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What is Orthorexia nervosa and explain in detail?
Orthorexia nervosa  (also known as orthorexia) (ON) is a proposed eating disorder characterized by an excessive preoccupation with eating healthy food. The term was introduced in 1997 by American physician Steven Bratman, M.D. He suggested that some peoples dietary restrictions intended to promote health may paradoxically lead to unhealthy consequences, such as social isolation, anxiety, loss of ability to eat in a natural, intuitive manner, reduced interest in the full range of other healthy human activities, and, in rare cases, severe malnutrition or even death.In 2009, Ursula Philpot, chair of the British Dietetic Association and senior lecturer at Leeds Metropolitan University, described people with orthorexia nervosa as being "solely concerned with the quality of the food they put in their bodies, refining and restricting their diets according to their personal understanding of which foods are truly pure." This differs from other eating disorders, such as anorexia nervosa and bulimia nervosa, where those affected focus on the quantity of food eaten.Orthorexia nervosa also differs from anorexia nervosa in that it does not disproportionally affect one gender. Studies have found that orthorexia nervosa is equally found in both men and women with no significant gender differences at all. Furthermore, research has found significant positive correlations between ON and both narcissism and perfectionism, but no significant correlation between ON and self esteem. This shows that high-ON individuals likely take pride over their healthy eating habits over others and that is the driving force behind their orthorexia as opposed to body image like anorexia.
Orthorexia nervosa is not recognized as an eating disorder by the American Psychiatric Association, and so is not mentioned as an official diagnosis in the widely used Diagnostic and Statistical Manual of Mental Disorders (DSM).

Signs and symptoms
Symptoms of orthorexia nervosa include "obsessive focus on food choice, planning, purchase, preparation, and consumption; food regarded primarily as source of health rather than pleasure; distress or disgust when in proximity to prohibited foods; exaggerated faith that inclusion or elimination of particular kinds of food can prevent or cure disease or affect daily well-being; periodic shifts in dietary beliefs while other processes persist unchanged; moral judgment of others based on dietary choices; body image distortion around sense of physical "impurity" rather than weight; persistent belief that dietary practices are health-promoting despite evidence of malnutrition."

Cause
There has been no investigation into whether there may be a biological cause specific to orthorexia nervosa. It may be a food-centered manifestation of obsessive-compulsive disorder, which has a lot to do with control.

Diagnosis
In 2016, formal criteria for orthorexia were proposed in the peer-reviewed journal Eating Behaviors by Thom Dunn and Steven Bratman. These criteria are as follows:
Criterion A.  Obsessive focus on "healthy" eating, as defined by a dietary theory or set of beliefs whose specific details may vary; marked by exaggerated emotional distress in relationship to food choices perceived as unhealthy; weight loss may ensue, but this is conceptualized as an aspect of ideal health rather than as the primary goal.  As evidenced by the following:

Compulsive behavior and/or mental preoccupation regarding affirmative and restrictive dietary practices believed by the individual to promote optimum health. (Footnotes to this criteria add: Dietary practices may include use of concentrated "food supplements." Exercise performance and/or fit body image may be regarded as an aspect or indicator of health.)
Violation of self-imposed dietary rules causes exaggerated fear of disease, sense of personal impurity and/or negative physical sensations, accompanied by anxiety and shame.
Dietary restrictions escalate over time, and may come to include elimination of entire food groups and involve progressively more frequent and/or severe "cleanses" (partial fasts) regarded as purifying or detoxifying. This escalation commonly leads to weight loss, but the desire to lose weight is absent, hidden or subordinated to ideation about healthy food.Criterion B. The compulsive behavior and mental preoccupation becomes clinically impairing by any of the following:

Malnutrition, severe weight loss or other medical complications from restricted diet
Intrapersonal distress or impairment of social, academic or vocational functioning secondary to beliefs or behaviors about healthy diet
Positive body image, self-worth, identity and/or satisfaction excessively dependent on compliance with self-defined "healthy" eating behavior.A diagnostic questionnaire has been developed for orthorexia sufferers, similar to questionnaires for other eating disorders, named the ORTO-15. However, Dunn and Bratman critique this survey tool as lacking appropriate internal and external validation.

Epidemiology
Results across scientific findings have yet to find a definitive conclusion to support whether nutrition students and professionals are at higher risk than other population subgroups, due to differing results in the research literature. There are only a few notable scientific works that, in an attempt to explore the breadth and depth of the still vaguely-understood illness, have tried to identify which groups in society are most vulnerable to its onset. This includes a 2008 German study, which based its research on the widespread suspicion that the most nutritionally-informed, such as university nutrition students, are a potential high-risk group for eating disorders, due to a substantial accumulation of knowledge on food and its relationship to health; the idea being that the more one knows about health, the more likely an unhealthy fixation about being healthy can develop. This study also inferred that orthorexic tendencies may even fuel a desire to study the science, indicating that many within this field might suffer from the disorder before commencing the course. However the results found that the students in the study, upon initial embarkation of their degree, did not have higher orthorexic values than other non-nutrition university students, and thus the report concluded that further research is needed to clarify the relationship between food-education and the onset of ON.Similarly, in a Portuguese study on nutrition tertiary students, the participants orthorexic scores (according to the ORTO-15 diagnostic questionnaire) actually decreased as they progressed through their course, as well as the overall risk of developing an eating disorder being an insignificant 4.2 percent. The participants also answered questionnaires to provide insight into their eating behaviours and attitudes, and despite this study finding that nutrition and health-science students tend to have more restrictive eating behaviours, these studies however found no evidence to support that these students have "more disturbed or disordered eating patterns than other students" These two aforementioned studies conclude that the more understanding of food one has is not necessarily a risk factor for ON, explaining that the data gathered suggests dietetics professionals are not at significant risk of it.
However, these epidemiologic studies have been critiqued as using a fundamentally flawed survey tool that inflates prevalence rates. Scholars have questioned both the reliability and validity of the ORTO-15.Most scientific findings tend to agree, however, young adults and adolescents are extremely susceptible to developing eating disorders. One study found that there was no relationship between BOT score and college major, which may indicate the prevalence of mental health issues and eating disorders on college campuses and that health and science majors are no longer the only ones affected More studies have also been conducted on the link between increased Instagram use and Orthorexia nervosa. The social media based healthy community has recently grown in popularity especially on platforms such as Instagram. The hashtag #food is one of the top 25 most popular hashtags on Instagram. A study that investigated this relationship found that increased use of Instagram correlated between symptoms of ON with no other social media platform having the same effect. With young adults and adolescents making up the majority of social media users, exposure to this type of content can lead to developing unhealthy behavior.

History
In a 1997 article in the magazine Yoga Journal, the American physician Steven Bratman coined the term "orthorexia nervosa" from the Greek ορθο- (ortho, "right" or "correct"), and όρεξις (orexis, "appetite"), literally meaning correct appetite, but in practice meaning correct diet. The term is modeled on anorexia, literally meaning "without appetite", as used in the definition of the condition anorexia nervosa