Feeling hungry, which is what happens to many people when they want to lose weight, does not simply mean that the body has run out of energy and needs food. Behind appetite lies a complex network of signals involving the brain, the stomach, the intestines, adipose tissue, and different hormones, but also other factors seemingly unrelated to eating, such as sleeping poorly, going through a period of stress, or even seeing and smelling appealing food.
That is why we can be hungry shortly after eating, temporarily lose appetite in certain situations, or feel a strong craving for a particular food even when the body has enough energy. And this complexity also helps explain why losing weight and, above all, keeping the lost pounds off can be difficult for some people.
Hunger and appetite: the brain has a lot to say
One of the main structures involved in regulating hunger is the hypothalamus, a small region of the brain that receives and integrates information from different parts of the body. Among these signals is ghrelin, a hormone produced mainly in the stomach. Its levels typically rise before meals and fall after eating, thereby playing an important role in the onset of hunger.
It is not merely an association. A clinical trial published in The Journal of Clinical Endocrinology & Metabolism administered ghrelin or a saline solution to nine healthy volunteers. When they received ghrelin, all of them consumed more energy at a free-choice buffet: on average, intake increased by a 28 %. Participants also reported feeling more appetite.
At the other end appears one of the hormones most known in weight research: leptin. It is produced mainly in adipose tissue and transmits to the brain information related to the available energy reserves.
But it would be too simplistic to describe ghrelin as the “hunger hormone” and leptin as the “satiety hormone.” The system is much more sophisticated. A scientific review on leptin and eating behavior explains that this hormone participates in both homeostatic mechanisms —those related to energy needs— and hedonic systems linked to the pleasure and reward that food provides. Additionally, the intestine and pancreas release other signals, among them GLP-1, PYY, cholecystokinin (CCK), amylin and insulin, which contribute in different ways to regulate when we start eating and when fullness occurs.
«The brain does not function as an exact calorie-counter. It interprets internal signals and external stimuli, and its response changes according to health status or after weight loss. For this reason, two people facing the same meal can experience very different levels of hunger», explains Christian Alvarado, medical director of Drop, the Sanitas digital unit specialized in treatment and follow-up of obesity.
Why do we feel hungry even after we’ve just eaten?
Because we do not eat solely to replenish energy. The brain also has circuits related to reward, pleasure, emotions, memory, and learning. These systems help explain something almost everyone has experienced at some point: not feeling particularly hungry and yet starting to crave food after seeing an appetizing plate, walking by a bakery, or encountering a food associated with a pleasant memory.
A review on the hormonal and neural mechanisms of food reward precisely recounts how hormones such as ghrelin, leptin, and insulin interact with brain circuits related to reward. Context, therefore, matters. The smell of a food, its appearance, a photo or a video on social media, and even certain social situations can act as signals capable of awakening the desire to eat.
This does not mean that hunger is “psychological” as opposed to “real.” It means that human appetite is the result of the interaction of biological, psychological, and environmental mechanisms.
Sleep deprivation can also increase appetite
Sleep is another factor that enters into this equation. The relationship between sleep deprivation, appetite, and obesity has been studied for decades. Studies have found that lack of sleep can affect the hormones related to hunger, but also food reward and decisions about what and how much to eat.
One of the classic experiments in this field was conducted by researchers at the University of Chicago. After young, healthy men slept only four hours for two nights, researchers observed an 18% decrease in leptin and a 28% increase in ghrelin. Participants also reported increased appetite and a stronger desire for high-calorie and carbohydrate-rich foods.
Eve Van Cauter, professor of Medicine at the University of Chicago and one of the investigators responsible for the work, explained then: “This is the first study that shows that sleep is an important regulator of these two hormones and that the magnitude of the hormonal changes relates to the magnitude of the change in hunger.”
Sleeping more can change how much we eat
Years later, another experiment went a step further. Instead of artificially restricting sleep in a laboratory, researchers studied what happened when people who habitually slept little managed to increase their sleep hours in everyday life.
The randomized clinical trial published in JAMA Internal Medicine included 80 overweight adults who habitually slept less than 6.5 hours per night. Those assigned to a personalized intervention to increase sleep managed to sleep about 1.2 hours more per night. The striking thing is that they did not receive instructions to diet or to increase their exercise. Yet they reduced their energy intake by about 270 kilocalories per day compared with the control group.
Esra Tasali, director of the UChicago Sleep Center and lead author of the research, summarized the results this way: “In our earlier work we understood that sleep is important for regulating appetite. Now we have shown that, in real life and without making other lifestyle changes, one can extend sleep and consume fewer calories. This could really help people who are trying to lose weight.”
Sleeping more can change how much we eat
Researchers, however, urge caution. The experiment lasted only a few weeks and its results do not mean that sleeping more automatically leads to long-term weight loss. They do suggest that rest is another piece of appetite regulation and deserves to be considered alongside diet, physical activity, and other factors.
Why hunger increases after weight loss
After a significant weight loss, the body can activate adaptive responses aimed at conserving energy and promoting intake. In other words: the body does not necessarily behave the same after slimming down as before. A well-known study published in The New England Journal of Medicine followed 50 overweight or obese people enrolled in a weight loss program. The researchers measured, among other parameters, leptin, ghrelin, PYY, GLP-1, amylin, CCK and insulin, as well as the hunger reported by the participants.
After an initial average weight loss of 13.5 kilograms, notable changes occurred in several of these signals. Leptin and PYY decreased, ghrelin increased, and participants reported more hunger. Most importantly, one year after the initial weight loss, changes in several appetite-regulating hormones persisted, as well as an increased hunger compared to the initial values. The authors concluded that alterations in appetite mediators that favor weight regain had not returned to pre-diet levels even after a year.
This helps debunk an overly simplistic explanation of obesity based solely on “eat less and move more.” “Weight loss usually triggers adaptive responses in the body. After a substantial reduction, it is possible that hunger increases and some hormonal signals related to eating regulation change. This does not mean that regaining weight is inevitable, but it helps explain why maintaining it requires long-term follow-up and individualized adjustments,” notes Alvarado.
The existence of these mechanisms does not mean that regaining weight is inevitable. It also does not imply that all people respond in the same way. What it shows is that maintaining weight loss may involve facing real physiological responses and that reducing the whole process to a matter of willpower does not adequately reflect what we know about the biology of appetite.
Too-restrictive diets: when control can backfire
The way we organize our eating also plays an important role. Following a structured nutritional plan by itself does not cause an eating disorder. However, there is a difference between organizing eating and developing an extremely rigid relationship with it.
Absolute prohibitions, guilt after eating, or the need to “compensate” for any deviation can increase concern about food. «When a person interprets any change to the diet as a failure, they can enter a cycle of restriction and bingeing. The aesthetic pressure and weight stigma add distress and, at times, delay seeking help», notes Virginia del Palacio, psychologist at Blua of Sanitas.
The scientific evidence also highlights weight stigma. A systematic review aggregating 242 studies found a consistent relationship between greater exposure, anticipation, or internalization of weight stigma and a higher presence of problematic eating thoughts and behaviors, including binge eating, body dissatisfaction, dietary restriction, and unhealthy weight-control practices. This does not allow us to claim that stigma is the sole cause of these behaviors —most of the studies reviewed were observational—, but it does show a sufficiently consistent association to consider.
What to do to better regulate hunger and the relationship with food
In light of these situations, Sanitas specialists recommend:
- Avoid intense restrictions and compensations. Skipping meals or eliminating foods without professional guidance can increase subsequent hunger and make it harder to identify satiety signals. A one-off deviation from the plan should not automatically become a reason to fast or exercise as compensation.
- Observe when hunger appears. Poor sleep, periods of stress, exposure to certain stimuli, or long gaps without eating can modify appetite. Identifying these patterns can be useful, provided it does not become obsessive vigilance of every food item.
- Tend to sleep and emotional well-being. Rest is part of the factors that can influence appetite regulation. Stress, guilt, and body dissatisfaction can also alter the relationship with food. When this discomfort persists or affects daily life, it is advisable to seek professional help.
- Gradually incorporate physical activity. Exercise offers benefits far beyond burning calories: it helps maintain functional capacity and muscle mass and can support rest and psychological well-being. It should be adapted to health status and personal preferences and not be used as punishment for having eaten.
- Avoid reducing the issue to willpower. Hunger depends on hormones, brain circuits, learning, sleep, emotions, and environmental cues. Habits remain important, but they develop within this biological context.
- Seek a comprehensive assessment. The approach to obesity may require medical, nutritional, and psychological care, in addition to tailored physical activity, depending on each person’s needs. A full evaluation allows consideration of history, medications, sleep, eating behavior, comorbidities, and mental health.
- Consult for recurrent episodes of loss of control. Repeated binge episodes and cycles of restriction and overeating require a specific evaluation. If there is a suspicion of an eating disorder, treatment should not focus solely on losing weight.
Hunger is much more than an empty stomach
As we have seen, the science of appetite shows that it is all much more complex than an empty stomach sending the brain the order to eat. Ghrelin, leptin, GLP-1, and other hormones; the hypothalamus and reward circuits; sleep, stress, memory, and environment form part of one network. Even weight loss can temporarily or persistently modify some of the signals that participate in this system.
Understanding how hunger works does not mean that habits stop mattering, but it helps explain why not all people feel the same level of appetite, respond differently to a diet, or encounter the same difficulties in maintaining weight loss.
