The association between breastfeeding and childhood obesity/underweight: a population-based birth cohort study with repeated measured data

Based on a population-based birth cohort study in north China, the present study found that compared with exclusive formula feeding, breastfeeding was inversely associated with the risk of childhood obesity from 2 to 6 years old, and there was a trend from mixed feeding to exclusive breastfeeding. Another finding was that infant exclusive formula feeding might be a risk factor for childhood underweight at 3 and 5 years old.

The impact of breastfeeding on childhood obesity has long been under debate, with most research showing significant association [8, 9], and others showing weak or no association between breastfeeding and childhood obesity [10]. The variation in the effects of breastfeeding on childhood obesity resulted from the differences of obesity definition, breastfeeding definition, study design (cross sectional study or cohort study), residual confounding by other potential factors, and so on. In addition, the difference of children’s age across different studies might also be an influencing factor, as the association of breastfeeding with childhood obesity may be diluted over time [15]. A recent meta-analysis has shown that breastfeeding is inversely associated with a risk of early obesity in children aged 2 to 6 years (OR 0.83, 95% CI 0.73, 0.94) [16]. The present study, based on a population-based birth cohort study and with 6 times of repeated measured data, added evidence to the inverse association between breastfeeding and obesity in preschool children. In particular, the association of exclusively breastfeeding was larger as compared with that of mixed feeding. Moreover, all of the previous studies focused on the association between breastfeeding and childhood obesity, and no studies to our knowledge have assessed the association between breastfeeding and childhood underweight. The present study suggested that infant exclusive formula feeding might be a risk factor for childhood underweight at 3 and 5 years old.

Mechanistic pathways linking breastfeeding to childhood obesity are explained previously. Some researchers gave nutritional explanations. First, breast milk provides a moderate amount of calories and nutrients for infant, while formula milk provides higher levels of fat and protein than the baby’s needs [17]. Second, breast milk contains bioactive substances such as adiponectin, leptin and ghrelin, which can influence the proliferation and differentiation of the infant’s adipocytes [18], and have a protective effect against future obesity [19]. Other researchers suggested psychological and behavioral explanations. Breastfeeding can help an infant establish a healthy eating habit to control food intake, while formula feeding may teach him to neglect satiety cues [20]. Moreover, breastfed infants are more likely to delay the introduction of solid foods, which decreases the odds of childhood obesity [21].

To our knowledge, no studies have focused on the mechanism of breastfeeding on childhood underweight. Some studies suggested that lactation could promote the proliferation of beneficial bacteria including Bifidobacteria [22], and more than 6 months of exclusive breastfeeding could reduce significantly the risk for episode of gastrointestinal infection(s) during months 1–9 [23]. Based on these studies, we hypothesized that Children who are exclusively breastfed have a better gastrointestinal microenvironment and less gastrointestinal infection, and therefore are less likely to be underweight. The mechanism of breastfeeding on childhood underweight should be further investigated.

Breast milk is considered the ideal food for infants, as it provides adequate energy and nutrients to meet the infants’ needs [7]. Breastfeeding has long-term benefits throughout a child’s lifetime [7]. The WHO recommends infants should be exclusively breastfed for the first 6 months of life, and continue to breastfeed supplemented with additional foods for the first 2 years and beyond [7]. However, nearly 2 out of 3 infants are not exclusively breastfed for the recommended 6 months-a rate that has not improved in 2 decades [7]. In China, the crude and weighted exclusive breastfeeding rate under 6 months was 20.7% and 18.6% based on a national representative survey in 2013 [24]. The present study reported the rate of exclusive formula feeding, mixed feeding, and exclusive breastfeeding were 10.0%, 59.4% and 30.6%, respectively. As shown in Table 1, both maternal (age, BMI, education, smoking status, and history of gestational diabetes) and child characteristics (sex, birth weight, and gestational age) can affect the prevalence of breastfeeding. Other affecting factors include maternal race/ethnicity, breast diseases, inadequate breast milk production, employment, length of maternity leave, inadequate knowledge regarding breastfeeding, lack of familial and societal support, and lack of guidance and encouragement from health care professionals [16, 25]. To strengthen breastfeeding practices, families, employers, professional workers and society as a whole should fully support breastfeeding mothers.

The major strength is that, to our knowledge, we are the first to report the association of breastfeeding with childhood body size based on a population-based birth cohort study and with 6 times of repeated measured data. We are also the first to evaluate the association between breastfeeding and childhood underweight. Moreover, all the information used in the present study was collected from the electronic medical records, which can avoid the recall bias. Our study has certain limitations. First, almost 70% of the children screened were excluded as they lacked some of the key information. Though we compared the general characteristics between the included and excluded children, we could not exclude the possibility that the observed effect sizes in our study had departure from the true value. Second, we do not have the information of infant feeding modalities after 6 months old, so we cannot evaluate the association between the duration of breastfeeding and childhood obesity/underweight. Third, even though our analyses adjusted for an extensive set of confounding factors, residual confounding due to unmeasured factors cannot be excluded, including maternal information (chronic diseases and/or medication, employment, marital status, living alone or co-habiting, etc.) and child information (method of child birth, proportion of preterm births, need for neonatal intensive care unit, chronic diseases and/or anomalies, and childhood physical activity and dietary factors, etc.).

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