WHO and United Nations International Children's Emergency Fund (UNICEF) recommend that breastfeeding be initiated within one hour of birth, that it continue with no other foods or liquids for the first six months of life, and be continued with complementary feeding until at least 24 months of age. Globally, the overall rate of EBF for infants under six months old is 40% in 2017 (16). But in China only 29.2% of the infants under 6 months were breastfed exclusively in 2017 (17). The problem of low breastfeeding rates is even more serious in the United States. For example, in 2015, the mean prevalence of women EBF their children by the age of six months was 24.9% in the United States (18). Currently, for 89.8% of children and adolescents in Shibei District of Qingdao, the breastfeeding duration is more than 6 months and 57.5% of children and adolescents’ breastfeeding durations were more than 12 months, which is significantly higher than the overall level of the world. This decision of how long breastfeeding lasts is strongly influenced by economic, environmental, social, and political factors, such as inadequate healthcare support, marketing of baby foods, and workplace support for women (16). In addition, our study indicated that infants who are non-first birth or vaginal delivery are more likely to breastfeed for more than 12 months. A similar conclusion was reached in Du Li et al.’s study (19). This may have to do with the fact that infants delivered normally are more likely to stimulate breast milk secretion, and that mothers with two or multiple children are more aware of the advantages of breastfeeding.
Up to now, most studies have confirmed that prolonging the time of breastfeeding in infancy can reduce the prevalence of overweight and obesity in children and adolescents. For example, studies conducted in Hong Kong (20) and Spain (21) suggested that the BMI of formula-fed infants increased faster than breastfed infants. And Spain’s birth cohort study also found that babies in the region who have never been breastfed will have a 7.8% increased risk of childhood obesity and overweight, and that every week of breastfeeding will reduce their childhood BMI by 3.5% (21). Our study proved that the longer breastfeeding duration was during infancy, the lower the BMI would be in childhood and adolescent years. But in a cohort study involving 5 years 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) (22). Durmus et al. also reported similar findings (OR = 1.20; 95%CI: 0.98-1.47) in Dutch children at the age of 3 (14). These inconsistency in the results of different studies may be attributed to children’s age, parental country of birth, parental age, parental smoking, education and cultural differences (22). The related biological mechanism of obesity in children may be closely related to protein intake and energy metabolism. First, the hormones in breast milk, such as leptin, ghrelin, and adiponectin, were identified to be involved in energy balance regulation and to protect children from obesity (23). Second, infant formula generally has a higher protein content than breastmilk, and protein intakes in excess of metabolic needs in early life may stimulate the secretion of insulin and insulin like growth factor type one, which in turn, promotes weight gain in infancy (24). Finally, breastfed infants are more likely to accept low-calorie complementary foods such as vegetables, and tend to form a low-calorie diet in later stages, thus reducing the total dietary calorie intake of children and adolescents (25).
In recently published articles, consensus has not been reached about the time 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 overweight (26). Australian researchers conducted a survey on 2868 infants and found that children 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 (27). The research in Croatia indicated that breastfeeding for more than 6 months is a protective factor for overweight and obesity in children aged 6 to 11 (28). But our study considered infants who were breastfed for more than 12 months were significantly less likely to be overweight and obese in childhood and adolescent years. At present, the reasons for the differences in the optimal timing of breastfeeding in different studies are still unclear.
In addition, we discovered that prolonged breastfeeding could reduce the prevalence of childhood obesity, especially among boys aged 9 to11. 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 the late childhood (at the age of 9 to 11) (29). Meanwhile, from German logistic regression analyses which classified subjects into age groups, also indicated that the protective effects of breastfeeding is the most significant at the age of 7 to10 (30). The reason why this correlation is more obviously in boys may be 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 (31).
Surely, there are some limitations in the present study. First, the participants in this study is only from Shibei district of Qingdao, which may have selection bias and the questionnaire response rate is 38%. However, the sample size of this study is large, and it is still representative to a certain extent. Second, part of the data in this study were collected through questionnaires, which is prone to recall bias. We excluded the data with missing variables, which reduced the recall bias to a certain extent. Third, due to the lack of data of the weight of parents, BMI of parents was not included in the multivariate statistical analysis, but other confounding factors such as maternal pregnancy and birth history were included in the analysis, and a variety of stratified analysis was conducted on the data, and the results of the study were stable and reliable.