Biological Explanations of Anorexia

Biological Explanations of Anorexia Nervosa

Anorexia nervosa (AN) is a serious psychiatric condition characterised by severe food restriction, intense fear of weight gain, and distorted body image. It has one of the highest mortality rates of any psychiatric disorder — approximately 5–10% over ten years — and is highly treatment-resistant. Biological explanations propose that genetic, neurobiological, and evolutionary factors contribute to its aetiology, complementing psychological and social accounts.

This benchmark examines scientific explanations of anorexia nervosa for educational purposes. If you or someone you know is affected by disordered eating, please speak with a healthcare professional or contact a relevant support service.

Genetic Factors

Twin studies provide the strongest evidence for a genetic contribution to anorexia nervosa. Wade et al. (2000) found concordance rates of approximately 58% in monozygotic (MZ) twins compared with 11% in dizygotic (DZ) twins — a pattern strongly suggestive of genetic influence. Bulik et al. (2006) estimated heritability of anorexia nervosa at approximately 56–74%, indicating that genetic factors explain more than half the variation in liability. As with schizophrenia, AN is not caused by a single gene — it is polygenic and highly complex. Genome-wide association studies (GWAS) are in early stages; a 2019 GWAS (Watson et al.) identified a significant locus on chromosome 12, near genes involved in metabolic regulation, suggesting AN has both psychiatric and metabolic genetic components.

AN shows familial aggregation: first-degree relatives of individuals with AN have approximately 11 times the risk of developing AN compared with the general population, and elevated rates of other eating disorders, anxiety disorders, and OCD — suggesting shared genetic liability across conditions.

Neural Explanations: Serotonin

Serotonin (5-HT) dysregulation is strongly implicated in anorexia nervosa. Kaye et al. (1998, 2005) found that individuals with AN, both during the ill state and after weight restoration and recovery, show elevated serotonin activity in the brain — particularly elevated 5-HT2A and 5-HT1A receptor alterations. Elevated serotonin activity is associated with anxiety, harm avoidance, and inhibitory behavioural control. Kaye proposed that food restriction may serve as a self-regulatory behaviour: eating increases blood tryptophan (the serotonin precursor), which raises brain serotonin; restricting food intake lowers tryptophan and brain serotonin, reducing anxiety and emotional arousal. This creates a vicious cycle — restriction reduces anxiety, reinforcing further restriction. The observation that serotonin alterations persist after recovery (and are thus not simply consequences of malnutrition) suggests they represent a pre-existing neurobiological vulnerability.

Neural Explanations: Dopamine and Reward

Frank et al. (2005) found altered dopamine functioning in AN, with evidence of enhanced dopamine D2 receptor activity in the caudate nucleus. This region is involved in reward processing and habit formation. In healthy individuals, food is a primary biological reward — it activates the mesolimbic dopamine system. In AN, there may be altered reward processing such that food does not activate the reward system normally — or weight loss itself becomes rewarding. Frank et al. proposed that in AN patients, restricting food triggers dopaminergic reward rather than distress, potentially explaining why starvation is experienced as rewarding or neutral rather than aversive.

Evolutionary Explanation: Adapted to Flee Famine

Guisinger (2003) proposed an evolutionary account — the 'adapted to flee famine' hypothesis: AN symptoms (hyperactivity, food restriction, denial of hunger) may represent an ancestral adaptive response to famine conditions. When food is scarce, surviving individuals would benefit from increased activity (to migrate to better food sources) and reduced appetite (to avoid wasting energy seeking food that does not exist). AN may represent a maladaptive triggering of this 'flee famine' response in modern conditions where food restriction is social or psychological rather than environmental.

 Key Takeaways

  • Twin studies (Wade et al., 2000): MZ concordance ~58% vs DZ ~11%. Bulik et al. (2006): heritability ~56–74%. AN is polygenic — no single 'AN gene'.
  • Familial aggregation: first-degree relatives have ~11× elevated AN risk, plus elevated rates of anxiety, OCD — shared genetic liability across conditions.
  • Serotonin dysregulation (Kaye et al., 1998, 2005): elevated 5-HT activity → anxiety; food restriction lowers tryptophan → reduces serotonin → reduces anxiety. Restriction becomes reinforcing via anxiety reduction.
  • Serotonin alterations persist after recovery — not just consequences of malnutrition. Likely represent a pre-existing neurobiological vulnerability.
  • Dopamine and reward (Frank et al., 2005): altered D2 receptor activity in caudate nucleus — food restriction may activate reward rather than distress. Weight loss itself becomes rewarding.
  • Guisinger (2003) — adapted to flee famine: AN symptoms (hyperactivity, food restriction, denial of hunger) may be a maladaptive triggering of an ancestral famine-response in a modern context.