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What causes atypical anorexia nervosa?

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Atypical anorexia nervosa (AAN) is a serious eating disorder that shares many of the same features of anorexia nervosa (AN), except one: extremely low body weight. An eating disorder is described as atypical if it has features that closely resemble one eating disorder but does not meet the precise diagnostic criteria of that disorder.

 minutes read
Last updated on 
October 27, 2022
In this article

Understanding atypical anorexia nervosa

Individuals who have AAN are usually within the “normal” weight range or at a higher weight. Because they appear to be “healthy,” and may even be praised for losing weight, they may not even realize they have AAN themselves and are often undiagnosed or misdiagnosed. This is an example of weight-bias at work in healthcare and eating disorder treatment and how misconceptions about weight, body size, and health can have a negative effect on health and access to care. (1,2)

But because atypical anorexia shares many of the same features as anorexia, AAN can be just as serious and life threatening as “typical” anorexia.

To recognize atypical forms of eating disorders, it’s essential to first understand the typical conditions. Anorexia nervosa has a high mortality rate. (3) Someone who has anorexia restricts eating severely and loses an extreme amount of weight, resulting in a dangerously low body weight. 

This starvation results in malnutrition and serious physical consequences, including heart problems, endocrine problems, digestive disorders, and long-term reproductive disorders–even organ failure and death. There is a large psychological component to the disorder as well, with a high risk of suicide. Even when the patient’s weight has stabilized, it does not mean their mental state has stabilized. (4) Relapse is common, with rates between 23 and 33%, and treatments are often ineffective. (3,4)

Atypical anorexia nervosa was added to the fifth edition of the Diagnostic and Statistical Manual of Mental Disorders (DSM-5) in 2013. (5) This text is the standard that all American psychiatrists use for diagnosing mental disorders.

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Comparing atypical anorexia to anorexia

People who have AAN and AN both severely restrict the calories they consume. They are also intensely afraid of gaining weight and may refuse to gain weight. Their self-worth is tied to their body shape and weight. They have a distorted body image. 

The main difference between the two disorders is that body weight may be in the normal range for AAN. It’s possible that someone with AAN may have had weight loss in the past that occurred over time instead of suddenly. 

In addition, the prevalence of atypical anorexia is much higher. AAN is found in up to almost 14% of females, and up to almost 5% in men. That’s a lot higher than the incidence of AN, which is about 4% in females and 0.3% in men. (6)

Symptoms of atypical anorexia nervosa

Like AN, symptoms of AAN may include:

  • Skipping meals or eating smaller portions of food
  • Drinking excess amounts of water or non-caloric beverages
  • Stomach cramps or other GI symptoms
  • Cutting out certain foods or groups of foods
  • Avoiding social events and functions that involve food
  • Feeling uncomfortable when others are in the same vicinity while eating

Causes of anorexia nervosa/atypical anorexia nervosa 

Anorexia and atypical anorexia, like all eating disorders, are very complex and have many possible causes, including biological and environmental. 

 Biological causes of AN may include:

  • Structural changes in the brain
  • Disordered reward and inhibitory systems in the brain
  • Genetics
  • Changes in gut microflora

Environmental factors that may cause AN include:

  • Personality traits
  • Family environment
  • Psychological states of mind
  • Stress and trauma
  • Peer pressure
  • Competition

Biological factors in anorexia nervosa

The biological factors that have been identified in AN are:

  • Changes in the brain’s gray matter in areas that control emotions, motivation, and goal-directed behavior. 
  • Disordered reward and inhibitory systems of the brain that result in compulsive behavior.
  • Genetics, with a high rate of heritability for disordered eating symptoms found in twins–up to 82% –and differences in serotonin and DNA methylation genes. (8,10,11,12)
  • Imbalance of bacteria levels in the gut due to malnutrition that correlate with anxiety, depression, obsession-compulsion, and immune reactions that disrupt hunger and satiety cues. (13,14,15)

While some of these factors may contribute to both anorexia and atypical anorexia, another study found no changes in gray matter in newly diagnosed AAN patients, which researchers believed was due to the different degree of weight loss in AAN patients. (16) 

However, more research is needed to explore the biological causes of atypical anorexia nervosa further.

Environmental factors of anorexia nervosa and atypical anorexia nervosa

Early research on anorexia attributed the environment as the biggest cause of eating disorders, including psychological, emotional, and societal  factors.  

Psychological factors for atypical anorexia

Personality traits, such as perfectionism and obsessive-compulsiveness, as well as emotional disorders, such as anxiety or depression, may lead to distorted thinking and behaviors that can cause AN. Being competitive, not being able to deal with conflict, and having low self-esteem are also contributing factors. And experiencing trauma, such as sexual, physical or emotional abuse, financial instability or poverty, death of a loved one, divorce, or discrimination of any kind, such as sex, gender, race, or weight, can lead to an eating disorder.

Both anorexia and atypical anorexia share some of these causes. Trauma often leads to AAN, just as it does for AN. The Adverse Childhood Experiences Questionnaire (ACE) measures the incidence of trauma an individual experiences during childhood. Adolescents who had high ACE scores (greater than or equal to 4) had more than five times higher odds of having AAN than those who had lower ACE scores in one study. There was no difference between groups on measures of low self-esteem and non-accidental self-injury. (17) 

Another study found no difference in the frequency of psychiatric comorbidities (43% vs 38% in AN) or suicide ideation (43% vs 39%). However, distress related to eating and body image was more severe in AAN. (19)

Social factors for atypical anorexia

Our society’s fixation on fitness and focus on appearance can contribute to the development of anorexia, atypical anorexia, and other eating disorders. Diet culture and weight stigma also play a big role in disordered eating. The pressure to conform, compete, be thin, and be perfect in many arenas–home, work, school, or social media, for example, can trigger disordered eating behaviors. Being in industries or occupations in which weight, body shape, and appearance are a focus may also contribute. These include flight attendants, pilots, modeling, acting, gymnastics, figure skating, wrestling, running, and ballet.

Beware of myths and misconceptions

Paradoxically, “atypical” anorexia nervosa is far more prevalent than anorexia nervosa. As cited above, AAN occurs in up to 14% of women and 5% of men vs. 4% and 0.3% occurrence of AN in men and women, respectively. 

So the label “atypical” may be a misnomer. This may be due to myths and misconceptions about weight and what a person with anorexia “should” look like. (1,2) That is, the commonly held belief that people with anorexia are typically young, emaciated, white and female. 

Since people with atypical anorexia are typically not thin, but of “normal,” or “straight” weight or higher, they may not realize they’re sick and healthcare professionals may not think they have anorexia. While they may lose weight, they still may not be very thin. Their efforts to lose weight may be praised as an attempt to get “healthier,” thanks to diet culture and weight stigma. But, weight is not an indicator of health. And, in reality, they are getting sicker. In fact, prescribing weight loss feeds the disorder.

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Treatment for atypical anorexia nervosa

No specific approach in treating AN has shown any clear superiority. (18) And there haven’t been any studies to date on AAN treatments to determine which works best. But treatment approaches to date have found combining re-nourishment and psychotherapy specific to AN can be effective.

Atypical anorexia is a very complex eating disorder that can have life-threatening implications. Weight stigma and misconceptions about what a person with anorexia “should” look like mean many people with atypical anorexia nervosa are not aware they have a problem and go undiagnosed or misdiagnosed for longer, which can make the condition even more dire. Raising awareness is an important part of diagnosis and treatment. But there is hope, and help is available. 
If you or someone you love is looking for clinically-superior, and compassionate care for an eating disorder, Within Health is here to help. Call our clinical care team to learn more about our virtual treatment programs for eating disorders.

Disclaimer about "overeating": Within Health hesitatingly uses the word "overeating" because it is the term currently associated with this condition in society, however, we believe it inherently overlooks the various psychological aspects of this condition which are often interconnected with internalized diet culture, and a restrictive mindset about food. For the remainder of this piece, we will therefore be putting "overeating" in quotations to recognize that the diagnosis itself pathologizes behavior that is potentially hardwired and adaptive to a restrictive mindset.

Disclaimer about weight loss drugs: Within does not endorse the use of any weight loss drug or behavior and seeks to provide education on the insidious nature of diet culture. We understand the complex nature of disordered eating and eating disorders and strongly encourage anyone engaging in these behaviors to reach out for help as soon as possible. No statement should be taken as healthcare advice. All healthcare decisions should be made with your individual healthcare provider.


  1. Puhl, R.M., Lessard, L.M., Himmelstein, M.S., Foster, G.D. (2021). The roles of experienced and internalized weight stigma in healthcare experiences: Perspectives of adults engaged in weight management across six countries. PLoS ONE 16(6): e0251566. https://doi.org/10.1371/journal.pone.0251566
  2. Puhl, R.M., Latner, J.D., King, K.M., Luedicke, J. (2014). Weight bias among professionals treating eating disorders: attitudes about treatment and perceived patient outcomes. (https://pubmed.ncbi.nlm.nih.gov/24038385/) International Journal of Eating Disorders. 47(1):65-75. https://doi.org/10.1002/eat.22186 
  3. Bulik, C.M., et al. Reconceptualizing anorexia nervosa. Psychiatry Clin Neurosci. 2019 Sep; 73 (9):518-525. https://pubmed.ncbi.nlm.nih.gov/31056797/
  4. Olivo, G., Gaudio, S., Schioth, H.B. Brain and cognitive development in adolescents with anorexia nervosa: A systematic review of fMRI studies. Nutrients. 2019 Aug 15;11(8):1907. https://pubmed.ncbi.nlm.nih.gov/31443192/ 
  5. Moskowitz, L., Weiselberg, E. Anorexia nervosa/atypical anorexia nervosa. Current Problems in Pediatric and Adolescent Health Care. April 2017; 47(4), pg 70-84. https://www.sciencedirect.com/science/article/abs/pii/S1538544217300470 
  6. Van Eeden, A.E., van Hoeken, D, Hoek, H.W. Incidence, prevalence and mortality of anorexia nervosa and bulimia nervosa. Curr Opin Psychiatry. 2021 Nov 1;34(6):515-524. https://pubmed.ncbi.nlm.nih.gov/34419970/ 
  7. O’Brien, K.M., et. al. Predictors and long-term health outcomes of eating disorders. PLoS One. 2017 Jul 10;12(7):e0181104. https://pubmed.ncbi.nlm.nih.gov/28700663/ 
  8. Thornton, L.M., Mazzeo, S.E., Bulik, C.M. The heritability of eating disorders: methods and current findings. Curr Top Behav Neurosci. 2011: 6: 141-156. https://www.ncbi.nlm.nih.gov/pmc/articles/PMC3599773/
  9. Kipman, A., et al. Genetic factors in anorexia nervosa. Eur Psychiatry. 1999 Jul;14(4):189-98. https://pubmed.ncbi.nlm.nih.gov/10572347/
  10. Baker, J.H., et. al. Genetics of anorexia nervosa. Curr Psychiatry Rep. 2017 Sep 22;19(11):84. https://pubmed.ncbi.nlm.nih.gov/28940168/
  11. Booij, L., et. al. DNA methylation in individuals with anorexia nervosa and In matched normal-eater controls: A genome-wide study. Int J Eat Disord. 2015;48:874-82. https://pubmed.ncbi.nlm.nih.gov/25808061/ 
  12. Bulik, C.M., Kleiman, S.C., Yilmaz, Z. Genetic epidemiology of eating disorders. Curr Opin Psychiatry. 2016 Nov;29(6):383-388. https://www.ncbi.nlm.nih.gov/pmc/articles/PMC5356465/#R34
  13. Seitz J, et al. Gut feelings: How microbiota might impact the development and course of anorexia nervosa. Nutrients. 2020 Oct 28;12 (11): 3295. https://pubmed.ncbi.nlm.nih.gov/33126427/ 
  14. Borgo F, et al. Microbiota in anorexia nervosa: The triangle between bacterial species, metabolite and psychological tests. PLoS One. 2017 Jun 21;12(6):e0179739. https://pubmed.ncbi.nlm.nih.gov/28636668/
  15. Carbone EA, et al. A systematic review on the role of microbiota in the pathogenesis and treatment of eating disorders. Eur Psychiatry. 2020 Dec 16;64(1):e2. https://pubmed.ncbi.nlm.nih.gov/33416044/
  16. Olivo G, et al. Atypical anorexia nervosa is not related to brain structural changes in newly diagnosed adolescent patients. Internat J Eating Disord. Jan 2018, Vol. 50, No. 1, pg 39-45. https://onlinelibrary.wiley.com/doi/abs/10.1002/eat.22805
  17. Pauls A, et al. Psychological characteristics and childhood adversity of adolescents with atypical anorexia nervosa versus anorexia nervosa. Eating Disorders. October 2020. https://www.researchgate.net/publication/346395751_Psychological_characteristics_and_childhood_adversity_of_adolescents_with_atypical_anorexia_nervosa_versus_anorexia_nervosa
  18. Zipfel S, et al. Anorexia nervosa: Aetiology, Assessment, and treatment. The Lancet Psychiatry. Dec 2015, Vol. 2, Issue 12, pgs 1099-1111. https://www.sciencedirect.com/science/article/abs/pii/S2215036615003569
  19. Sawyer SM, et al. Physical and psychological morbidity in adolescents with atypical anorexia nervosa. Pediatrics. April 01, 2016, Vol. 137, (4). https://publications.aap.org/pediatrics/article-abstract/137/4/e20154080/81504/Physical-and-Psychological-Morbidity-in?redirectedFrom=fulltext


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Further reading

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