Can breastfeeding in the first six months prevent childhood obesity? Childhood obesity is becoming a worldwide concern given the potential health implications in the future. Obese children are more likely to suffer physical and mental health problems and are likely to develop into obese adults (Labayen, Ruiz et al. 2012), thereby increasing the long term risk of developing chronic conditions such as diabetes, cardiovascular diseases and stroke. The cause of childhood obesity is multifactorial, including hereditary factors, comorbidities, dietary habits and physical activity. There is much debate as to the impact of breastfeeding during the early stages of life and how it correlates with childhood obesity compared with formula-fed newborns. Breast milk is nutritionally balanced to provide infants with all dietary requirements during the early stages of life. It also provides antibodies to reduce infection risks in newborns. Breast milk constitutes the appropriate amounts of protein, water, fat and sugar for a newborn and changes composition over time to adapt to a growing child’s needs. Formula tends to be higher in protein and fat than the baby actually requires and this excessive intake has been linked with adiposity (Hernell 2011). Marseglia et al have reviewed the potential impact of key breast milk constituents thought to play a role in reducing obesity risk (Marseglia, Manti et al. 2015). There have been a number of recent reviews discussing the association between breastfeeding and childhood obesity, all of which have concluded that breastfeeding confers a protective effect against childhood obesity and being overweight (Horta and Victora 2013, Aguilar Cordero, Sánchez López et al. 2014, Lefebvre and John 2014, Yan, Liu et al. 2014). The largest reduction in obesity risk was 81%, reported in a study of females aged 11 years of who had been breastfed for more than three months compared with controls who had never been breastfed (Panagiotakos, Papadimitriou et al. 2008). The males in the same study had a reduced risk of 72% and both results were statistically significant. However, other literature reports either no association between breastfeeding and childhood obesity (Burdette, Whitaker et al. 2006, Huus, Ludvigsson et al. 2008, Jing, Xu et al. 2014), or an increased risk of obesity following breastfeeding of 9% (Kwok, Schooling et al. 2010), 10% (Novaes, Lamounier et al. 2012), 11% (Buyken, Karaolis-Danckert et al. 2008), 14% (Sabanayagam, Shankar et al. 2009), 18% He (2000), 29% (Al-Qaoud and Prakash 2009), 34% (Neutzling, Hallal et al. 2009), 40% (Toschke, Martin et al. 2007) and 83% (Araújo, Victora et al. 2006), although none of which were statistically significant. Some studies suggest that there is a dose-response relationship, with increased duration of breastfeeding resulting in a decreased prevalence of being obese in childhood (von Kries, Koletzko et al. 2000, Fallahzadeh, Golestan et al. 2009, Griffiths, Smeeth et al. 2009, Yan, Liu et al. 2014). In contrast, other studies have reported no significant association between breastfeeding and its duration and obesity prevention (Burke, Beilin et al. 2005, Al-Qaoud and Prakash 2009, Sabanayagam, Shankar et al. 2009, Vehapoglu, Yaz?c? et al. 2014). One meta-analysis analysed the association between breastfeeding duration and obesity (Yan, Liu et al. 2014). As eligible studies reported different durations, the review categorised breastfeeding duration into less than three months, 3-4.9 months, 5-6.9 months and seven or more months. Those exclusively breastfed for at least seven months had a 21% decrease in the risk of childhood obesity, whilst those fed for less than three months only showed a 10% decrease. They concluded that the duration of breastfeeding was associated with a decreased likelihood of childhood obesity and reported a stepwise gradient of decreasing risk with increasing duration of breastfeeding. Single studies report a significant protective effect against childhood obesity when breastfeeding is done for at least one to three months (Goldfield, Paluch et al. 2006), three months (Twells and Newhook 2010), 13-25 weeks (McCrory and Layte 2012), four months (Scholtens, Gehring et al. 2007, Griffiths, Smeeth et al. 2009, Chivers, Hands et al. 2010), nine months (Nelson and Sethi 2005), 12 months (Burke, Beilin et al. 2005) and two or more years (Rathnayake, Satchithananthan et al. 2013). However, the differences in study design make it difficult to directly compare findings as the comparator groups can be formula-fed babies or babies’ breastfed for short durations. For studies investigating the impact of breastfeeding for at least six months on childhood obesity, the comparator group can be either newborns breastfed for less than six months (i.e. mixed feeding of variable durations) or newborns exclusively formula-fed. Additionally, the age of the children being assessed also differs in studies. When comparing those breastfed for at least six months with those breastfed less than six months, studies report a reduction in obesity risk of 60% when assessing two year olds (Weyermann, Rothenbacher et al. 2006), 54% and 43% in four year olds (Komatsu, Yorifuji et al. 2009, Simon, Souza et al. 2009), and 67% in six year olds (Thorsdottir, Gunnarsdottir et al. 2003). This suggests that the age of assessment affects the degree of risk reduction observed. However, when comparing against formula-fed newborns there are studies reporting reductions of 14%, 28% and 67% for three year olds (Poulton and Williams 2001, Armstrong, Reilly et al. 2002, Taveras, Rifas-Shiman et al. 2006), 6% for four year olds (Moschonis, Grammatikaki et al. 2008), 45% for seven year olds (Yamakawa, Yorifuji et al. 2013), 60% for nine year olds (Toschke, Martin et al. 2007), 64% for 11 year olds (Poulton and Williams 2001), 21% for 21 year olds (Poulton and Williams 2001) and 6% for 45 year olds (Michels, Willett et al. 2007). This data suggests that observing adults to determine the impact of breastfeeding on obesity is not advisable. Only one study reported an increased risk of obesity for newborns breastfed more than six months compared with formula-fed newborns, reporting a non-significant 40% increased risk of obesity in nine year olds (Toschke, Martin et al. 2007). Interestingly, very few detailed, for those breastfeeding for at least six months, whether the feeding duration was exclusively breastfeeding or mixed. Only two studies (Simon, Souza et al. 2009, Yamakawa, Yorifuji et al. 2013) reported on exclusive breastfeeding. There is evidence that exclusive breastfeeding also results in a decreased prevalence of being obese in childhood (Fallahzadeh, Golestan et al. 2009, Simon, Souza et al. 2009, Lefebvre and John 2014). Mayer-Davis et al (2006) compared exclusively breastfed newborns with exclusively formula-fed newborns and found that the breastfed children were significantly less likely to be overweight (34%) and that the results were not affected by maternal weight or diabetes status (Mayer-Davis, Rifas-Shiman et al. 2006). When exploring the differences between studies who defined breastfeeding as “Never – ever” and those reporting “exposure” to breastfeeding (implying mixed feeding practices of different types), a systematic review found a reduced likelihood of obesity in the exclusive feeding group of 20% and in the mixed group of 27% (Yan, Liu et al. 2014). This was supported by another review comparing “ever” breastfed with “exclusively breastfed for a specific number of months”, the latter showing a 27% decreased risk compared with the former at 21% (Horta and Victora 2013). That review postulated that if there is no critical window effect, but rather a cumulative effect of breastfeeding, studies that compared ever vs. never breastfed subjects will tend to underestimate any association. Any observed association between breastfeeding and later obesity does not prove causality (Butte 2001). There may be any number of potential confounders impacting on the relationship including geography, social deprivation status, parental weight status, smoking, marital status and education, ethnicity, gender, number of hospital admissions during the early stages of life, diet, sleep duration and physical activity. Whilst a number of studies discuss their impact, very few studies actually provide control for these factors in their analysis. The issue of geography is a potential confounder of any association between breastfeeding and obesity. In high-income countries, the babies usually receive formula, whereas many non-breastfed infants in low and middle income countries receive whole or diluted animal milk (Horta and Victora 2013). However, Hancox et al have reported that whilst breastfeeding reduced the risk of obesity slightly, there was no evidence that an association between breastfeeding and body mass index (BMI) was different in lower income countries compared with higher income countries (Hancox, Stewart et al. 2014). The socio-economic status of the mother may also contribute to the child’s weight status in childhood. The World Health Organisation (WHO) review analysed obesity risk in studies also controlling for social deprivation and found a further 3% decrease in the risk of obesity to 37% compared with studies which did not (34%) (Horta and Victora 2013). Armstrong et al reported that the reduced prevalence in obesity for breastfed children also persisted after adjustment for socio-economic status, birth weight and gender (30% reduction) (Armstrong, Reilly et al. 2002). The impact of gender was prominent as Nelson et al reported that breastfeeding for at least nine months reduced the risk of being overweight more in girls than in boys (Nelson and Sethi 2005). A similar gender inequality was reported by Panagiotakos et al with girls breastfed for more than three months having a larger reduced risk of obesity than the boys (Panagiotakos, Papadimitriou et al. 2008). Sibling studies have been unable to rule out the impact of confounders on childhood obesity. One study which controlled for this as part of a sibling study reported the adolescent BMIs were 0.39 standard deviations lower in the breastfed sibling than the non-breastfed sibling (Metzger and McDade 2010). However, another study of sibling pairs was unable to prove a protective effect for breastfeeding (Nelson and Sethi 2005). As well as the lack of control for confounders, other study limitations may affect the results reported. Definitions of obesity vary from a BMI of ?90th to ?97th, making any direct comparison of the outcome problematic. During their meta-analysis Yan et al investigated the association of breastfeeding and obesity, stratifying by the definitions of obesity and found a lower adjusted odds ratio for the BMI ? 97th group (25%) than the BMI ? 95th group (22%) (Yan, Liu et al. 2014). Most studies varied in the time when obesity was measured. As the definition of childhood can extend from one year olds to adolescents, there is an increasing influence of external and genetic factors on a child’s weight as potential confounders for any weight gain. When Scholtens et al looked at children breastfed for at least four months they reported a significantly lower BMI at age 1 compared to children not breastfed, but at age 7 this difference was no longer significant (Scholtens, Gehring et al. 2007). The WHO review reported a 38% decreased risk of obesity when assessing 10-19 year olds compared with 23% for 1-9 year olds and 11% for adults aged 20 and over, suggesting that endpoint for analysis is critical in determining the impact of breastfeed on obesity at various stages in childhood (Horta and Victora 2013). Finally, study design and follow up can affect the findings as high dropout rates affect long term follow ups, and the methodology used to analyse the results can produce unreliable results. Beyerlein et al investigated the impact of breastfeeding on children’s BMI in Germany but was unable to make any firm conclusions as the results differed according to whether they used linear or logistic regression (Beyerlein, Toschke et al. 2008). To summarise, there is a wealth of literature reporting the decreased risk of childhood obesity for newborns who are breastfed, although there was limited literature exploring those breastfed for at least six months. However, most studies cannot completely control for confounding maternal, child, cultural, genetic and environmental factors. The WHO recommend that infants should be exclusively breastfed for the first six months and that it should be supplemented with additional foods for the first two years (World Health Organisation 2015). Following close examination of the literature, we would conclude that breastfeeding for at least six months should reduce the risk of obesity in early childhood, although the protective effect may be lost in latter childhood depending upon the child’s upbringing.
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