· Elementary school students gained weight during the lockdown that was implemented due to the coronavirus disease 2019 (COVID-19) pandemic in Japan. In contrast, junior high school students lost weight during this time. The COVID-19 pandemic had an unfavorable effect on weight gain, particularly in younger school-age children.

The novel coronavirus disease 2019 (COVID-19), which emerged in Wuhan, China at the end of 2019, has changed society worldwide. The number of infected people with COVID-19 in Japan has increased steadily since the country's first case was reported on January 16, 2020. On February 27, 2020, the Prime Minister abruptly requested that schools be closed temporarily. As the infection continued to spread, the first nationwide state of emergency was declared on April 16, 2020. At the time of this announcement, all kindergartens and schools were closed nationwide. Most schools closed the following week (of April 20, 2020). Children were forced to live extraordinary lives instead of their daily routine of going to school. On May 25, 2020, the state of emergency was ended. Schools resumed their regular operations on June 1, 2020. Most schools didn't start the new semesters until the end of May 2020, and even in areas where schools started in June, school attendance was initially low. Online classes were continued because of the ongoing presence of COVID-19. Students finally returned to school after the summer vacation.

Several studies have shown that these restrictions resulted in major changes in children’s lifestyles, such as decreased sports participation, increased media exposure, increased anxiety and fear, and changes in sleep patterns and eating behavior [1-3]. Recently, one group suggested that the COVID-19 pandemic increased the risk of weight gain by restricting children from attending school [4]. Because lifestyle changes are predictors of weight gain and obesity, almost all studies showed a considerable increase in childhood obesity during the COVID-19 pandemic [5,6].

Since 1900, physical measurements have been taken on a regular basis in Japan's public schools [7]. Currently, students are required to have a physical examination at school twice a year. In 2015, the Japanese Society of School Health published the Health Examination Manual for Children (revised edition), and developed a software known as the Children's Health Management Program [8]. This software was distributed on compact discs to elementary, junior high, and high schools across the country. This software program creates growth and obesity curve graphs. Additionally, this software has a screening function that automatically searches for 9 categories of growth abnormalities, such as short stature, tall stature, obesity, and underweight.

Although the COVID-19 pandemic was associated with weight gain in adults [9,10], little is known regarding its effects on children’s physiques in Japan. To assess the pandemic-related changes in the physique of school-age youths, we compared data from school physical examinations performed during the pandemic in 2020 with those performed prior to the pandemic. We hypothesized that factors that negatively affect physique in schoolchildren were exacerbated during the pandemic-related lockdown.

Many studies have reported that the COVID-19 lockdown led to weight gain and exacerbated obesity in adults and children [4-6,9,10]. Long-term lockdowns negatively affect school children’s health and well-being. Children were unable to attend school as a result of the lockdown, and had to forego exercise or other physical activity. Therefore, the lockdown led to obesity in some children. These findings are consistent with the findings of previous studies regarding the effect of summer vacations on weight [16,17].

However, we found that the lockdown due to the COVID-19 pandemic did not promote obesity, but rather improved obesity in Japanese junior high school students aged 12–14 years. In contrast, severe obesity significantly increased in elementary school boys during the lockdown. Although the rate of obesity significantly increased from 2018 to 2019 in boys and girls, it did so at a slower rate than it had before the pandemic [12]. The COVID-19 lockdown increased the proportion of overweight and obesity in elementary school children, presumably because they became less physically active, and spent more time using screen-based media and snacking than they had prior to the pandemic [18,19]. This deterioration is most noticeable in elementary school children with a high degree of obesity.

In this study, the lockdown had little effect on adolescents’ weight gain, although this finding is limited to Japan. The rate of underweight declined, and almost all children appeared to maintain a healthy weight even during the pandemic. The short stature rate decreased from 2019 to 2020, whereas the tall stature rate increased. These findings indicate that the children's physique improved despite the pandemic. The only other report of weight loss due to the pandemic effect is that of older people, which suggested malnutrition and sarcopenia [20].

Obesity among boys and girls has increased in Japan since 1977, but has been declining since approximately 2003 [21-23]. However, in Osaka City, schools have increasingly used growth and obesity curves and detected physical abnormalities on a yearly basis [12]. The adoption of this growth curve software has increased the detection rate of physical developmental abnormalities. The detection rate of physical abnormalities has leveled off because all elementary and junior high schools have recently adopted this software. The annual rate of children with obesity has increased by 20%–30% up to 2019 [12]. However, the rate of increase from 2019 to 2020 reduced to ≤20%, except for a 40% increase in elementary school boys. The increased proportion of overweight and obese children reflects ongoing changes in body composition during the lockdown. Therefore, the rate of increase in obesity among Japanese elementary school students may be due to a fundamental cause, rather than an effect of the COVID lockdown.

Although the weight of elementary school students continues to rise, several factors may explain weight loss among junior high school students. Young children were more affected by the confinement of the lockdown than were older children. During the COVID-19 lockdown, Hourani et al. reported weight gain of 2.3 kg in children aged 6–12 years versus 1.7 kg in children aged 13–17 years [24].

Mulugeta et al. [25] reported a higher net increase in the body mass index of children younger than 12 years than in those over 12 years old. In the Philadelphia region, children aged 5–9 years were more obese than were teenagers [26]. This disparity between ages may be partially explained by the higher daily physical activity and energy needs in younger children compared to those of older children. In elementary school children, particularly boys, obesity worsened during the lockdown because of reduced physical activity and increased sedentary hours [27]. We hypothesize that this disparity is due to the fact that the majority of junior high school students, especially boys, are in puberty. It is speculated that the degree of obesity did not increase in junior high school students because growth in height exceeded that in weight. Obesity in elementary school students has long been recognized, but it does not appear to have been addressed. During the lockdown, many elementary school students may have been tempted to consume junk food, whereas junior high school students who followed a proper diet during the pandemic greatly reduced their weight.

In this study, the rate of short stature significantly increased from 2018 to 2019, but decreased after this time in elementary and junior high school children (including boys and girls). This finding may be explained by the increased use of the growth curve software prior to the pandemic. This abrupt change from an increase to a decrease underscores the magnitude of the COVID-19 pandemic's impact. The rate of a short stature might also have decreased because students slept longer and were better nourished during the lockdown than they were prior. Further research is required to explore these possibilities.

Rates of underweight in elementary and junior high school children significantly increased from 2018 to 2019 before the COVID-19 pandemic, but have remained stable since that time. The rate of underweight in school-age children declined significantly in elementary school children only.

There are some limitations to this study. First, the data were extracted on the basis of physical measurements that were taken at school; therefore, we could not determine each student's life patterns. Second, we performed a statistical comparison of the yearly data collected by each school, but did not perform individual data comparisons. Finally, this was a survey that was conducted until 2021; therefore, further follow-up will be useful once the lockdown is fully released.

In conclusion, elementary school students gained weight during the lockdown that was implemented because of the COVID-19 pandemic in Japan. In contrast, junior high school students lost weight during the lockdown. The COVID-19 pandemic had an unfavorable effect on weight gain, particularly in younger school-age children.