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Introduction
In recent years, the number of people dieting for weight loss has been increasing worldwide. However, some have raised concerns that dieting itself may be accelerating the rise in obesity. 

For example, some female actors and television announcers appear to have gained weight compared to the past. However, it is unlikely that they are habitually overeating; rather, they may be restricting their food intake in an effort to manage their weight.

In this article, I would like to explore whether the spread of dieting may be related to the rise in obesity, based on findings from observational and intervention studies. 

(1) In 1992, a panel of experts convened by the U.S. National Institutes of Health concluded that, with continued participation in weight-loss programs conducted in controlled settings, participants typically lose about 10% of their body weight.

However, within one year after weight loss, one-third to two-thirds of the lost weight is regained, and within five years, almost all of it is regained [1].

Studies on long-term outcomes have also shown that at least one-third of dieters regain more weight than they lost [2]. This has raised concerns that dieting may, paradoxically, produce outcomes that are directly opposite to its original purpose [2,3].

    
(2) The 1983 book “Dieting Makes You Fat” proposed the idea that dieting to lose weight is counterproductive for weight control because people may regain more fat than they lose with each cycle of weight loss and regain [4]. Since then, whether dieting contributes to long-term weight gain remains a controversial and actively debated topic among scientists [5,6,7].

(4) Thus, the proportion of people attempting to lose weight has increased alongside the rising prevalence of overweight and obesity. However, this observation alone does not establish a causal relationship between the two.

Some researchers have suggested that the association between dieting and weight gain is, at least in part, causal [16,17]. Others, however, have argued that dieting is merely a proxy marker for individuals who are already prone to weight gain, and that without dieting, they would likely have gained even more weight [18].　

     
(5) Several prospective studies have suggested that dieting is a strong predictor of future weight gain among adolescents [17,19,20], middle-aged women [21], and even individuals who were initially of normal weight [5,21,22]. 

Note 1: Unhealthy weight-control behaviors include weight-loss practices that are generally not recommended, such as fasting, skipping meals, eating very little, and using food substitutes or diet pills.

In particular, “skipping meals” and “eating very little” were the most commonly reported unhealthy weight-control behaviors and were significantly associated with greater increases in BMI in both sexes. 

The use of food substitutes (e.g. powdered products or special beverages) among males and the use of diet pills among females were associated with greater subsequent increases in BMI.

Interestingly, among girls who were overweight (25 ≤ BMI < 30) at baseline, those who continued to engage in unhealthy weight-control behaviors experienced the greatest increase in BMI over the 10-year follow-up (more than 5 BMI units), whereas those who never engaged in such behaviors showed only a minimal increase in BMI [17].

In conclusion, these findings suggest that dieting and unhealthy weight-control behaviors during adolescence may contribute to long-term weight gain [17].

Previous observational studies on dieting and subsequent weight change have yielded inconsistent findings [22].

Many longitudinal observational studies have reported subsequent weight gain among self-reported dieters [17,20,21,22]. On the other hand, some studies have also reported that dieting predicts both weight loss and weight gain [24,25]. Several factors may help explain these differences in the findings.
      

Many studies have examined whether participants were dieting at baseline, or had a history of dieting. However, relatively few have investigated the specific methods used for weight loss [21,22,26].

   
In addition, the terms “dieting” and “weight-loss attempts” are somewhat ambiguous and may be interpreted differently by different individuals [17].
People who adopt healthy eating and exercise habits—for example, eating more natural foods such as vegetables, reducing ultra-processed foods, eating breakfast, and exercising regularly—may be able to achieve and maintain weight loss. 

In contrast, those who engage in unhealthy weight-control behaviors, such as skipping meals, eating very little, or using food substitutes (e.g. diet shakes or nutritional drinks), may experience only temporary weight loss, fail to maintain it over the long term, and eventually regain the weight. 

The duration of follow-up and the timing of dieting may also influence whether long-term weight change can be evaluated accurately.

Several studies have assessed changes in body weight or BMI several years later (e.g. 2, 5, or 10 years) based on participants' dieting status at baseline [20,21,22,26].

However, people who repeatedly go on and off diets are more likely to experience substantial fluctuations in body weight. 

From the perspective of body-weight homeostasis, as described by the body-weight set-point theory, individuals who were dieting at baseline may have been temporarily below their natural body weight (set point). Simply discontinuing the diet could therefore lead to subsequent weight gain.

Conversely, individuals who began dieting shortly before the end of the follow-up period may have experienced substantial weight loss because of the temporary effects of dieting. 

Furthermore, when the follow-up period is as long as five or ten years, weight-control behaviors that were started or discontinued during that interval may not have been captured. 

   
*Although not discussed here, observational studies also require caution regarding the limitations of self-reported data and the influence of confounding factors that may contribute to weight gain.

Moreover, in studies of adolescents, the natural increases in muscle mass and body weight that occur during growth should also be taken into account.

In conclusion, observational studies alone cannot establish a causal relationship between dieting and an increased risk of weight gain. However, for the following reasons, I believe that dieting is likely to contribute to weight gain.

One interpretation is that dieting is simply a proxy marker reflecting the global trend toward increasing body weight, rather than a cause of weight gain itself. From this perspective, it is argued that without dieting, people would gain even more weight [18].

However, the 10-year study of adolescents discussed in Section 1(5) made it possible to compare the weight trajectories of participants who were dieting at baseline but had stopped dieting five years later with those who continued dieting. 

The researchers found that those who stopped dieting gained substantially less weight than those who continued dieting. Based on these findings, they did not support the claim that people would gain even more weight if they did not diet [17].

Another explanation for the association regarding dieting and obesity is that dieting itself does not cause subsequent weight gain. Rather, people who are genetically prone to obesity may simply be more likely to go on a diet [6].

However, the 10-year study of adolescents yielded findings that were not consistent with this claim.
Among girls who were already overweight at baseline, those who continued to engage in unhealthy weight-control behaviors showed significantly greater increases in BMI than those who had never engaged in such behaviors. The researchers suggested that dieting itself may contribute to subsequent weight gain [17]. 

           
Furthermore, a longitudinal twin study conducted in Finland examined changes in body weight among twins with different numbers of intentional weight-loss episodes of at least five kg.

Although the study could not rule out the influence of genetic and shared family factors on weight gain, it also suggested that dieting itself may contribute to subsequent weight gain [16].

Recent research has greatly improved our understanding of energy homeostasis and the mechanisms that regulate body weight. 

Severe dietary restriction accompanied by substantial weight loss can trigger biological starvation responses involving changes in metabolism, hormones, and the nervous system, potentially leading to increased appetite and overeating. As a result, body weight tends to return toward its previous level and may even exceed the pre-weight-loss level in some individuals [27,28].

     
In addition, fasting for weight control (going without food for 24 hours) has been suggested to be associated with a greater risk of developing binge-eating disorder in the future than less severe dieting behaviors [29]. 

Furthermore, an overshoot in body weight during the recovery period has been documented in normal-weight participants in both the classic Minnesota Starvation Experiment and the U.S. Army Ranger multistressor study [30,31].

My intestinal starvation theory suggests that when all ingested food has been completely digested within the intestinal tract, the body may perceive this as a state in which no food is present. This may be described as a modern form of hunger that can occur even in affluent societies—a condition that may have become more likely with advances in food processing and the spread of ultra-processed foods.

Calorie-restricted dieting, especially unhealthy weight-control behaviors such as skipping meals, eating very little, or using food substitutes instead of regular meals, may further promote this intestinal starvation. 

Not all dieting leads to weight gain. Some people successfully lose weight and maintain their weight loss by adopting healthy dietary patterns and regular physical activity.

In fact, adherence to the Mediterranean diet has been reported to be inversely associated with the risk of overweight, obesity, and long-term weight gain [32].

Recent research has shown that severe dietary restriction accompanied by substantial weight loss can trigger biological starvation responses involving changes in metabolism, hormones, and the nervous system [27,28,34]. These changes may help explain why body weight often returns after weight loss and, in some cases, even exceeds the pre-weight-loss level.

         
In addition to these biological responses, I believe that unhealthy weight-control behaviors, particularly dietary restriction, are also likely to promote intestinal starvation, thereby contributing to an upward shift in the body-weight set point. 

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