In 2019, twenty adults spent a month in a clinical research center run by the U.S. National Institutes of Health. For two weeks they ate mostly ultra-processed food, and for two weeks minimally processed food. Researchers deliberately designed the offered diets to be similar in calories, protein, fat, carbohydrates, sugar, sodium and fiber. Participants were not told to lose weight and were not forced to finish their portions — they could eat as much as they wanted.
The result was striking enough to be difficult to explain with willpower alone. On the ultra-processed diet, the same people ate about 500 kcal more per day on average and gained roughly 0.9 kg in two weeks. On the minimally processed diet, they lost about the same amount.
Nobody replaced their character. Nobody added or removed discipline. What changed was the environment in which each person made hundreds of small decisions about food.
This experiment does not prove that ultra-processed food is the single cause of obesity, and the small sample does not allow its exact numbers to be applied to everyone. But it raises an uncomfortable question: if the same person, under different conditions and without any special intention, begins to eat more or less, where does “willpower” end and biology and environment begin?
The modern answer is much more complicated than “eat less and move more.” Behavior matters. Choice matters. But body weight is shaped by a system in which appetite, the brain, genetic susceptibility, food availability, sleep, stress, physical activity, medications, disease, culture and dozens of repeated decisions operate at the same time.
Willpower can influence weight, but it is not an independent master regulator of it.
Energy balance is real. But it is not yet an explanation of behavior
Debates about obesity often produce two opposite claims. The first says: “weight is just calories.” The second says: “calories do not matter at all.”
Both statements confuse different levels of the problem.
For energy to accumulate in the body, the relationship between energy intake and expenditure does matter. The World Health Organization also describes overweight and obesity in terms of an energy imbalance, while at the same time emphasizing that obesity is usually a multifactorial condition involving interactions among genetic variants, psychosocial factors and the environment.
That distinction matters. Saying “a person gained weight because, over time, energy intake exceeded expenditure” describes the physical outcome. It does not yet explain why that particular person repeatedly took in more energy than they used.
It is like saying a bathtub overflowed because more water flowed in than drained out. Formally true. But if we want to prevent the next flood, we still have to ask why the tap opened, why the drain failed and who was controlling the system.
When it comes to body weight, that “tap” is not a single human trait called willpower.
Appetite does not wait for us to consciously decide to become hungry
A person can decide at eight in the morning to eat less. But throughout the day, that decision interacts with a system of appetite regulation that existed long before diets, calorie charts and fitness apps.
The brain receives information about food availability, smell and appearance, energy stores, signals from the digestive system and hormonal changes. Modern research on body-weight regulation treats weight as the result of interaction between internal biological systems and the surrounding environment. A 2025 review of body-weight regulation models in Nature Reviews Endocrinology specifically highlights the interaction between an obesogenic environment — conditions that promote weight gain — and individual biological susceptibility.
It is this difference between people that makes a simple moral model so weak.
One person can finish dinner and barely think about food for several hours. Another notices a smell, an advertisement, an open package or dessert on the table and feels a much stronger pull. From the outside, we see only the last second — whether the person takes a bite or refuses it. We do not see how difficult that decision was.
Imagine two people standing in front of the same cake. Both refuse it. The outcome is identical. But if one thought about the cake for two seconds while the other fought the desire for twenty minutes, it would be hard to say that their biological task was the same.
Willpower measures a decision. It measures the cost of that decision poorly.
Genes do not determine weight in advance — but they are not silent either
Twenty years ago, talk about an “obesity gene” could sound as though there were a single switch somewhere in DNA that made a person thin or heavy. Real genetics turned out to be far more interesting and far less convenient.
Researchers have identified more than a thousand genetic loci associated with obesity risk and body weight. Some of these signals are linked to the central nervous system and mechanisms of appetite regulation. But for most people there is no single “guilty gene”: risk reflects the combined influence of many variants, each with a small effect.
A particularly revealing example was a 2025 study based on genetic data from up to 5.1 million people. The polygenic score developed by the authors explained 17.6% of the variation in body mass index among UK Biobank participants of European ancestry. Its predictive performance, however, differed substantially between populations.
This does not mean that “17.6% of a person’s weight is written in the genes.” That would be an incorrect interpretation. What it shows is that genetic differences have a measurable association with body weight at the population level, but no modern polygenic score turns an individual person’s weight into a predetermined script.
And here lies an important paradox. If genes were destiny, the global prevalence of obesity could not have changed so rapidly over a few decades — the human genome did not change that quickly. If genes did not matter at all, we would not see such clear genetic associations.
The best explanation lies between those extremes: the environment changes quickly, while people enter it with different levels of biological vulnerability.
One of the strongest arguments against “just control yourself” appears after weight loss
Weight gain is often treated as evidence of weak control. But it is even more revealing to look at someone after successful weight loss.
They have already done what the old logic demanded. They changed their diet. They tolerated restriction. They lost weight.
It would seem that the only task left is to keep behaving the same way.
But after weight loss, the body does not necessarily remain a neutral observer. In a classic study published in the New England Journal of Medicine, 50 people lost an average of 13.5 kg after ten weeks of intensive weight loss. One year later, changes in several appetite-related hormonal signals persisted, while subjective hunger remained higher than before weight loss.
It was a small study, and modern understanding of weight regain cannot rest on it alone. But newer reviews support the broader picture: after weight loss, hormonal and neural changes in appetite, adaptive reductions in energy expenditure and other mechanisms may promote weight regain.
At that point, the moral interpretation begins to look especially strange. A person may demonstrate enough self-control to lose weight and then spend years in a situation where maintaining the new weight is biologically harder than making the initial decision to lose it.
Weight regain does not prove that behavior is unimportant. It shows that behavior does not happen in a vacuum.
Why does one person gain weight in the same world while another does not?
This is the strongest argument for the willpower explanation: if the same food is sold everywhere, everyone sees advertising and everyone can order pizza, why does obesity not develop in everyone?
That is exactly why the environment cannot be the only cause.
People differ in genetics, appetite, reward sensitivity, early development, health, medications, physical activity, sleep, income, occupation, stress and many other factors. Even the same food can create different behavioral difficulties for different people.
The modern model looks less like one cause and more like a combination lock. For one person, easy access to energy-dense food may be a major component. For another, genetic susceptibility. For someone else, medications, chronic sleep loss, disease or a combination of several factors.
WOW:
That is why “the environment causes obesity” is just as incomplete as “weak willpower causes obesity.”
In its updated 2025 description, WHO classifies obesity as a chronic, relapsing disease arising from a complex interaction among genetics, neurobiology, eating behavior, access to healthy food, market forces and the broader environment.
There is still a place for individual behavior in that model. Behavior simply stops being the only explanation.
Obesity medicines created a very uncomfortable experiment for our beliefs
For decades, people with obesity heard almost the same advice: you need to want food less.
Then medicines appeared that can affect biological signals involved in appetite and satiety.
And for some people, behavior changes not after a lecture about discipline, but after a change in a biological signal.
This does not mean the medicines “proved that willpower does not exist,” nor that they are the right solution for everyone. The evidence does not justify that conclusion. But the effectiveness of biologically targeted treatment is difficult to reconcile with the idea that obesity is only the result of insufficient desire to control food.
At the end of 2025, WHO issued its first global guidance on the use of GLP-1 medicines for obesity in adults. The organization did not propose replacing nutrition or physical activity with medication: drug therapy is treated as one part of comprehensive long-term care.
That is a revealing change in language. Medicine is moving from “make yourself lose weight” toward “which mechanisms are sustaining excess fat accumulation in this person, and what can be done about them?”
But throwing willpower out completely would also be a mistake
After everything above, it is easy to swing to the opposite extreme: if genes, hormones and environment matter so much, then a person decides nothing at all.
That is also false.
People can change what they buy, how they structure meals, physical activity, sleep, which foods are available at home, how they respond to stress and many other repeated behaviors. Behavioral interventions can produce weight loss and health improvements. Medication and surgery also do not happen without behavior: people continue to eat, move, make decisions and interact with their environment.
The mistake is not believing that willpower exists.
The mistake is making it the explanation for everything.
Consider myopia. A person decides whether to put on glasses, but we do not treat visual acuity as a moral trait. Or blood pressure: behavior can influence it, but nobody seriously proposes defining hypertension by the strength of someone’s character.
Culturally, we still often do exactly that with body weight.
The environment can turn a hundred small decisions into one large difference
One extra bite seems trivial. A slightly larger portion does too. A drink left on the table. Food visible from a desk. Delivery that takes three taps. An evening after five hours of sleep. One more snack during a series.
None of those moments alone explains obesity.
But body weight is not shaped by one dramatic decision per year. It can reflect thousands of small decisions repeated in the same environment.
That is why the ultra-processed-food experiment at the beginning of this article matters so much. Participants were not told: “overeat today.” They were simply given a different set of foods and spontaneously consumed more energy.
That does not eliminate choice. It shows that the architecture of choice can systematically tilt decisions in one direction.
If one person has to refuse an easily available reward twenty times every evening while another encounters it twice, comparing their body weights as a test of character makes very little sense.
What changes when we stop treating weight as a moral issue
The biggest practical difference appears not when we tell a person, “you have no role in this,” but when we stop fighting an abstract flaw in character and start looking for specific mechanisms that can be changed.
- If the problem is constant food availability, changing the environment may sometimes work better than practicing refusal hundreds of times a day.
- If eating is tied to habitual situations, it is useful to identify the triggers themselves: screens, evenings, work breaks, large packages or a particular route.
- If hunger becomes much stronger after weight loss, that is not automatically proof of a “failure of character”; maintaining weight may require a different long-term strategy.
- If weight changes substantially without a clear reason, a universal explanation based on overeating can miss medical factors or medication effects.
- If obesity is already affecting health, the issue may go far beyond a self-directed diet experiment and require professional assessment.
All of these steps still involve decisions. But they move control to points where one action can change dozens of future situations.
Instead of refusing the food in front of you a hundred times, sometimes the stronger move is deciding once what will be in front of you.
Maybe we have been asking the wrong question
When two people have different body weights, it is very easy to ask: “which one controls themselves better?”
The question is attractive because it turns a complex system into one human quality. It allows us to look at the outcome and invent the cause afterward: thin means disciplined, obesity means not disciplined.
But the science of weight regulation shows a different picture. Genetic risk differs. Appetite differs. Responses to the environment differ. After weight loss, the body can alter hunger signals and energy expenditure. Food itself can change energy intake even when someone did not intend to eat more.
So a better question is:
not “why could this person not control themselves strongly enough?”, but “what forces do they have to control every day — and how strong are those forces?”
That is where willpower returns to its proper place. It does not disappear. It becomes one tool among many.
And body weight stops being a test of character.
Disclaimer. This material is informational and analytical and is not medical advice, a diagnosis or a personalized recommendation for weight loss. Obesity, major or unexplained changes in body weight, pronounced appetite disturbances and related conditions can have different causes and may require individual professional assessment. Medicines used to treat obesity are prescribed for appropriate medical indications.
