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12/30/2025
Biological Responses Driving Weight Rebound After Weight Loss
Summary
・When individuals lose weight through calorie restriction, adaptive responses involving coordinated changes in metabolism, neuroendocrine function, autonomic regulation, and behavior are triggered, which may promote weight rebound. These responses help explain why calorie-restricted diets often fail to produce lasting weight loss.
(1) Metabolic adaptation
Calorie-restricted weight loss reduces resting energy expenditure beyond what would be predicted from changes in body composition. This adaptation occurs in both formerly obese and naturally lean individuals, creating conditions that favor weight rebound.
(2) Endocrine function
Hormones secreted by the gastrointestinal tract and adipose tissue, including leptin and ghrelin, regulate appetite, food intake, and energy expenditure. Calorie restriction can reduce satiety and increase hunger, thereby promoting overeating.
(3) Food reward and addiction-like processes
Palatable foods activate reward-related neural circuits through neurotransmitters such as dopamine. The desire to experience this pleasure again can motivate further eating. Calorie restriction and fasting may heighten the reward value of food, especially energy-dense, highly palatable foods.
(4) Inhibitory control and binge eating
Short-term dieting success may result from enhanced inhibitory responses that temporarily suppress the desire to eat. However, prolonged dietary restriction may strengthen reward-related responses and weaken inhibitory control, making cravings increasingly difficult to resist.
(5) Fat cell size and number
Weight loss reduces the size of adipocytes (fat cells), but their number generally remains unchanged. Smaller fat cells may break down less fat and become more prone to storing it again, thereby promoting the regain of lost body fat.
(6) Intestinal starvation
Unlike the responses described in (1)–(5), my proposed intestinal starvation hypothesis suggests that the body may interpret the complete digestion of food, leaving no undigested matter in the intestinal tract, as a signal of starvation.
Conclusion
Some researchers argue that the biological forces promoting weight rebound after weight loss are extremely powerful and difficult to overcome.
In my view, rather than trying to overcome these responses, long-term weight management requires adopting eating and lifestyle habits that minimize their activation in the first place.
【Full text】
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Contents
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- Various mechanisms that promote weight rebound
(1)Metabolic adaptation
(2) Endocrine Function
(3) Food reward and addiction-like processes
(4) Inhibitory control and binge eating
(5) Fat cell size and number
(6) Intestinal starvation - Conclusion
- Various mechanisms that promote weight rebound
Introduction
The conventional prescription that people with obesity should “eat less and exercise more” remains widely used as a standard approach to weight management, despite repeated reports of its low long-term success rate [1].
At the same time, recent findings from genetics, epidemiology, physiology, and related fields have increasingly shown that body weight and body fat are biologically regulated within a certain range. These findings are helping to establish a biological framework that may explain why long-term success with weight-loss dieting is so difficult to achieve.
Following weight loss, adaptive responses are triggered through coordinated changes in metabolism, neuroendocrine function, autonomic nervous system activity, and even behavior. These responses have been shown to resist the maintenance of weight loss [4].
In this article, I will briefly introduce the biological mechanisms that may promote post-weight-loss weight rebound—or, in some cases, further weight gain. I will also explain how these mechanisms relate to, and differ from, my proposed “intestinal starvation” theory.
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１．Various mechanisms that promote weight rebound
(1) Metabolic adaptation
Energy restriction is associated with a reduction in resting energy expenditure (REE) [5]. Many studies have reported that behavioral weight loss leads to a greater decrease in both resting and total energy expenditure than would be predicted based on changes in body composition and the thermic effect of food [4,6].
This phenomenon, known as adaptive thermogenesis (AT) or metabolic adaptation, creates conditions that favor regaining lost weight [7].
Metabolic adaptation can be viewed as the body's survival response: when the body perceives a state of starvation, it reduces the energy cost of living in an attempt to prolong survival.
Interestingly, this response also appears to occur in individuals with obesity and does not seem to be diminished by the amount of energy stored as body fat [7,8].
(Author: rawpixel.com / Source: Freepik)
However, regarding the timing of its onset, the evidence is inconsistent [9]. Some studies have detected adaptive thermogenesis (AT) within a week of energy restriction, which has been associated with rapid declines in insulin secretion, depletion of glycogen stores, and loss of intracellular and extracellular fluid [10].
In contrast, a growing body of evidence suggests that underfeeding-associated AT takes weeks to develop, in association with reduced leptin secretion following the loss of body fat, among other physiological adaptations [9,11,12].
Although the persistence of AT also remains a subject of debate [7], some studies indicate that this metabolic adaptation may continue for years even after energy balance has been reestablished at a lower weight [13].
(2) Endocrine function
A number of hormones secreted from the gastrointestinal tract and adipose tissue are known to play key roles in regulating appetite, food intake, energy expenditure, and body weight [14,15].
Leptin is a hormone secreted by fat cells that helps regulate body weight by suppressing appetite and influencing energy expenditure. High leptin levels signal to the brain that energy stores are sufficient, whereas low leptin levels indicate that energy stores are low [16].
It has been shown that leptin levels drop within 24 hours of energy restriction [17]. Interestingly, many studies have reported a greater reduction in leptin levels than would be expected based on the loss of body fat [18,19].
It has been suggested that the primary role of leptin may be the prevention of starvation, rather than weight regulation per se [15,20]. When leptin levels fall below a certain threshold—the point at which specific physiological responses are triggered (Note1)—starvation defense mechanisms are activated, even if substantial fat stores remain [17]. This leads to a reduction in metabolic rate and physical activity, as well as an increase in hunger [21,22].
Furthermore, in individuals who have lost weight, an increase in the appetite-stimulating hormone ghrelin, along with decreases in the post-meal satiety signals peptide YY (PYY) and cholecystokinin (CCK), has been observed [23]. As a result, diet-induced weight loss may simultaneously reduce satiety and increase hunger, potentially promoting overeating [15].
Note 1: It has been suggested that this threshold rises as fat mass increases [17].
(3) Food reward and addiction-like processes
Food reward refers to the brain's reward response to food, which generates pleasure and satisfaction from eating, as well as the motivation to eat again. This process involves activation of the brain's reward circuitry, where neurotransmitters such as dopamine are released, leading to feelings of pleasure and increased appetite.
(Author: rawpixel.com / Source: Freepik)
The regulation of food intake is influenced by a close interaction between homeostatic and non-homeostatic (hedonic) factors.
The former is driven by the body's nutritional needs, monitoring available energy in the blood and fat stores to maintain energy balance. The latter, in contrast, is largely associated with the brain's reward system [24,25].
Although food intake is primarily regulated by homeostatic mechanisms, reward-related signals can easily override normal satiety signals that help maintain a stable body weight, potentially leading to overeating [25,26].
Modern neuroimaging studies using fMRI have shown that both nutritional status (e.g. hunger vs. satiety) and different food stimuli (e.g. high- vs. low-calorie foods, appetizing vs. bland foods) can alter activity in the brain's reward circuitry [27–29].
Recent studies in healthy individuals indicate that short- or long-term caloric restriction, as well as fasting, may increase the reward value of food—especially high-calorie, palatable foods [27,30].
These findings may help explain why calorie-restricted diets often fail in the long term [28,30].
Food addiction: similarities and differences from drug addiction
While drugs and food share certain characteristics, they also differ in important ways.
Drugs of abuse, such as cocaine, act directly on the brain's dopamine circuitry, whereas food influences these circuits more indirectly. Signals from taste and smell, nutrient sensors in the digestive tract [31], and hormones released during digestion and nutrient absorption all communicate with the brain, thereby influencing the dopamine system [25].
Although it remains debated whether specific food components such as sugar, sweeteners, salt, or fat can promote addiction-like processes [25], highly palatable and calorie-dense foods—such as chocolate, ice cream, cookies, and salty snacks—can serve as powerful rewards.
In today's stress-filled society, these foods provide pleasure and comfort, leading some researchers to draw parallels between "food addiction" and drug addiction [32,33].
(4) Inhibitory control and binge eating
Food intake is primarily regulated by three interacting neural systems: the homeostatic, reward-related, and inhibitory systems [15].
The inhibitory system—mainly involving the brain region responsible for self-control and decision-making—helps regulate eating behavior and inhibit excessive food intake [34].
Cognitive control of food reward
In humans, the urge to seek and consume palatable foods can be moderated by executive functions—the cognitive processes involved in self-control and decision-making.
One of the central dilemmas in daily life is balancing one's internal goals (e.g. cutting back on sweets to maintain health and weight control) against the immediate reward of eating tempting foods. This conflict is particularly challenging when highly desirable foods, such as donuts or pizza, are readily available [25].
(Source: Freepik)
The short-term success of dieting suggests that increased inhibitory neural responses can temporarily override the neurobiological drive to consume highly palatable, high-calorie foods [35].
However, recent evidence indicates that reward-related neural activity also increases alongside inhibitory neural activity [36].
In simple terms, as dietary restriction continues, it may become increasingly difficult to resist the urge to eat appetizing, high-reward foods.
Prospective studies in young individuals, as well as rodent experiments, suggest that severe caloric restriction, characterized by 24-hour fasting or fat-free diets, may increase the risk of developing binge eating and bulimia in the future [37,38].
(5) Fat cell size and number
Weight-loss dieting may reduce the size, but not the number, of fat cells [39]. It remains unclear whether hyperplasia (an increase in adipocyte number) contributes to weight rebound in weight-suppressed individuals [15]. However, in a study of obese rats, adipocyte hyperplasia was observed following refeeding after fasting [40].
In humans, a similar possibility has been suggested.
Normally, when energy availability is low, triglycerides stored in adipose tissue are broken down to supply energy to the body's cells.
However, the rate of lipolysis (fat breakdown) appears to be related to adipocyte size and cell surface area [41], meaning that as fat cells shrink, their rate of lipolysis tends to decline.
If size-reduced adipocytes undergo functional changes that favor fat storage over fat breakdown, they may gradually re-expand, potentially promoting the regain of lost body fat.[15,42,43].
(Author: brgfx / Source: Freepik)
(6) Intestinal starvation
The mechanisms described in Sections (1)–(5) are generally thought to represent a series of anti-starvation (or anti-weight-loss) responses (Note 2) that occur in association with energy restriction and weight loss [15].
In contrast, my intestinal starvation hypothesis proposes that when all ingested food is completely digested and no undigested matter remains in the intestinal tract, the body may interpret this condition as a starvation signal.
Intestinal starvation may occur not only during strict dietary restriction for weight loss (e.g. skipping meals or eating extremely small amounts of food), but also during more moderate dieting or even everyday eating habits that are not intended for weight loss, such as skipping breakfast, eating a light lunch, having dinner late at night, or eating only two meals a day.
Furthermore, in my intestinal starvation hypothesis, adaptive responses to intestinal starvation may lead to weight gain suggestive of an upward shift in the body-weight set point. This weight gain may involve not only body fat but also lean body mass, including skeletal muscle. Therefore, this mechanism may differ from conventional models of obesity, in which weight gain is explained primarily by an increase in body fat.
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Note ２: Because these responses are activated despite adequate energy stores, some researchers prefer the term “anti-weight-loss” rather than anti-starvation mechanisms [15].
２. Conclusion
At present, the causal relationship between the biological responses described in Sections (1)–(5) and post-weight-loss weight rebound has not yet been fully established [15]. Nevertheless, many people who have experienced weight rebound after dieting may find that these mechanisms are consistent with their own experiences.
Some researchers have pointed out that the biological forces that resist weight loss and promote the recovery of lost weight are extremely powerful and difficult to overcome for most individuals attempting to lose weight through behavioral interventions. They have also suggested that achieving long-term weight loss will require the development of interventions that weaken these biological responses themselves [15].
I generally agree with this perspective, although my approach differs somewhat.
In my opinion, achieving long-term weight loss depends not only on developing interventions that weaken these biological responses, but also on adopting dietary and lifestyle habits that minimize activation of the body's anti-starvation (or anti-weight-loss) mechanisms in the first place.
Specifically, it is important to appropriately regulate energy intake while reducing the consumption of refined carbohydrates and ultra-processed foods.
At the same time, I believe that naturally derived foods—such as vegetables, seaweed, dairy products, minimally processed meat and fish, and nuts—should be actively included in the diet.
In particular, I consider it beneficial to consume adequate amounts of foods containing less digestible components, as well as foods that take longer to digest (Note 2).
Maintaining this type of dietary pattern may help prolong satiety and reduce feelings of hunger. Over the long term, I propose that it may also facilitate the transmission, via the gut-brain axis, of information to the brain indicating that food remains sufficiently available.
Note 2. I do not believe that foods high in fat should necessarily be avoided. Depending on the overall composition of the diet and how such foods are consumed, they can be incorporated appropriately.
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