The rising prevalence of childhood obesity has become a pressing global health concern. A comprehensive analysis reported a substantial increase in obesity rates among children and adolescents across more than 200 countries between 1990 and 2020, with prevalence reaching 7.5% in boys and 5.4% in girls by 2020 [1]. The prevalence of overweight or obesity among children in OECD (Organization for Economic Co-operation and Development) countries has reached approximately 25%, posing a significant public health challenge [2]. Obesity in children and adolescents, akin to its effects in adults, is linked to a range of adverse outcomes—including type 2 diabetes, metabolic dysfunction-associated steatotic liver disease, sleep apnea, and mental health conditions—while also increasing the risk of type 2 diabetes and cardiovascular disease in early adulthood [3,4]. Importantly, children with overweight or obese are more likely to become adults with obesity, facing a heightened risk of numerous serious health complications [5,6].

Adiposity rebound (AR), first described by Rolland-Cachera et al. [7], represents the point in childhood when body mass index (BMI), after declining during infancy, reaches its lowest point (nadir) around 5 to 7 years of age before beginning to increase again through adolescence. This shift from a decreasing to an increasing BMI trajectory is defined as AR. When AR occurs earlier than expected, a phenomenon termed early AR, it is recognized as a significant risk factor for obesity in childhood, adolescence, and adulthood. Early AR has also been strongly linked to an elevated risk of metabolic disorders, including type 2 diabetes [7-10]. These associations are mediated by several interrelated mechanisms, such as early proliferation of adipocytes, metabolic programming [11], persistent unhealthy dietary and physical activity patterns, hormonal dysregulation [12], genetic predisposition, and cumulative environmental exposures [11]. These interconnected processes can lead to lasting alterations in body composition and metabolic function, predisposing individuals to obesity and related metabolic disorders later in life. Recent longitudinal cohort studies have demonstrated that early AR, occurring between 1.5 and 3 years of age, is associated with increased BMI and waist-to-height ratio in adulthood [13]. Moreover, additional studies have linked early AR to obesity and metabolic syndrome in middle age, particularly after the age of 40 [8]. Evidence further indicates that early AR is associated with elevated triglycerides, apolipoprotein B, and blood pressure, with sex-specific differences observed—boys tend to exhibit higher blood pressure, while girls are more prone to lipid abnormalities [8]. Early AR has also been shown to accelerate fat accumulation during childhood, with significant implications for long-term metabolic health [7]. These effects are especially pronounced in children born small for gestational age or preterm, who face heightened risks of obesity and cardiovascular complications in adulthood [14,15]. This growing body of evidence underscores the importance of identifying and addressing early AR to mitigate its long-term impact on metabolic health.

Given the long-term risks associated with early AR, early identification and intervention targeting modifiable lifestyle factors—such as reducing screen time and promoting healthy dietary habits and physical activity—are considered [8,10,16]. Despite the global rise in childhood obesity, large-scale cohort studies focusing on factors related to early AR remain limited [11]. This study leverages a robust, large-scale population-based cohort from Japan’s health check-up system to assess the prevalence of early AR at age 3 years and to explore individual and social factors that may be associated with its early occurrence.

The final analysis included 74,466 children with complete check-up data at both 1.5 years and 3 years, resulting in approximately 10.0% missing data from the 1.5-year cohort and 10.4% missing data from the 3-year cohort. Median BMI was 16.2 (interquartile range [IQR], 15.4–17.0) kg/m² at 1.5 years and 15.7 (IQR 15.0–16.4) kg/m² at 3 years. The median BMI z score was 0.46 (IQR. -0.16 to 1.03) at 3 years and 0.12 (IQR, -0.43 to 0.68) at 3 years. At age 3 years, based on WHO age‑ and sex‑specific BMI z scores, obesity was present in 4.0% (n=2,732) and overweight in 9.7% (n=7,198); at 1.5 years, the corresponding prevalences were 7.5% (n=5,596) and 17.2% (n=12,816), respectively.

Early AR, operationalized as ΔBMI >0 between the 2 visits, was observed in 25.0% (n=18,673) of children. As shown in Fig. 1, the distribution of ΔBMI (3 years–1.5 years) was approximately symmetric (skewness≈0.00); 25.0% showed an increase (AR (+)) and 75.0% did not. Among the nonincreasing group (ΔBMI ≤0 kg/m²), 71.0% (n=52,832) had a decrease (ΔBMI ≤-0.10 kg/m²) and 4.0% (n=2,961) were essentially unchanged (-0.10 < ΔBMI [kg/m²] ≤ 0).

The following other categorical values were observed for the study population (Table 1): 37,975 (51%) male children; median birth weight of 3,000 g (IQR, 2745–3260 g); 6,041 (8%) had single-parent households; 55,119 (83%) had other siblings at home; 64,556 (87%) attended childcare; 67,362 (91%) had a routine nap time; 53,368 (72%) slept <10 hours; 13,809 (18%) were able to brush their teeth alone; 71,707 (96%) routinely ate breakfast; 19,198 (26%) had dental caries at age 3 years, and 1887 (3%) had dental caries at 1 year; 19,479 (26%) routinely drank juice; 63,538 (86%) routinely ate fast food; 14,229 (19%) used a car seat; and 65,818 (88%) had a primary care physician.

At 3 years of age, 25% of children had early AR. Besides the factors detailed in Table 2, a factor significantly associated with early AR on logistic regression analysis was obesity at age 1.5 years (OR, 4.32; 95% CI, 4.00–4.67). On multiple logistic regression analysis, obesity, birth weight categorization, and mandatory vaccine rate >80% at 1.5 years of age were positively associated with early AR. The following factors, however, were negatively associated with early AR for children: male sex, single-parent status, worker’s role in a family, routine nap time, and routine consumption of breakfast, juice, or fast food. Among these children, male sex (OR, 0.59; 95% CI, 0.57–0.61) and routine napping habits (OR, 0.84; 95% CI, 0.79–0.90) were significant factors that decreased the risk of early AR, while obesity at 1.5 years strongly predicted early AR (OR, 4.32, 95% CI 4.00–4.67). Routine juice intake or fast-food consumption showed no significant associations. Supplementary Table 2 documents the presence of a stable group with similar BMI at both time points and compares characteristics across AR (+), stable, and decrease categories.

This study is the first to investigate the prevalence and risk factors of early AR for children at age 3 years in a large pediatric population. Although the prevalence of obesity was only 4% in this large pediatric cohort of Japanese children at age 3 years, 25% of children had already experienced AR. After controlling for factors related to early AR, obesity, birth weight categorization, and mandatory vaccine rate >80% were positively associated with early AR. By contrast, male sex, a single-parent household, parental employment status, and routine napping were associated with lower odds of early AR, whereas regular juice and fast‑food consumption showed no significant associations.

The mechanisms driving the development of early AR are complex and multifaceted, arising from intricate interactions between neurodevelopment, metabolic demands, endocrine fluctuations, and genetic and environmental factors [11]. One hypothesis suggests that changes in the brain's energy requirements during critical developmental periods may result in excess energy being stored in adipose tissue, thereby contributing to early AR [20]. Hormonal variations, including changes in growth hormone and insulin-like growth factor-1 levels, are also thought to influence adipose tissue metabolism and AR timing [12,21]. Additionally, early-life factors such as nutritional status, physical activity levels, and genetic predispositions likely play essential roles in determining the onset of early AR [19]. Understanding why early AR occurs is vital for identifying at-risk populations and for informing potential prevention strategies. Further research in diverse populations is necessary to explore how genetic, cultural, and environmental factors collectively influence the timing and consequences of early AR.

Our study is the first to specifically examine early AR among 3-year-old children, revealing that 25% had already experienced early AR by this age. However, the long-term consequences of early AR in this population remain unclear. Previous research in Japan indicates that AR typically occurs around the age of 5 years [8], which is earlier than what has been observed in European populations [7,15]. This variability highlights the potential influence of genetic, cultural, and environmental differences on the timing of AR. Furthermore, a recent retrospective cohort study from a Japanese academic university hospital suggested that some children may already show signs of early AR by their 3-year health check-ups [13]. These findings underscore the need for further international studies to investigate early AR in different populations, as they may uncover new insights into its mechanisms and implications, particularly when considering variations across regions and cultures.

Our study demonstrated that routine consumption of breakfast and naptime habits were associated with lower odds of early AR. It has been widely reported that routine breakfast consumption is associated with the prevention of childhood obesity [18]. No previous studies have been conducted to investigate whether napping prevents early onset of AR or obesity in children younger than 3 years. In another regard, short sleep time has been linked to obesity in preschool children [22]. More than 70% of the study participants did not meet the WHO recommendation of 10 h of sleep or more [22], suggesting that adequate total sleep duration may play a more critical role than napping itself in mitigating early AR.

Sex differences appear to influence childhood growth trajectories and subsequent obesity risk. Based on data collected from 1983 to 2010 for 5000 boys and 2000 girls in Poland, AR occurred earlier in girls [23,24]. Other studies have similarly reported that girls, particularly those with higher BMI percentiles, tend to experience earlier AR than boys [25]. Conversely, some studies found negligible or nonexistent sex differences. Of concern, girls with increased BMI before the age of 3 were more likely to develop insulin resistance by age 12 compared to boys [26]. These findings highlight the importance of heightened vigilance among parents and caregivers regarding early AR in daughters.

By contrast, neither routine juice consumption nor fast-food consumption was associated with an increased risk of early AR. The timing of AR has not been demonstrated to be linked to high protein intake in several studies [27,28]. Additionally, no significant associations have been reported between the timing of AR and nutritional intake of energy foods, fats, or carbohydrates [29]. These results underscore the multifactorial nature of early AR and suggest that dietary patterns alone may not fully explain its onset.

The strengths of this study are 3 key factors: it was the first study with a large sample size design conducted in Japan; it had a high rate of child participation in routine health check-ups; and the data accuracy was ensured by data collection performed by trained nurses, physicians, and dentists. This study has several limitations. First, its retrospective design limited the ability to establish temporal relationships and causality. Specifically, we did not have access to information about maternal and paternal BMI or the educational backgrounds of the parents or guardians. Second, information on children’s dietary and sleep habits was based on caregiver self-reports, which may be subject to recall and social desirability bias. Additionally, we did not gather data on physical activity during these health examinations for children younger than 3 years, which is important because early AR appears to reflect a positive energy balance during the preschool years [30]. However, the exact contributions of physical activity, sedentary behavior, and food intake to the positive energy balance and to the timing of AR remain unclear [10]. Another key limitation is that we did not prespecify a threshold for a clinically meaningful increase in BMI (e.g., ΔBMI [kg/m²] ≥0.10 or ≥0.20). Future work should evaluate such thresholds; the absence of this criterion may attenuate effect estimates. Finally, our 2 measurement points (1.5 years and 3 years) cannot establish the true BMI nadir; therefore, some children categorized as early AR may simply be on an upward trajectory without having reached the nadir. This potential misclassification likely biases associations toward the null rather than creating spurious findings. Despite these limitations, this study represents the largest investigation to date on the prevalence and risk factors of early AR in Japanese preschool children. By incorporating a variety of socioeconomic factors commonly observed in developed countries, this research provides valuable insights into early AR. These findings may significantly inform future studies of childhood obesity, particularly in understanding the interplay of socioeconomic, behavioral, and biological factors. Future research should further examine these relationships and assess strategies that may help address early AR. Understanding why early AR occurs is vital for identifying at-risk populations and informing prevention efforts, while recognizing that additional longitudinal studies are needed to clarify causal pathways and underlying mechanisms.

In conclusion, in this population-based cohort of 74,466 children, approximately 25% had early AR by age 3. Female sex and nonroutine napping (shorter sleep) were associated with higher odds of early AR, while routine breakfast consumption and napping habits were associated with lower odds after adjustments, suggesting actionable prevention targets.