The diagnosis hinges on one number: body weight. Learn the anorexia versus bulimia split, the physical signs, refeeding syndrome, and the two drugs the clinical medicine love (fluoxetine in, bupropion out).
The Setup
It All Comes Down to Weight
Both anorexia and bulimia can binge and purge, so the behaviors do not separate them. The body weight does, and starvation physiology explains the exam.
Two 19-year-old women both binge and then vomit to compensate. The first has a BMI of 15.5 and a deep fear of gaining weight. The second has a BMI of 22 (normal), binges and purges several times a week, and judges herself almost entirely by her shape. Both have calluses on their knuckles.
What single feature best separates anorexia nervosa from bulimia nervosa here?
The whole topic hinges on one number: body weight. Both anorexia and bulimia can involve bingeing and purging, so those behaviors do not separate them. Anorexia nervosa is defined by a significantly low body weight from restricted intake, an intense fear of gaining weight, and a distorted body image. Bulimia nervosa occurs at a normal or higher weight, with recurrent binges followed by compensatory behavior. The scale decides the diagnosis.
Starvation explains the physical exam. When the body is starved, it slows down to conserve energy: the heart rate drops (bradycardia), blood pressure and temperature fall, and the body grows fine downy hair (lanugo) to hold warmth. Sex hormones shut down, so periods stop (amenorrhea) and bone thins (osteoporosis). Anorexia carries the highest mortality of any psychiatric disorder, from cardiac complications and suicide.
Three core diagnoses. Flip each card.
Anorexia nervosaTap to flip
Low weightRestricted intake leading to significantly low body weight, intense fear of weight gain, and distorted body image. Subtypes: restricting, or binge-eating and purging. Severity is graded by BMI. Highest mortality of any psychiatric illness.
Bulimia nervosaTap to flip
Normal weightRecurrent binges plus compensatory behavior (vomiting, laxatives, fasting, or over-exercise) at least once a week for 3 months, with self-worth tied to shape and weight, at a normal or higher weight.
Binge-eating disorderTap to flip
Most commonRecurrent binges with a sense of loss of control, at least once a week for 3 months, but no compensatory behavior. The most common eating disorder, often with obesity. First-line: CBT; lisdexamfetamine is approved.
At the Bedside
What the Body Records
Lanugo, Russell's sign, parotid swelling, dental erosion, and a hypokalemic alkalosis each tell you what the patient has been doing.
The body keeps a record of the behaviors. Tap each physical sign to see what it reveals.
Lanugo
Tap to reveal
Fine, downy body hair that grows when the starved body tries to retain heat. A sign of significant undernutrition, classic for anorexia nervosa.
Russell's sign
Tap to reveal
Calluses or scarring on the knuckles from the teeth during self-induced vomiting. It points to recurrent purging, seen in bulimia and in the binge-purge subtype of anorexia.
Parotid swelling
Tap to reveal
Painless enlargement of the salivary glands (chipmunk cheeks) from repeated vomiting. A clue to purging behavior.
Dental erosion
Tap to reveal
Acid from vomiting strips enamel off the inner surfaces of the teeth, causing erosion and cavities. Another marker of chronic purging.
Bradycardia and low temperature
Tap to reveal
A slow heart, low blood pressure, and low body temperature from the starved body conserving energy. Severe bradycardia is a key reason to hospitalize in anorexia.
Hypokalemic metabolic alkalosis
Tap to reveal
Repeated vomiting loses stomach acid and potassium, producing low potassium with a high bicarbonate. The low potassium can trigger dangerous arrhythmias and a prolonged QT.
Compare the two that get confused.
Feature
Anorexia nervosa
Bulimia nervosa
Body weight
Significantly low
Normal or higher
Core fear
Gaining weight; distorted body image
Self-worth tied to shape and weight
Vital signs
Bradycardia, hypotension, hypothermia
Often normal
Classic labs
Low gonadotropins, leukopenia, low T3
Hypokalemic metabolic alkalosis from vomiting
Mortality
Highest of any psychiatric disorder
Lower, but arrhythmia risk from electrolytes
Confirming It
Sorting by Behavior and Weight
The forks come down to weight, refeeding safety, and the contraindicated drug. Get those and the management is clear.
Three forks decide the management calls. Work each before revealing it.
A patient binges and purges several times a week but maintains a completely normal body weight. Which diagnosis fits?
Bulimia nervosa. Binge plus compensatory purging at a normal weight is bulimia. The same behaviors at a significantly low weight would instead be the binge-purge subtype of anorexia. Normal weight plus binge-purge equals bulimia.
A severely underweight teen with anorexia is admitted and aggressive refeeding is started. On day 3 she becomes weak with cardiac arrhythmia. Which electrolyte abnormality is the hallmark?
Hypophosphatemia is the hallmark of refeeding syndrome. As carbohydrates return, an insulin surge drives phosphate (and potassium and magnesium) into cells, depleting serum levels and risking cardiac and respiratory failure. Refeed slowly and replete phosphate. Refeeding syndrome: watch and replace phosphate.
Which antidepressant is contraindicated in patients with eating disorders that involve purging?
Bupropion is contraindicated because it lowers the seizure threshold, and patients with purging-type eating disorders (often with electrolyte derangements) are at increased seizure risk. Fluoxetine is actually first-line for bulimia. Avoid bupropion in eating disorders; it lowers the seizure threshold.
The three diagnoses, lined up by behavior and weight.
Disorder
Binge?
Compensatory behavior?
Weight
Anorexia, restricting type
No
Restriction or exercise
Significantly low
Anorexia, binge-purge type
Yes
Yes
Significantly low
Bulimia nervosa
Yes
Yes
Normal or higher
Binge-eating disorder
Yes
No
Often higher
The Plan
Restore, Treat, and Avoid the Trap
Weight restoration leads in anorexia, family-based therapy for teens, CBT plus fluoxetine for bulimia, and never bupropion in a purging patient.
For anorexia, weight restoration comes first. The immediate priority is nutritional rehabilitation and weight restoration, done carefully to avoid refeeding syndrome. For adolescents, family-based therapy (the Maudsley approach) is first-line. SSRIs do not work well at very low weight, so they are not the answer in acute, underweight anorexia. Olanzapine can help with weight gain in some cases.
For bulimia, therapy plus a specific SSRI. Cognitive behavioral therapy is first-line, and the medication of choice is fluoxetine, the one SSRI approved for bulimia and used at a higher dose than for depression. For binge-eating disorder, CBT is first-line and lisdexamfetamine is approved.
Disorder
First-line treatment
Anorexia nervosa
Weight restoration first; family-based therapy for adolescents; olanzapine may aid weight gain
Bulimia nervosa
CBT plus fluoxetine (the SSRI of choice)
Binge-eating disorder
CBT; lisdexamfetamine is approved; an SSRI may help
From the AttendingHospitalize anorexia for medical danger, not just a low number: a heart rate under 40, dangerous electrolytes, unstable vitals, or roughly under 75 percent of expected body weight. Do not refeed too fast. Start low, advance slowly, and watch the phosphate, or you trade starvation for refeeding syndrome.
Board TrapBupropion is contraindicated in eating disorders with purging because it lowers the seizure threshold in patients already prone to electrolyte-driven seizures. If a bulimia stem offers bupropion, it is the wrong choice; the SSRI answer is fluoxetine. And the hallmark of refeeding syndrome is hypophosphatemia, not the sodium or calcium distractors.
Prove It
Board Walkthrough
Original clinical vignettes, 5 dealt per round, answer choices shuffled, never repeating within a round. Tap a wrong answer first to see why it almost works, then read the glowing clues.
Medically reviewed by Fatima Ali, DO and Kaitlyn Cocuzzo, MD · Last reviewed June 2026
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