Orthorexia nervosa 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 people's 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. 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.

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