Overweight and obesity among children and adolescents have long been a focus of public concern. It is well-known that overweight and obesity are affected by many factors, such as genetic factors, socioeconomic factors, educational level, diet habits, and so on [19, 20]. In this study, the related factors included age, gender, parity, delivery mode, birth weight, birth length and breastfeeding (Supplementary Table 1). We found that students aged 6 to 11 years had a higher risk of being overweight and obesity than students aged more than 12 years. Meanwhile, boys were more likely to suffer overweight and obesity than girls. The possible reason for the gender differences is that boys consumed more fast food and spend more time in sedentary activities, including watching and using computers, phones and tablets [21, 22]. We also found the prevalence of overweight and obesity in students delivered by cesarean section were higher. A recent study has shown cesarean section could affect the colonization pattern of intestinal bacteria in infants, which might lead to overweight and obesity in children and adolescents . First born was one risk factor for overweight and obesity in our study, although the reasons weren't clear. It has been widely proved that the physical development situation of infants has long-term effect on children's physical growth, which even persists into their adolescence [24, 25]. Breast milk is the main food source in infancy, which is closely related to infant nutrition and physical development [26, 27]. But the research on breastfeeding and overweight or obesity among children and adolescents is inadequate [27, 28]. Some new findings which differ from previous studies have been obtained in this study.
The WHO and United Nations International Children's Emergency Fund (UNICEF) recommend that breastfeeding be initiated within one hour of birth, that it continues with no other foods or liquids for the first six months of life, and that it be continued with complementary feeding until at least 24 months of age. Globally, the overall rate of EBF for infants under six months old was 40% in 2017 . However, in China only 29.2% of infants under 6 months were breastfed exclusively in 2017 . The problem of low breastfeeding rates is even more serious in the United States. For example, in 2015, the mean prevalence of EBF in children until the age of six months was 24.9% in the United States . Currently, for 89.8% of the children and adolescents in the Shibei District of Qingdao, the breastfeeding duration was more than 6 months and 57.5% of the children and adolescents’ breastfeeding durations were more than 12 months, which is significantly higher than the overall level worldwide. The decision of how long breastfeeding lasts is strongly influenced by economic, environmental, social, and political factors, such as inadequate health care support, the marketing of baby foods, and workplace support for women . In addition, our study indicated that infants who are not the first-born child or those who are delivered vaginally are more likely to breastfed for more than 12 months. A similar conclusion was reached in Du Li et al.’s study . This may have to do with the fact that infants delivered vaginally are more likely to stimulate breast milk secretion, and that mothers with two or more children are more aware of the advantages of breastfeeding.
To date, most studies have confirmed that prolonging the duration of breastfeeding in infancy can reduce the prevalence of overweight and obesity in children and adolescents. For example, studies conducted in Hong Kong  and Spain  suggested that the BMI of formula-fed infants increased faster than that of breastfed infants. Spain’s birth cohort study also found that babies in the region who had never been breastfed will had a 7.8% increased risk of childhood obesity and overweight, and that every week of breastfeeding reduced their childhood BMI by 3.5% . Our study proved that the longer the breastfeeding duration was during infancy, the lower the BMI would be in childhood and adolescence. However, in a cohort study involving 5-year-old Swedish children, Huus et al. found no relationship between EBF and the risk of overweight including obesity (OR = 1.22; 95% CI: 0.81-1.83) . Durmus et al. also reported similar findings (OR = 1.20; 95% CI: 0.98-1.47) in Dutch children at the age of 3 . These inconsistences in the results of different studies may be attributed to the children’s age, parental country of birth, parental age, parental smoking status, education level and cultural differences . The related biological mechanism of obesity in children may be closely related to protein intake and energy metabolism. First, studies have confirmed that breast milk itself contains hormones that regulate energy metabolism and food intake, such as leptin and adiponectin. Breastfeeding can regulate the feeding centre and help to establish a dietary habit of consuming energy on demand in infancy, thereby reducing the occurrence of overeating . Second, the protein and calorie content of breastmilk is significantly lower than that of formula milk powder, which can prevent the excessive secretion of insulin and insulin-like growth factors, thereby reducing the deposition of fat and the increase of fat cells . Third, breastfeeding can provide enough nutrition for the infant so that solid foods do not need to be introduced until he or she is 6 months old. The early introduction of solid foods has also been linked to higher risks of obesity . Finally, recent studies suggest that breastfed infants are more likely to accept the low-calorie complementary foods such as vegetables, and are more inclined to form a low-calorie diet in later stages of life, thus reducing the total dietary calorie intake of children and adolescents .
In recently published articles, a consensus has not been reached about the duration at which breastfeeding significantly reduces the incidence of overweight and obesity in children. For example, Fallahzadeh et al. found that breastfeeding for more than 24 months was a protective factor for children from becoming overweight . Australian researchers conducted a survey with 2868 infants and found that children who breastfed for less than 4 months had a significantly increased risk of childhood weight exceeding the 95th percentile of children of the same age and sex . Research in Croatia indicated that breastfeeding for more than 6 months was a protective factor for overweight and obesity in children aged 6 to 11 years . However, our study considered that infants who were breastfed for more than 12 months were significantly less likely to be overweight and obese in their childhood and adolescent years. At present, the reasons for the differences in the optimal duration of breastfeeding in different studies are still unclear.
In addition, we discovered that prolonged breastfeeding could reduce the prevalence of childhood obesity, especially boys between the ages of 9 and 11 years. In a longitudinal study with 1,037 children, Poulton and Williams also pointed out that the protective effects of breastfeeding on childhood overweight were relatively weak up to the age of 7 years and then strengthened in late childhood (from the ages of 9 to 11 years) . Meanwhile, German logistic regression analyses that classified subjects into age groups, also indicated that the protective effects of breastfeeding were the most significant from the ages of 7 to10 years . The reason why this correlation is more pronounced in boys is currently unclear but there are several possible reasons. First, due to girls’ intrinsically high insulin resistance, girls have been found to have higher levels of triglycerides and lower concentrations of high-density lipoprotein than boys of the same age, which indicates that metabolic disturbances are more advanced in girls than in boys . Second, this correlation be affected by factors such as family education and social culture. At the ages of 9 to 11 years, girls’ self-awareness begins to mature gradually, and they tend to pay attention to their own image and consciously avoid obesity. However, Chinese culture has few restrictions on the body shapes of boys . Third, epidemiological studies have shown that the prevalence of overweight and obesity in boys is higher than that in girls [19, 20], and this study also had the same findings. This may make it easier to find the relationship between overweight in boys obesity and breastfeeding.
There are several strengths in this study. First, this study was carried out in the urban area of an emerging city in China, where there is currently a lack of research on the relationship between breastfeeding and obesity and overweight. Second, among previous studies, some only involved children, and some only focused on adolescents. Our subjects included both children and adolescents aged from 6 to 16 years. Surely, there are some limitations in the present study. First, the participants in this study were only from the Shibei District of Qingdao, which may have led to selection bias. However, the sample size of this study was large, and it is still representative to a certain extent. Second, some of the data in this study were collected through questionnaires, which are prone to recall bias. We excluded the data with missing variables, which reduced the recall bias to a certain extent. Third, Although we tried our best to identify relevant factors that may affect overweight and obesity, our data are still far from perfect, which may change the results from adjusting to potential confounding. Our future research will focus on these covariates.