What is anorexia?

Definition of Anorexia

Anorexia refers to two distinct clinical entities: (1) anorexia nervosa, a psychiatric eating disorder characterized by severe food restriction, intense fear of weight gain, and body image distortion; and (2) anorexia as a symptom, meaning loss of appetite that occurs across numerous medical conditions including infection, cancer, and chronic disease. 1

Anorexia Nervosa (The Psychiatric Disorder)

DSM-5 Diagnostic Criteria

According to the American Psychiatric Association, anorexia nervosa requires three core features 1:

• Persistent restriction of energy intake leading to significantly low body weight (below what is minimally expected for age, sex, developmental trajectory, and physical health) 1
• Intense fear of gaining weight or becoming fat, or persistent behavior that interferes with weight gain despite already being at significantly low weight 1
• Disturbance in body image perception, with undue influence of body shape and weight on self-evaluation, or persistent lack of recognition of the seriousness of current low body weight 1

Key Changes from Previous Diagnostic Criteria

The DSM-5 eliminated two previously required criteria 1:

• The 85% expected body weight threshold was removed 1
• Amenorrhea (loss of menstrual periods) is no longer required for diagnosis 1

Clinical Subtypes

Two distinct presentations exist 1:

• Restricting type: Weight loss achieved through dietary restriction, fasting, and/or excessive exercise 1
• Binge eating/purging type: Regular episodes of binge eating or purging behaviors (self-induced vomiting, laxative abuse, diuretic abuse) 1

Epidemiology

• Peak age of onset is early to mid-adolescence 1
• Lifetime prevalence in adolescent females is 0.3% 1
• Female-to-male ratio is 9:1, though increasing numbers of males are being recognized 1
• Hospitalizations for eating disorders in children younger than 12 years increased 119% from 1999 to 2006 1

Anorexia as a Symptom (Loss of Appetite)

Sickness-Associated Anorexia

Sickness-associated anorexia is the reduction in appetite seen during acute infection, representing an evolutionarily conserved response triggered by the same inflammatory cytokines that induce immune responses 1. This is not a disorder but rather a physiological adaptation to illness 1.

Cancer-Related Anorexia

In cancer patients, anorexia results from 2, 3:

• Cytokine-mediated hypothalamic disruption: Proinflammatory cytokines (IL-1, IL-6, TNF-α) alter central nervous system appetite regulation centers 2
• Altered hypothalamic neurotransmitter control rather than direct neurotransmitter depletion 2
• Tumor-derived factors that spill into systemic circulation and directly affect neuroendocrine control of appetite 2

Disease-Associated Anorexia

Anorexia represents a consistent clinical manifestation during acute and chronic pathophysiological processes including 4:

• Infection and inflammation 4
• Injury and toxins 4
• Immunological reactions 4
• Malignancy and necrosis 4

The mechanisms involve cytokine action on hypothalamic feeding centers, prostaglandin-dependent pathways, neurotransmitter system modifications, and gastrointestinal, metabolic, and endocrine factors 4.

Critical Distinctions for Clinical Practice

Anorexia Nervosa vs. Medical Anorexia

The key differentiator is intent and psychological features 1:

• Anorexia nervosa patients have deliberate food restriction driven by fear of weight gain and body image distortion 1
• Medical anorexia involves involuntary loss of appetite without the psychological features of eating disorders 4

Common Pitfall: Misdiagnosis

Patients presenting to gastroenterologists with weight loss and gastrointestinal symptoms often do not have the typical presentation of anorexia nervosa 1. Significant caution should be exercised to avoid escalating to invasive nutrition support in patients with functional symptoms, especially those with high or normal BMI, as this risks iatrogenesis without improving global function or quality of life 1.

Temporal Considerations

• Temporary anorexia during acute disease may be beneficial by restricting nutrients that support bacterial growth 4
• Long-term anorexia during chronic disease is deleterious and may lead to cachexia and death 4

Clinical Manifestations of Severe Anorexia

Regardless of etiology, severe anorexia leads to 5:

• Involuntary weight loss exceeding 5% over 6 months or 10% beyond 6 months 5
• BMI below 20 kg/m² in adults or below 18.5 kg/m² 5
• Visible skeletal structures with loss of muscle and adipose tissue 5
• Severe fatigue and reduced physical capacity 5
• Markedly reduced food intake 5