Joint association of overweight/obesity, high electronic screen time, and low physical activity time with early pubertal development in girls: a case–control study

To examine the joint association of electronic screen time (EST), moderate-to-vigorous physical activity time (MVPA) and overweight/obesity with early pubertal development (EPD) in girls. A case–control study of 177 EPD girls and 354 girls with normal pubertal development was conducted between October 2019 and August 2022. Overweight/obesity was defined as body mass index ≥ 85th percentiles for age and sex. We found a non-significant increase of EPD risk among girls with high EST alone [OR: 2.75 (0.65–11.58)] or low MVPA alone [OR: 2.54 (0.74–8.69)], but a significant increase of EPD risk among girls with overweight/obesity alone [OR: 4.91 (1.01–23.92)], compared to girls without any of the three risk factors (low MVPA, high EST and overweight/obesity). Girls with any two of the three risk factors faced increased risk of EPD, and girls with all three risk factors faced the highest risk of EPD [OR and 95% CI: 26.10 (6.40–106.45)]. Being overweight/obesity might be more important than having low MVPA or high EST as a correlate of EPD compared to girls without any of the three risk factors, but the co-presence of low MVPA, high EST and overweight/obesity would largely increase the risk of EPD in girls.


Data collection methods
We obtained information about the participants by using a questionnaire.The investigator filled out the questionnaires during a face-to-face interview with the children and their guardians.The questionnaire was conducted by the researchers after reviewing the current evidence and revised by several pediatricians based on their expertise (a copy of the questionnaire was presented in the Supplementary Material 1).The questionnaire contained five sections including: (1) General information (age, date of birth, and ethnicity); (2) Parental information (height and weight of parents, mother's age of first menarche, and father's age of first spermatogenesis); (3) Diet information in 1 month prior to the survey and the average per week was calculated, by asking "How many eggs did you eat, including all kinds of cooking like poaching, frying, and being included in other foods ?", "How often did you eat snacks, including chips, cookies, candy, cake, chocolate, ice cream and other desserts ?"; (4) Behavior information in 1 month prior to the survey, and the average time per week was calculated, including: electronic screen watching time, MVPA time, and time spent on the roadside (including parking, walking, or biking on the roadside); the proportion of plastic bottled water in total drinking water was also surveyed.Moderate physical activity is defined as levels 12-14 of 20 on the RPE scale 22 , and the intensity is 3.0 to 5.9 metabolic equivalent (MET), such as jogging, skating, cycling at normal speed, etc. Vigorous physical activity is defined as levels 15 or above of 20 on the RPE scale, and the intensity is ≥ 6 MET, such as carrying heavy objects, running fast, cycling fast, etc.; (5) Sleep habits in 1 month prior to the survey, and the average time per day was calculated, including: wakeup time in the morning, bedtime at night, average time of night sleeping, and average time of day sleeping.The total sleeping time was calculated by night sleeping time plus day sleeping time.
Children's height, weight, and waist circumference were measured according to standardized procedures.Body weight was measured to the nearest 0.01 kg by using a digital scale (TCS-60, Tianjin Weighing Apparatus Co., China).Standing height was measured to the nearest 0.1 cm using a stadiometer (SZG-180, Shanghai Zhengdahengqi, China).Children's waist circumference was measured midway between the lower rib margin and the iliac crest, and the measurement was rounded to the nearest 0.1 cm.Children's body fat percentage was measured using a body composition analyzer (Inbody J-20, South Korea).Body mass index (BMI) was obtained by dividing weight in kilograms by the square of height in meters.Children's Z scores of BMI for age were calculated according to the WHO age-and gender-specific growth reference (0 ~ 60 months 23 and 5 ~ 19 years old 24 .Childhood overweight/obesity was defined as BMI ≥ 85th percentiles (Z score of BMI for age ≥ 1.035) which included both overweight and obesity to improve the statistical power of test.The period of data collection included both of the periods before (between October 2019 and December 2019) and during (between January 2020 and August 2022) the COVID-19 Pandemic, and this confinement period might be a strong influencing factor.However, only 13 EPD girls and 62 controlled girls were recruited before the COVID-19 Pandemic, so we did a sensitivity analysis by excluding participants who were recruited before the COVID-19 Pandemic.

Ethical approval
The studies involving human participants were reviewed and approved by the Ethics Committee of Tianjin Women and Children's Health Center (approval number: ky-20190119).Written informed consent to participate in this study was provided by the participants' legal guardians.

General characteristics
In all participants, the age of girls in the case group and the control group was 7.72 ± 1.0 years and 7.60 ± 1.0 years, respectively, with no statistical difference between the two groups (t = − 1.300, P = 0.194).According to the general information shown in Table 1, there were significant differences of mother's age of menarche, father's age of first spermatogenesis, height, weight, BMI, waist circumference, EST, MVPA, time on the roadside, proportion of plastic bottled water in total drinking water, bedtime at night, night sleeping time, day sleeping time, and total sleeping time (all P values < 0.05).In the sensitivity analyses after excluding participants who were recruited before the COVID-19 Pandemic, girls in the case control group was older than those in the control group, and other comparison between the two groups almost did not change.

Independent association of overweight/obesity, low MVPA and high EST with EPD
EPD was independently associated with overweight/obesity, low MVPA and high EST in both univariate and multivariate analyses (Table 2), and the multivariable-adjusted ORs and 95% CIs were 2.41 (1.43-4.06),3.12 (1.72-5.65),and 4.01 (2.44-6.57)respectively as compared to their reference counterparts.In the sensitivity analyses after excluding participants who were recruited before the COVID-19 Pandemic, results almost did not change.

Joint association of overweight/obesity, high EST and low MVPA with EPD
The relative contributions of exposures to overweight/obesity and low MVPA and both for the risk of EPD were showed in Fig. 1A.Using girls with normal weight and high MVPA as the referent group, the OR of EPD increased from 1.00 to 3.30 (1.15-9.49)for the isolated presence of overweight/obesity, and to 3.67 (1.69-7.95) for the isolated presence of low MVPA, and to 8.07 (3.38-19.27)for the co-presence of overweight/obesity and low MVPA.The relative contributions of exposures to overweight/obesity and high EST and both for the risk of EPD were shown in Fig. 1B.Compared to girls with normal weight and low EST, the OR of EPD increased from 1.00 to 2.87 (1.42-5.77)for the isolated presence of overweight/obesity, and to 4.51 (2.49-8.16)for the isolated presence of high EST, and to 8.92 (4.03-19.73)for the co-presence of overweight/obesity and high EST.In the sensitivity analyses after excluding participants who were recruited before the COVID-19 Pandemic, results almost did not change (Supplementary Material 2).
To examine the joint association of overweight/obesity, high EST and low MVPA with EPD, girls were divided into eight groups, and those without any of the three risk factors (low MVPA, high EST and overweight/obesity) were designated as the reference group (Fig. 2).We found a non-significant increase of EPD risk among girls with high EST alone [OR: 2.75 (0.65-11.58)] or low MVPA alone [OR: 2.54 (0.74-8.69)], but a significant increase of EPD risk among girls with overweight/obesity alone [OR: 4.91 (1.01-23.92)],compared to girls without any of the three risk factors (low MVPA, high EST and overweight/obesity).Girls with any two of the three risk factors faced increased risk of EPD, with ORs and 95% CIs of 13.08 (3.77-45.40)for the co-presence of high EST and low MVPA, 6.45 (1.33-31.27)for the co-presence of high EST and overweight/obesity, and 6.21 (1.68-23.00)for the co-presence of low MVPA and overweight/obesity.Girls with all of the three risk factors faced the highest risk of EPD [OR and 95% CI: 26.10 (6.40-106.45)].In the sensitivity analyses after excluding participants who were recruited before the COVID-19 Pandemic, results almost did not change (Fig. 3).

Discussion
In this case-control study, overweight/obesity, low MVPA and high EST were all risk factors for EPD in girls.We found a non-significant increase of EPD risk among girls with high EST alone or low MVPA alone, but a significant increase of EPD risk among girls with overweight/obesity alone, compared to girls without any of the three risk factors.Being overweight/obesity might be more important than having low MVPA or high EST as a correlate of EPD compared to girls without any of the three risk factors, but the co-presence of low MVPA, high EST and overweight/obesity would largely increase the risk of EPD in girls.
Childhood obesity and EPD are both major public health problems worldwide, and the relationship between childhood obesity and early onset of puberty has drawn more and more attention.The present study confirmed the results of previous studies that increased adiposity is associated with earlier onset of puberty in girls 25 .As this association is more complex in boys 25 and the prevalence of EPD is higher in girls than in boys, we only focused on girls in the present study.The mechanisms underlying this association of obesity with EPD remain elusive.A recent review summarized the recent progress regarding how childhood obesity impacts on hypothalamicpituitary-gonadal (HPG) axis and pubertal onset 12 .Simply, the increase of fat can promote the secretion of leptin, ghrelin, insulin and other hormones, all of which can stimulate the hypothalamus and promote the development of gonads, leading to the onset of puberty.
After the outbreak of the novel COVID-19, China and other countries globally enacted lockdown policies as an epidemiological containment strategy to reduce the spread of COVID-19 infection.During the lockdown, unhealthy lifestyles and behaviors were observed including over-eating or skipping meals, sleep problems, decreased physical activity, increased use of screening devices, changes in body weight, smoking behavior, and alcohol consumption 26,27 .At the same time during the pandemic and lockdown for COVID-19, the incidence of EPD increased and puberty accelerated in female 28,29 .The period of data collection in our study included both of the periods before and during the COVID-19 Pandemic.As a case-control study, it aimed not to compare the exact rates of EPD and unhealthy behaviors between the periods before and during the COVID-19 Pandemic, but to examine the association of EST, MVPA and overweight/obesity with EPD in girls.This confinement period Table 1.General information for the case and control groups.The t-test was used to compare the continuous variables and the results were presented in mean ± SD.The χ 2 test was used to compare the categorical variables and the results were presented as frequency and percentage (%).EST electronic screen time, MVPA moderate-to-vigorous physical activity.*Data of body fat percent was missing for 19.8% (35/177) in the case group and 57.0%(200/351) in the control group.# Sensitivity analysis, by excluding participants who were recruited before the COVID-19 Pandemic.

All participants
Participants www.nature.com/scientificreports/might be a strong influencing factor, so we did a sensitivity analysis by excluding participants who were recruited before the COVID-19 Pandemic.The results of sensitivity analyses showed similar effects of EST, MVPA and overweight/obesity on EPD with those in total participants.Thus, we suggested that high EST, low MVPA and overweight/obesity would increase the EPD risk regardless of the COVID-19 Pandemic.This finding was consistent with the review presented at the 60 th Annual European Society for Pediatric Endocrinology Meeting in Rome, which suggested that the increase of EPD during the pandemic of COVID-19 might not have anything to do with the infection at all, and time spent during lockdowns scrolling through smart devices for hours on end could be to blame 18 .The present study found that the risk of EPD increased three times in girls who spent more than 7 h per week on electronic screen, and it supported the above view 18 .The mechanisms of light exposure correlating with early signs of maturity have not been confirmed among human beings, but they have been explained by animal studies, including sleep disruption and melatonin theory 15 .Sleep disruption could result in lack of sleep which is correlated with weight gain, and weight gain is correlated with early puberty 15 .Light exposure could reduce the secretion of melatonin, and then advance the secretion of pituitary gonadotropin which was normally suppressed by melatonin, leading to EPD 15 .
In this study, we also found that the risk of EPD increased more than two times in girls who did not exercise enough (MVPA ≤ 7 h/week).The main mechanism is that physical exercise specifically inhibits the pulsating release of hypothalamic gonadotropin-releasing hormone (GnRH), restricts the pituitary secretion of luteinizing hormone and, to a lesser extent, follicle-stimulating hormone, thereby limiting ovarian stimulation 16 .At the same time, physical activity may delay puberty by reducing the synthesis of sex hormones by reducing obesity, or by decreasing insulin levels, increasing sex hormone-binding globulin levels, and decreasing estradiol bioavailability 17 .
There are no studies assessing the joint association of overweight/obesity, EST and physical activity with EPD previously.Our findings regarding the joint effect of high EST, low MVPA and overweight/obesity on EPD in girls is biologically plausible.Observational studies have concluded that behaviors and lifestyles including EST and physical activity are associated with obesity.Gordon-Larsen et al. reported that obesity risk was proportional to TV-watching hours and inversely proportional to significant physical activity among adolescents 30 .Therefore, besides the potential mechanisms of light and physical activity mentioned above, both high EST and low MVPA could increase the risk of EPD by influencing obesity.
This study assessed the joint association of overweight/obesity, EST and physical activity with EPD in girls for the first time.However, it has some limitations.Firstly, most of the information including EST and MVPA were not obtained by a direct measure, but through interview questionnaires.This kind of data collection could lead to bias.Secondly, this study could not distinguish between central or peripheral EPD, as the sex hormone stimulation test was not carried out in all girls in the case group.And also not all participants underwent the bone age X-ray examination.Finally, the study participants were collected from one medical institution which might introduce selection bias.Thus, our findings may not be generalized to all populations of Chinese children.
The findings of this study need to be further confirmed by future studies.

Conclusion
In this case-control study, we found that overweight/obesity, low MVPA and high EST were all risk factors for EPD in girls.Being overweight/obesity might be more important than having low MVPA or high EST as a correlate of EPD in girls, as the isolated presence of low MVPA or high EST would not significantly increase the risk of EPD whereas the isolated presence of overweight/obesity would significantly increase the risk of EPD in girls.However, the co-presence of any two of the three risk factors would largely increase the risk of EPD in girls.The

Figure 1 .
Figure 1.Relative contributions from exposures to overweight/obesity, low MVPA and both (A), and overweight/obesity, high EST and both (B) to the risk of EPD in all participants.MVPA moderate-to-vigorous physical activity, EST electronic screen time.

Figure 2 .
Figure 2. Joint effects overweight/obesity, low MVPA and high EST on the risk of EPD in all participants.

Figure 3 .
Figure 3. Joint effects overweight/obesity, low MVPA and high EST on the risk of EPD in Participants recruited during the COVID-2019 Pandemic.
The criterion of statistical significance was < 0.05 (for two-sided tests).All statistical analyses used IBM SPSS Statistics 24.0 (IBM SPSS, Chicago, IL).The general characteristics between groups were compared by using student's t-test for continuous variables if their normal distribution was not rejected, and Chi-square test for categorical variables.

Table 2 .
Odds ratios and 95% confidence intervals of electronic screen time and physical activity for precocious puberty.EST electronic screen time, MVPA moderate-to-vigorous physical activity.*Adjusted for mother's age of menarche, father's age of first spermatogenesis, age, time on the roadside, proportion of plastic bottled water in total drinking water, night sleep time, and day sleep time.# Sensitivity analysis, by excluding participants who were recruited before the COVID-19 Pandemic. No.