When the Brain Overcomes Evolution: The Neural Mechanism Behind Anorexia Nervosa (by Danielle Steinbach)
Aug 07, 2025By Danielle Steinbach
If you have read my previous blogs, you may know that I often reflect on the ways our species has changed over the past hundreds of thousands of years of its existence. We’ve come a long way and morphed quite a bit from our ancestors, whose shelters were mere caves with walls illuminated by firelight. But there are some primitive threads within us that tether us to those old early humans of ages past. One of those primitive threads is the desire to eat, to stay alive.
Through millennia of evolution, we have retained fundamental neural feedback circuits that drive us to eat for the sake of keeping our body fat stores above a certain threshold. It’s a complex interplay between ghrelin, leptin, and dopamine that drives us to eat.
But every once in a while, something in that neural feedback circuit goes amiss. Sometimes – someway, somehow – the brain learns to reward starvation and fear the very thing that keeps us alive. Sometimes an entirely different feedback loop takes control inside the brain, one that rewards the continuous decline of fat stores to whatever end. It’s a strong and clever feedback loop. When this loop takes control in someone’s brain, it warps their perception of reality so that they don’t see their underweight body for what it is or pay attention to their freezing limbs, the loss of hair, the aging of their face, or the cessation of their menstrual cycle. It’s a neural circuit that turns the mirror into a funhouse mirror accentuating any small bit of fat.
And this is the nature of anorexia nervosa. It’s the reality of millions of people every day.
There’s something very unique about a psychiatric condition like that – there’s almost nothing like it. A condition that twists one’s understanding of the world so much that they believe they are thriving and happy when they are truly on the road to dying. It’s a cruel psychiatric disease that completely severs the connection between physiology and psychology, with the two becoming completely at odds so that one becomes satisfied with themselves as they become sicker and panics should they nourish themselves to become healthier.
It’s difficult to detect too because it often happens quite gradually. Very few people with anorexia simply wake up one morning finding themselves fearful of eating. It often begins innocently and insidiously with a decision to simply cut out a small set of certain foods deemed unhealthy.
Oh, I won’t have a slice of cake, thank you. I’m trying to cut out sugar.
Oftentimes, these changes truly are for good. Things like reducing one’s intake of processed sugars and altering one’s diet to maximize lean proteins that will keep one satiated for longer. But then, somewhere along the road, the restrictions start to accumulate.
No pasta for me, please. I don’t need the extra carbs.
I’m going to stop putting milk in my coffee because I don’t want to drink my calories.
I think I’ll have an apple instead of a banana because bananas are the most calorie-dense fruit.
I don’t think I need sauce on this chicken actually. It barely tastes any different dry, and I can’t count the calories in the sauce.
The rules and strategies pile up, and the purpose of everything in life slowly boils down to “How can I become thinner?”. It’s a slow mental spiral that leads the brain to override millions of years of evolution that drive us to seek out food and stability in our fat stores.
So where in the brain does it begin? And how? And why? And under what circumstances?
Well, like most human behaviors – both typical and atypical – it begins in the reward system. There’s no driver to action quite like dopamine.
In the case of anorexia, unfortunately, that dopamine signaling occurs in response to images of unhealthily thin individuals. A particular region in the brain has been isolated where this dopamine surge occurs: the ventral striatum. An area associated with decision-making and evaluating costs and rewards, the ventral striatum is the mass of tissue we hinge most of our choices on.
As the sightline of an anorexic patient narrows in on losing weight, stimuli that would typically cause excitement and joy – things like favorite foods, images of loved ones, being rewarded with money – are less likely to trigger a release of dopamine comparable to the average person. This suppressed dopaminergic response induces a state of anhedonia, a lack of pleasure even in the presence of pleasant stimuli. This is where the danger lies. When humans are deprived of dopamine, we often seek alternative ways to supply ourselves with that dopamine. In the case of someone suffering with anorexia, that dopamine rush is obtained through starvation – more specifically, through the thrill of watching the numbers of the scale creep ever lower. And so, those with anorexia often use starvation as a mechanism to find the elation and thrill that they don’t find in other things in life.
This anhedonia is why people recovered from anorexia often report that they weren’t really themselves while they were ill – it’s a disease that steals away your passion for common joys of life and overtakes your senses until all your focus is dedicated to dieting.
Functional magnetic resonance imaging (fMRI) studies also reveal lower levels of cortical connectivity and signaling in the brains of anorexic patients. Disparate brain regions need to be able to communicate with each other in a holistic system for proper decision-making that allows for self-inhibition and complete integration of sensory stimuli. Without this complete cortical connectivity, there is less inhibitory signaling to check the extreme reward felt in response to losing weight, and the feeling of starvation feels ever more euphoric.
Among these abnormalities in the reward system, people with anorexia have aberrant conditioning patterns that lead them to become fixated on certain actions that were once rewarded even if they are no longer currently being rewarded. In other words, what was once rewarded in the past will always be considered a “good action”, even if circumstances change such that this action becomes detrimental. And this is why so many cases of anorexia start off as diets that go entirely too far. For many people that develop anorexia, they begin with an initial weight loss or diet change that truly is beneficial for their health, but then they become addicted to the positive reward of seeing their body become ever thinner with their minds hard-wired to always view weight loss as a victory.
Beyond just the ventral striatum, however, there’s another player in the brain that contributes to the obsessive desire to diet seen in anorexia: there’s a dangerous convergence between hunger signaling and reward signaling that occurs in the insula. When presented with images of their underweight bodies, those with anorexia exhibit extremely high levels of activation in the insula. The insula has the dual role of controlling both motivation and introspection about internal states, like hunger signaling – this makes it a prime hub for misfiring in anorexia, where the perception of hunger drives further dieting, rather than re-feeding.
Of course, momentary feelings of reward can only override someone’s primal need for food for so long. There’s another pillar to the illness that allows someone to sustain self-inflicted starvation for years: fear. An overwhelming, unrelenting fear of gaining weight.
That’s where the amygdala comes in.
The amygdala – the fear hub.
While images of underweight bodies induce a spike of reward in anorexic individuals, images of themselves at a heavier weight trigger frantic firing in the amygdala.
Even if there exists a subconscious fear of dying somewhere later on down the road from malnutrition, there’s a more immediate, louder fear of gaining weight that outcompetes it. This is how getting truly healthy and eating more keeps getting delayed, over and over again.
What’s even worse is that oftentimes the fear intensifies as the distortion of one’s self-perception becomes more severe. When asked to judge how large their bodies are, anorexic patients will drastically over-estimate their own size. This phenomenon can be traced back to the precuneus, a region responsible for self-conscious reflection and visual-spatial processing. Aberrant signaling in the precuneus is hypothesized to be responsible for the ‘funhouse mirror’ effect mentioned earlier, which leads someone with anorexia to perceive themselves to be much larger than they are.
Thus, one of the greatest difficulties with anorexia is finding a way to convince the patient that they are in fact losing too much weight, as people with this condition often simply cannot see how much weight they’ve lost. In many ways, the choice to begin eating more and recovering from anorexia is taking a leap of faith to trust that the eyes of your loved ones are more reliable than your own.
It’s a leap of faith that many people who have recovered from anorexia report to be terrifying.
Terrifying, but ultimately worth it.
Because when the brain is no longer preoccupied with resisting food, the thing we are so deeply hard-wired to seek out, there is room for so much more in the mind – hobbies, family, friends, academic goals, career aspirations, and everything else that there is to life.
– Danielle Steinbach
Sources:
https://www.ncbi.nlm.nih.gov/books/NBK92777/
https://www.nationaleatingdisorders.org/statistics/
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