Clinical Psychopharmacology · Paper A
Eating disorder epidemiology
Four eating disorders each have a distinct fingerprint of WHO gets them and WHEN. Anorexia nervosa strikes early (mid-teens), overwhelmingly female, and kills more often than any other psychiatric disorder. Bulimia nervosa strikes a little later, stays close to normal weight, and is more common than anorexia. Binge eating disorder is the most common of all, hits later still, in a far more even mix of men and women. ARFID breaks the whole pattern by starting in childhood, with no weight or shape fear at all.
The age-of-onset and sex-ratio differences are not arbitrary labels — they trace back to what actually drives each disorder. Anorexia nervosa and bulimia nervosa are both driven by a fear of weight gain and body image disturbance, which is why both cluster around puberty and adolescence, when body change and sociocultural pressure around thinness peak, and why both are heavily female-predominant. The difference between them is what the fear produces: anorexia nervosa produces sustained restriction severe enough to cause a genuinely low body weight, while bulimia nervosa produces a binge-purge cycle where compensatory behaviour offsets the binge calories, so weight stays normal. Binge eating disorder has no compensatory behaviour and no weight criterion at all, which both raises its prevalenceThe proportion of a population who have a condition at a given point or period in time.Read more → (a lower bar to meet) and flattens its sex ratio (it isn't gated by the female-skewed body-image drivers of the other two). ARFID is driven by something else entirely — sensory sensitivityThe proportion of people who truly have a condition that a test correctly identifies as positive.Read more →, fear of an aversive consequence, or simple lack of interest in food — with no body-image component. This is exactly why it starts in childhood, before body-image concerns are developmentally even possible, and why its sex ratio does not skew female.
If a vignette gives you a patient's age, sex, weight, and eating pattern and asks you to name the eating disorder, you are not being asked to memorise a list — you are being asked to recognise a pattern. Four disorders (anorexia nervosa, bulimia nervosa, binge eating disorder, and ARFID) cover the vast majority of exam questions on this topic, and each one has a recognisable epidemiological shape: a typical age when it starts, a typical sex it affects, and a typical relationship with body weight. Getting the pattern right, rather than reaching for whichever disorder is most famous, is what separates a confident answer from a guess.
Eating disorders as a group are conditions where a person's relationship with food, eating, or their own body becomes seriously disrupted, to the point of causing physical harm or major distress. Some of these disorders (anorexia nervosa, bulimia nervosa, atypical anorexia nervosa, purging disorder) are driven by an underlying fear of gaining weight and a disturbed sense of one's own body shape. Others (binge eating disorder, night eating syndrome) involve disordered eating behaviour but no such fear. And a third group (ARFID, pica, disorder) has nothing to do with weight or shape at all — the disturbance is in the relationship with food itself, for entirely different reasons. Keeping these three families separate in your head is the single most useful thing you can do before trying to memorise individual facts.
Anorexia nervosa is diagnosed when a person restricts their food intake to the point of a significantly low body weight, driven by an intense fear of gaining weight and a disturbance in how they perceive their own body (they may see themselves as overweight even when severely underweight). It typically starts young — commonly in the early-to-mid teenage years — and affects females far more often than males, with historically quoted ratios around 10:1, although male cases are increasingly recognised and are probably under-diagnosed because clinicians are less likely to suspect the diagnosis in a boy.
Bulimia nervosa involves recurrent episodes of binge eating (eating an unusually large amount of food in a short time, with a sense of losing control over the eating) followed by compensatory behaviour to avoid weight gain, most often self-induced vomiting, but sometimes laxative misuse or excessive exercise. The crucial difference from anorexia nervosa is body weight: because the compensatory behaviour offsets the extra calories from bingeing, a person with bulimia nervosa usually stays at a normal or even above-normal weight. It tends to start a little later than anorexia nervosa, typically in the late teens to early twenties, and remains strongly female-predominant, though the skew is somewhat less extreme than anorexia nervosa's.
Binge eating disorder is recurrent binge eating, exactly as in bulimia nervosa, but without any compensatory behaviour afterwards — no vomiting, no laxatives, no excessive exercise. Because there is no compensation and no weight criterion in the diagnosis at all, binge eating disorder ends up being the most common eating disorder overall. It also tends to start later in life than anorexia nervosa or bulimia nervosa, sometimes not appearing until well into adulthood, and its sex ratio is far more even than the other two — for example, a middle-aged man with recurrent binges and obesity is a plausible and common presentation.
ARFID (avoidant/restrictive food intake disorder) looks superficially like anorexia nervosa — a person eating far less than they need, sometimes with significant weight loss or, in children, failure to grow as expected — but the resemblance stops there. In ARFID there is no fear of weight gain and no body image disturbance whatsoever; the avoidance instead comes from one of three sources: extreme to a food's texture, smell or taste (sensory sensitivity); a specific fear that eating will cause harm, most often after a frightening experience like choking (fear of aversive consequences); or simply very little interest in food or eating at all.
Because none of these drivers depend on the body-image pressures that shape anorexia nervosa and bulimia nervosa, ARFID typically starts in childhood, well before adolescence, and its sex ratio is much more even — some studies of children even find slightly more boys than girls affected. It is also frequently seen alongside autism spectrum conditions, anxiety disorders, and gastrointestinal problems.
One more diagnosis deserves its own mention because it is a classic exam trap: atypical anorexia nervosa. Imagine a teenager who previously had a higher-than-average body weight, and who then loses a substantial proportion of that weight — driven by the exact same fear of weight gain and body image disturbance that defines anorexia nervosa — but because they started heavier, their resulting lands within, or even above, the normal range. Every psychological feature of anorexia nervosa is present; only the number on the scale looks reassuring. This is exactly why it is dangerous to rule anorexia nervosa out just because someone's weight looks fine: the medical risk tracks how much weight was lost and how fast, not the final BMI reached in isolation.
Why do these disorders cluster the way they do? Anorexia nervosa and bulimia nervosa share the same underlying psychological engine — an intense fear of weight gain paired with a distorted body image — and that engine tends to switch on during puberty and adolescence, when the body is changing rapidly, identity is forming, and sociocultural pressure around thinness (particularly for girls and young women) is at its most intense. That is why both disorders cluster in the teenage-to-young-adult years and why both skew heavily female.
What separates them is simply what the fear produces: in anorexia nervosa it produces sustained restriction severe enough to cause real starvation and a low body weight; in bulimia nervosa it produces a binge-purge cycle where the purging cancels out the binge's calories, so the person's weight settles back to something close to normal even though the eating pattern is just as disordered.
Binge eating disorder and ARFID both break away from this engine, but in different directions. Binge eating disorder keeps the binge-eating half of bulimia nervosa but drops the compensatory half and the weight criterion entirely — which is exactly why it is both more common (a much lower bar to meet) and much more evenly split between men and women (it is not gated by the female-skewed body-image drivers that shape anorexia nervosa and bulimia nervosa). ARFID has essentially nothing in common with the fear-of-weight-gain engine at all; its drivers — sensory aversion, specific fear, or low interest in food — are unrelated to body image, which is precisely why it can and does start in early childhood, long before a child could even have developed adult-style body-image concerns, and why its sex ratio does not skew female the way the body-image-driven disorders do.
Consider a 16-year-old girl who previously weighed enough to have a in the overweight range. Over eight months, driven by an intense fear of gaining weight, she loses 18% of her body weight. Her BMI on presentation is 21 kg/m^2 — comfortably within the normal range — but she has developed bradycardia and low blood pressure, and blood tests show early signs of the metabolic disturbance seen in starvation.
A clinician who stops at the thought 'normal , therefore not anorexia nervosa' will miss the diagnosis entirely. Working through it properly: is there a fear of weight gain and body image disturbance? Yes. Is there an objectively large binge? No. Is the weight low relative to what is expected for this specific person, given how much she has actually lost and how fast? Yes, even though the absolute BMI number looks reassuring. That combination is atypical anorexia nervosa — exactly as medically serious as classic anorexia nervosa, just easier to miss because the scale doesn't immediately raise alarm.
Management decisions in the UK follow NICE guideline NG69 (Eating disorders: recognition and treatment, published 2017), which sets out that any of these eating disorders should prompt assessment and referral to a specialist eating disorder service, and that treatment should not be delayed or withheld purely on the basis of body weight or — a principle that exists specifically because of cases like atypical anorexia nervosa, where a normal-looking weight can mask a genuinely dangerous illness. First-line treatments differ by diagnosis (for example, family-based treatment is typically first-line for children and young people with anorexia nervosa, while guided self-help or -based approaches are commonly used for bulimia nervosa and binge eating disorder), but the shared principle across all of them is that recognition should not wait for a low BMI to appear, because by the time it does, medical risk has often already accumulated.
A few pairs are worth pinning down explicitly. Bulimia nervosa versus purging disorder: both involve purging, but bulimia nervosa requires a true, objectively large binge beforehand, while purging disorder involves purging without ever having had a real binge — the purge exists on its own.
Binge eating disorder versus night eating syndrome: both can occur in adults with obesity and no compensatory behaviour, but binge eating disorder's episodes can happen at any time of day, while night eating syndrome's eating is specifically clustered around the sleep period, with the person eating late at night or after waking, fully aware of doing so, and then having a reduced appetite the next morning. ARFID versus pica versus disorder: all three lack any weight/shape driver, but ARFID is avoidance of ordinary food, pica is the eating of non-food substances (like chalk or soil), and rumination disorder is the repeated regurgitation and re-chewing of food that has already been swallowed.
Before moving on, try to answer this without looking back: a 45-year-old man reports several years of eating an unusually large amount of food several times a week, with a clear sense of losing control each time, but no vomiting, no laxative use, and no fasting afterwards. What is the diagnosis, and which two features of the story (rather than his sex) actually rule out bulimia nervosa?
Work out the driver (weight/shape fear or something else), the weight (low, or normal despite the same psychology), and whether a binge and a compensatory behaviour are present, in that order, and the right eating disorder falls out — rather than defaulting to whichever one is most familiar.