) provide valuable insights on orthorexia nervosa.

Part III provides brief descriptions of the most frequently used self-reported questionnaires (ORTO-15, Eating Habits Questionnaire, Düsseldorf Orthorexia Scale, Teruel Orthorexia Scale, Barcelona Orthorexia Scale, Orthorexia Nervosa Inventory, ORTO-R, Orthorexia Self-Test), to outline those that seem to be suitable as self-assessment tools to measure orthorexia nervosa and those that are promising in research and clinical settings. It also proposes Orthorexia Nervosa Screening Questions to detect the risk of orthorexia nervosa. A summation of the highlights is included at the end of this chapter. The commentary of the invited international expert (Dr Adrian Meule, the University Hospital of the LMU Munich and the Schoen Clinic Roseneck, Germany ) provides valuable insights on orthorexia nervosa.

Orthorexia nervosa is a syndrome not yet recognised in major classifications such as the DM-5 or ICD-11. At first glance, it may not even appear problematic. Its core feature is the pursuit of ‘healthy’ eating. In the same way as exercise has many positive attributes and psychological effects, and indeed can be regarded as a form of an antidepressant, desiring a nutritious diet may be promoted as a means of improving mood and well-being. The Royal Australian and New College of Psychiatrists (RANZCP) in their most recent Clinical Practice Guideline (Malhi et al., 2021) for management of mood disorders indeed have a section recommending dietary practices to enhance other treatments for low mood. They state that ‘Diet with high proportions of vegetables, fruit, fish and grain but low animal fats appear helpful in depressive disorders’ (p. 38).