Everything You Need to Know About Eating disorders (AN, BN, BED) for Step 1

Eating disorders show up on Step 1 the way they show up in real life: often hidden, often denied, and packed with physiology, endocrinology, and psych clues. If you can quickly distinguish anorexia nervosa (AN) vs bulimia nervosa (BN) vs binge-eating disorder (BED)—and pair each with its hallmark labs, complications, and best treatments—you’ll pick up easy points on questions that try to distract you with weight, vitals, or electrolytes.

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Why Step 1 Cares (and How Questions Are Written)

USMLE loves eating disorders because they integrate:

• Psych diagnosis (behavior + body image + duration)
• Physiology (starvation vs purging)
• Electrolytes/acid–base (vomiting vs laxatives vs diuretics)
• Cardiac risks (arrhythmias, QT prolongation)
• Endocrine/repro (amenorrhea, osteoporosis)
• Treatment nuance (CBT, SSRIs, medical stabilization)

Classic stems include: teenage girl + athletics/dance, dental enamel erosion, parotid swelling, bradycardia/hypotension, or “normal BMI but…” purging.

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Quick Definitions (Know These Cold)

Anorexia Nervosa (AN)

Restriction of energy intake → significantly low body weight, plus:

• Intense fear of gaining weight or persistent behavior preventing weight gain
• Distorted body image / undue influence of weight on self-evaluation

Subtypes:

• Restricting type
• Binge-eating/purging type (important: still low weight)

Bulimia Nervosa (BN)

• Recurrent binge eating + compensatory behaviors (vomiting, laxatives, diuretics, excessive exercise)
• At least 1×/week for 3 months
• Self-evaluation overly influenced by body shape/weight
  Key: weight is usually normal or overweight.

Binge-Eating Disorder (BED)

• Recurrent binge eating with loss of control
• No compensatory behaviors
• At least 1×/week for 3 months
• Often associated with obesity, guilt, distress

Step trick: If there’s purging/excessive exercise → think BN (unless weight is significantly low → AN binge/purge subtype).

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Pathophysiology & Core Biology (What’s Actually Happening)

AN: Starvation physiology

• Hypothalamic suppression → ↓ GnRH → ↓ LH/FSH → amenorrhea (classically tested)
• Low leptin (low fat stores) → reinforces hypothalamic–pituitary suppression
• High cortisol (stress response)
• “Euthyroid sick” pattern: ↓ T3 (adaptive), normal/low T4, normal/low TSH
• Low insulin, low IGF-1 → impaired bone formation → osteopenia/osteoporosis

BN: Purging physiology

Primary issue is not starvation but binge–purge cycles, leading to:

• Electrolyte abnormalities (especially hypokalemia)
• Acid–base disturbances depend on method:
  • Vomiting → metabolic alkalosis
  • Laxatives → metabolic acidosis (often non-anion gap)

BED: Metabolic consequences

No purging → fewer electrolyte emergencies, but:

• Obesity-associated risks: insulin resistance, dyslipidemia, fatty liver disease, HTN, OSA

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Clinical Presentation: How They Look on Test Day

High-yield AN findings

• Low BMI, weight loss, restricting, excessive exercise
• Bradycardia, hypotension, hypothermia
• Lanugo, dry skin, hair loss
• Amenorrhea/infertility, low libido
• Osteopenia/osteoporosis, stress fractures
• Constipation, early satiety
• Depression/anxiety, perfectionism, obsessive traits

Danger zone: arrhythmias (electrolytes), severe malnutrition, suicidality.

High-yield BN findings

• Often normal weight
• Recurrent binge episodes + compensatory behaviors
• Parotid gland enlargement
• Dental enamel erosion (perimylolysis)
• Calluses on knuckles (Russell sign)
• Esophagitis, GERD, Mallory–Weiss tears

High-yield BED findings

• Recurrent binges with distress
• No purging
• Frequently overweight/obese
• Psychiatric comorbidity: depression, anxiety

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Diagnostic Criteria & Distinguishing Features (Table)

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Labs & Acid–Base: A Favorite USMLE Playground

Vomiting (common in BN; also AN binge/purge subtype)

Loss of gastric HCl → metabolic alkalosis:

• ↑ pH, ↑ $\text{HCO}_3^-$
• Hypochloremia
• Hypokalemia (via aldosterone + renal K wasting)

Laxative abuse

Loss of $\text{HCO}_3^-$-rich intestinal fluid → metabolic acidosis (often non-anion gap):

• ↓ pH, ↓ $\text{HCO}_3^-$
• Hypokalemia can still occur from GI losses

Diuretic abuse

Mimics loop/thiazide effects → metabolic alkalosis, hypokalemia.

Step 1 pro tip: If the stem screams BN and they show metabolic alkalosis + hypokalemia + hypochloremia, vomiting is the likely method.

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Complications You Must Know

AN complications (high yield)

• Cardiac: sinus bradycardia, hypotension, QT prolongation (esp. if electrolytes off), sudden death
• Bone: osteopenia/osteoporosis (low estrogen + low IGF-1)
• Heme: anemia, leukopenia (marrow suppression)
• Renal: dehydration, prerenal azotemia
• Neuro: seizures (severe electrolyte derangements)
• GI: delayed gastric emptying, constipation

Refeeding syndrome (very Step 1)

After reintroducing nutrition in a severely malnourished patient → insulin surge drives electrolytes into cells:

• Hypophosphatemia (key), hypokalemia, hypomagnesemia
• ↓ ATP → arrhythmias, heart failure, respiratory failure, rhabdo
• Treat/prevent with slow refeeding + electrolyte repletion (especially phosphate) + thiamine as indicated

BN complications

• Dental: enamel erosion, caries
• ENT: parotid hypertrophy
• GI: esophagitis, Mallory–Weiss tear, rare rupture
• Electrolytes: hypokalemia → arrhythmias

BED complications

• Type 2 diabetes, dyslipidemia, NAFLD, HTN, OSA
• Depression/anxiety comorbidity

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Differential Diagnosis (Common Traps)

• Avoidant/Restrictive Food Intake Disorder (ARFID): restriction without body image distortion (sensory issues, fear of choking, etc.).
• Major depressive disorder: decreased appetite/weight loss but no fear of weight gain/body image distortion.
• Hyperthyroidism: weight loss with heat intolerance, tremor, tachycardia (AN is often bradycardic/hypothermic).
• GI disease/malignancy: unintended weight loss—USMLE may test “rule out medical causes,” especially if older.

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Treatment: What to Do (and What Not to Miss)

First priority: medical stabilization (especially AN)

Treat emergent issues first:

• Vitals, orthostasis, dehydration
• Electrolytes (K, Mg, Phos), EKG (QTc)
• Inpatient if severe malnutrition, unstable vitals, suicidality, severe electrolyte abnormalities

AN treatment (core)

• Nutritional rehabilitation + psychotherapy
  • Family-based therapy is high yield in adolescents
  • CBT can help, but weight restoration is primary
• Meds: not primary; SSRIs may help comorbid depression/anxiety but often less effective when severely underweight.

BN treatment (high yield)

• CBT is first-line psychotherapy
• SSRIs (fluoxetine) are first-line pharmacotherapy
• Address dental/GI complications and correct electrolytes

BED treatment

• CBT and lifestyle interventions
• Consider pharmacotherapy in selected cases (often tested conceptually rather than by brand names)
• Treat comorbid mood/anxiety disorders

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High-Yield Associations & Classic Vignettes

AN associations

• Athletics/dance/gymnastics culture
• Perfectionism, obsessionality
• Amenorrhea + osteoporosis (hypogonadotropic hypogonadism physiology)

Vignette cue: “17-year-old runner, BMI 15, bradycardia, lanugo, denies seriousness.”

BN associations

• Normal BMI but intense shame and compensatory behavior
• Dental enamel erosion + parotid enlargement
• Hypokalemia → muscle weakness, arrhythmias

Vignette cue: “College student, normal weight, recurrent binges, uses laxatives, swollen cheeks, low K.”

BED associations

• Obesity + distress about bingeing + lack of control
• No purging behaviors

Vignette cue: “Middle-aged patient, BMI 35, eats large amounts rapidly when stressed, feels guilty, no vomiting.”

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First Aid Cross-References (Where This Lives)

Exact page numbers vary by edition, but you’ll find these topics clustered in:

• First Aid (Psychiatry)
  • Eating disorders: AN vs BN (clinical features, labs, complications)
  • Behavioral science/psych: CBT, SSRIs (fluoxetine for BN)
• First Aid (Endocrine/Repro)
  • Functional hypothalamic amenorrhea physiology (↓ GnRH → ↓ LH/FSH)
  • Osteoporosis risk with low estrogen states
• First Aid (Renal/Acid–Base)
  • Metabolic alkalosis from vomiting/diuretics
  • Non-anion gap metabolic acidosis from diarrhea/laxatives
• First Aid (Biochemistry/Nutrition)
  • Refeeding syndrome and hypophosphatemia consequences

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Rapid Review: Step 1 “Must Memorize” Bullets

• AN = low weight + fear of weight gain + body image distortion; bradycardia, amenorrhea, osteoporosis, lanugo.
• BN = binge + purge, usually normal weight, parotid enlargement, enamel erosion, Russell sign.
• BED = binge without compensatory behaviors, often obesity/metabolic syndrome.
• Vomiting/diuretics → metabolic alkalosis + hypokalemia + hypochloremia.
• Laxatives → non-anion gap metabolic acidosis (think diarrhea physiology).
• Refeeding syndrome → hypophosphatemia → arrhythmias, respiratory failure; refeed slowly.