Curr Pediatr Rep (2017) 5:150–155 DOI 10.1007/s40124-017-0140-9

OBESITY (S ARMSTRONG AND A PATEL, SECTION EDITORS)

The Intestinal Microbiome and Childhood Obesity Jessica McCann 1 & John Rawls 1 & Patrick Seed 2,3 & Sarah Armstrong 4

Published online: 2 August 2017 # Springer Science+Business Media, LLC 2017

Abstract Purpose of Review Pediatric obesity has reached epidemic proportions worldwide. The community of microbes inhabiting the human intestine affects differential nutrient absorption, metabolism, and weight status. However, the majority of our knowledge is derived from animal models and adults with obesity. This review discusses the role of the intestinal microbiome in the development and modification of pediatric obesity, with a focus on opportunities for modification of the microbiome through alteration of environmental factors. Recent Findings Recent evidence suggests that obesity is associated with phylogenetic changes in the gut microbiome, yet most of what we know about the role of the microbiome and obesity is from research on adults. A vast number of variables influence the gut microbial ecology early in life, including maternal weight status, breastfeeding, dietary manipulation, antibiotic exposure, and pre/probiotic use. Both in experimental animal and human studies, advances in genomic, proteomic, and metabolomics technologies have expanded our

This article is part of the Topical Collection on Obesity * Sarah Armstrong [email protected] 1

Department of Molecular Genetics and Microbiology, Center for Genomics of Microbial Systems, Duke University Medical Center, 213 Research Drive, Durham, NC 27710, USA

2

Department of Pediatrics, Northwestern Feinberg School of Medicine, Chicago, IL 60611, USA

3

Ann and Robert H. Lurie Children’s Hospital, Stanley Manne Children’s Research Institute, Chicago, IL 60611, USA

4

Department of Pediatrics, Division of Primary Care, Duke University Medical Center, 4020 North Roxboro Street, Durham, NC 27701, USA

capacity to understand the composition and phenotype of the gut microbiome and mechanistic factors that modulate human health. Summary The human intestinal microbiome is associated with both the environment and child obesity. Understanding the mechanisms behind microbial regulation of human metabolism during infancy and childhood is key to developing effective prevention and treatment of obesity. Keywords Microbiome . Pediatric obesity . Dysbiosis . Intestinal bacteria

Introduction Overview Pediatric obesity has remained a public health crisis for over two decades [1, 2]. Prevention strategies have done little to reduce the prevalence of obesity. Effective treatment has been elusive, particularly in low-income and racially diverse groups [3]. In the past, energy homeostasis was thought to be a simple “calories in-calories out” equation. However, recent evidence has demonstrated that energy expenditure varies greatly between individuals, and even within the same individual before and after weight loss [4]. One proposed variable to explain a predisposition to weight gain, metabolic dysregulation, and resistance to lifestyle modification is the “intestinal microbiome”. This term is used to describe the collection of microbes (bacteria, bacteriophage, fungi, protozoa, and viruses) that inhabit the intestinal tract and their collective genomes [5••]. For the purpose of this review, “taxonomy” refers to the rank-based classification of bacterial species from broad classification (“domain”) through

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specific classification (“species”); “phyla/phylum” refers to the classification order to which a bacteria belongs, just beneath “kingdom” and above “order.” Increasing evidence supports that intestinal microbiome composition and function can be influenced by environmental and host factors, and can contribute to the development of obesity at different life stages.

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but some predictions can be made based on microbial genomic content. A less diverse Firmicutes-driven microflora typically make less of the anti-inflammatory short chain fatty acids that can protect against inflammation in the gut [12]. Alternatively, the Bacteroidetes-dominant microbiome associated with lean body mass are replete with polysaccharide utilization loci (PUL) that encode enzymes and uptake machinery required to help digest plant-based polysaccharides [13].

The Pediatric Gut Microbiome Previous studies have suggested that after the first 1– 3 years of life, the intestinal microbiome is relatively stable. However, recent data suggest that although healthy children and adults harbor a similar number of microbial species and genetic material, the composition and functional capacity of their microbiomes was different [6]. In fecal analyses, children aged 7–12 years harbor more Bifidobacterium, Faecalibacterium, and Lachnospiraceae species than adults, and adults harbor more Bacteroides species. Using a predictive functional analysis of fecal communities, the preadolescent microbiome is predicted to have more capacity to synthesize vitamins, degrade amino acids, and perform other functions related to growth and development. On the other hand, the adult species are functionally enriched for genes involved in oxidative phosphorylation, bacterial flagella, lipopolysaccharide production, and steroid hormone production, each of which may interconnect with a predilection for obesity and inflammation [6].

Evidence from Animal Models In recent years, the gut microbiota has emerged as a key factor in the development of obesity among adult animals. The most convincing links between the gut microbiota and obesity come from comparing the microbiota in stool between obese and lean mice and microbial transplantation studies into germfree mice, as reviewed elsewhere [7•, 8]. Ley et al. [9] were among the first to demonstrate an association between obesity and certain dominant microbial phyla in the mouse gut. In mice genetically predisposed to obesity, the abundance of Bacteroidetes was approximately 50% lower than wild-type siblings fed the same diet. The obese mice had a proportional increase in Firmicutes. In a follow-up human study, Ley et al. reported a positive association between the abundance of Firmicutes and weight gain. Concurrently, they observed a proportional increase in Bacteroidetes relative to the amount of weight lost in human patients on low carb and low fat diets [10]. It should be noted, however, that not all subsequent studies comparing the obese and lean microbiota see the same shifts in representative phyla, suggesting that there is no simple “obese microbiome” [11•]. Further, the functional consequences of any shift in dominant phyla are not yet understood,

Evidence from Human Studies The small number of studies to date comparing the microbiota between obese and typical weight children and adolescents are are summarized in Table 1. One recent cross-sectional study of 36 typical weight (avg age 11+/−0.33) and 42 obese (avg age 11 +/− 2.9) Italian children found that, as in adults, Firmicutes are elevated and Bacteroides are depleted in the obese microbiota [7•]. Notably, several single bacterial taxa (operational taxonomic units or OTUs) correlated significantly either positively or negatively with an increasing BMI z score (or BMI normalized to age and sex), including Bacteroides vulgatus (R = −0.4321), Faecalibacterium praustnitzii (0.3508), and Bacteroides stercoris (−0.3252) [13]. In fact, a major outstanding question of microbiome research is whether any single taxon in the gut is consistently associated with a healthy weight, or conversely, with obesity. Two recent studies in adults have hinted at a strong association between the presence of key microbiota in stool and lean vs obese phenotypes [17, 18]. Additionally, B. vulgatus was positively associated with growth in mice following microbiome transplants from nourished and malnourished African children [19].

Enviromental Influences Maternal Factors Chronic disease risk is inextricably linked to the early life environment, where maternal, fetal, and childhood factors portend disease risk later in life. Currently, maternal obesity is a key predictor of childhood obesity and metabolic complications in adulthood [5••, 17]. Although the mechanisms are unclear, new and emerging evidence points to the maternal microbiome, where the microbial composition of the gut modulates the weight gain and altered metabolism that drives obesity. The pregnancy-associated gut microbiome is dynamic, undergoing changes that contribute to pregnancy-associated weight gain and normal relative insulin resistance [20]. Over the course of pregnancy, maternal gut bacterial load increases, and gut bacterial diversity changes and is influenced by prepregnancy- and pregnancy-related obesity [5••]. Alterations in the bacterial composition of the mother have been shown to

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Curr Pediatr Rep (2017) 5:150–155 Description and significant outcomes of recent pediatric microbiome-obesity studies

Study design

Significant results

Reference

Cross-sectional study, 36 typical weight (avg age 11+/−0.33) and 42 obese (avg age 11 +/− 2.9)

Correlations with increasing BMI: Bacteroides vulgatus (R = −0.4321), Faecalibacterium praustnitzii (0.3508), and Bacteroides stercoris (−0.3252) Obesity strongly correlated with Bifidobacterium/E. coli ratio < 1, positive correlation between ratio decrease and obesity (OR = 719.2, OR 95% C.I. = 81.57–6341.18) Rise in relative abundance of Enterobacteriaceae correlated negatively (R = −0.28) with increased BMI at 36 months.

[7•]

16S rRNA sequencing revealed significant increases in species of the genus Bifidobacterium and decreases in Bacteroides vulgatus (P = 0.005) and Faecalibacterium prausnitzii (P = 0.002) within the group who consumed OI.

[16]

Case-control, compared prevalence of Bifidobacteria and E.coli between 62 typical weight (avg age 6.8 +/− 2.4) and 64 obese (avg age 6.8 +/− 2.1) by quantitative PCR. Longitudinal study of 330 infants, fecal samples taken at 9, 18, 36 months after birth, qPCR-targeted assesment of 31 selected microbal targets, compared with growth measurements. Obese children aged 7–12 year given oligofructose-enriched inulin (OI, 8 g/day; n = 22) or maltodextrin placebo (isocaloric dose, controls; n = 20) once daily for 16 weeks. Fecal samples taken at baseline and 16 weeks.

affect the development and function of the gastrointestinal tract of her offspring [21]. How these microbial shifts influence the maternal-fetal-infant relationship, however, is unclear. Prematurity and Low Birth Weight Prematurity and low birth weight are linked to the later life development of obesity and the metabolic syndrome [22]. A variety of prenatal and maternal factors may lead to infant prematurity, and these factors may differentially affect the maturity and integrity of the infant’s gut microflora and immune system. Premature and low birth weight infants are more likely to be born by Caesarian section (C-section), to be fed with elemental or low-allergen formula, may experience delays in oral feeding, and are typically exposed to more interventions and medications— particularly antibiotics—than their full-term and healthy birth weight counterparts [23]. How a dysbiotic microbiota in the premature infant interacts with immature metabolic physiology and current medical care remains unknown, and strategies of microbiota preservation and restoration may provide clues to the import of the microbiota to the long-term metabolism of the former premature infant. Delivery Method Caesarian section (C-section) birth has been implicated in the development of child obesity, although study results are conflicting [24, 25]. Early studies found an association between C-section and obesity through 15 years of age [26]; however, more recent longitudinal data show that the association disappears by age 3–6 years [24]. In addition, a large retrospective cohort study found higher BMI but no increase in the development of metabolic risk factors by delivery type among individuals 23–

[14]

[15]

25 years of age [27]. Predicated on the idea that Csection conveys an altered microbiome to the infant, the hypothesis emerged that C-section leads to an early life microbiome that promotes obesity and the metabolic syndrome. However, studies of delivery method and the early life microbiome are inconsistent. Although small cohort studies suggested that the gut microbiome of infants delivered vaginally differs from those delivered by Caesarian section (recently reveiewed in [28]), a more recent study does not corroborate these findings and found no significant differences between infants born by vaginal or C-Section [29]. Breastfeeding Multiple studies demonstrate protection from obesity following breastfeeding, and these have been recently systematically reviewed [30]. However the dose relationship between maternal milk consumption and obesity prevention, excluding potential confounding effects, is controversial [31]. In animal models, where randomization to formula vs. maternal milk is possible, formula-feeding is associated with a reduced bio-diversity of the intestinal microflora as obtained through 16S ribosomal RNA sequencing [32]. The human breastmilk microbiome is similar in composition to the placenta, suggesting a transitional role from intra-uterine to extra-uterine infant nutrition [21]. In addition, infants who breastfeed directly for at least 75% of their daily milk intake obtain 27.7% of their intestinal bacteria from the milk, and 10.3% from the mother’s skin, suggesting a difference between breastfeeding and breastmilk feeding through a bottle [33]. Antibiotic Exposure Identifying modifiable risk factors for the development of obesity holds potential for prevention early in life.

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Antibiotic use may be one such modifiable factor, as antibiotics are frequently prescribed for non-indicated reasons, with the assumption that there is minimal harm. The association between intestinal microbiota composition and obesity, and the known alterations that antibiotics cause has led to several large-scale epidemiological studies that have examined the impact of antibiotics given early in life. However, these studies have conflicting outcomes [34]. In one study, the microbiome at 3 months and 5– 6 years was significantly different between children with a history of antibiotic use and those who were not exposed. Antibiotics were associated with a higher abundance of Streptococci and reduced Bifidobacteria; prior antibiotic exposure was associated with a higher BMI [35]. Alternatively, other studies that examine multiple early life exposures to specific antibiotics in mice and in children show correlation with development of obesity [36, 37]. In contrast, antibiotic administration did not significantly alter host microbiome or metabolism in a smaller, wellcontrolled study of healthy individuals [38]. Furthermore, a recent large-scale retrospective study of 38,522 children showed no association of antibiotic exposure in the first 6 months of life and the development of obesity through age 7 years [39]. Adding to the controversy, antibiotic administration early in life was associated with a decreased risk of obesity among children of overweight mothers [40]. Prebiotics and Probiotics Probiotics are live microorganisms that are believed to provide health benefits when consumed orally. In contrast, prebiotics are substances that are thought to support the growth of beneficial microbes when consumed. Neither probiotic nor prebiotic strategies have demonstrated efficacy in durably restoring a dysbiotic intestinal microbiome to a baseline state. Probiotics do have a broad spectrum of activity; however, the implications of this activity are inconclusive [41]. Studies have investigated the efficacy of probiotics for multiple conditions including metabolic disease, antibioticassociated and Clostridium difficile-associated diarrhea, IBS, constipation, IBD, chemotherapy-associated diarrhea, respiratory tract infection, ventilator-associated pneumonia, NAFLD, liver encephalopathy, periodontitis, depression, vaginosis, urinary tract infections, pancreatitis, incidence of ventilator-associated pneumonia, hospital infection, and stay in ICU, mortality of post-trauma patients, and NEC in premature infants [42]. A recent meta-analysis of these studies reports that probiotics are effective only for antibiotic- and Clostridium difficile-associated diarrhea, and respiratory tract infections [42]. One recent study, however, did see loss of fat mass and a gut microbial shift after giving prebiotics for 16 weeks to obese preadolescents (Table 1 and [16]). A consistent challenge remains, however, to pro- and prebiotic

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studies and that is a lack of definition of type and biological effect of probiotic strains, as well as the outcome in a disease state.

Potential Mechanisms The mechanisms underlying the microbiota and obesity, including more efficient energy harvest have been recently reviewed [43]. Inflammation of the gut and adipose tissue, likely derived from dysbiosis, is a hallmark of obesity in mouse studies [44–46]. As mentioned in the “Introduction” section, the microbiome of the healthy preadolescent is thought to be less capable of inducing inflammation than that of the adult but on average houses more Firmicutes and Actinobacteria than found in healthy adults [6]. Other hints at potential mechanisms behind how microbes may contribute to obesity have emerged. Microbially derived metabolites have been implicated in impact on circulating satiety hormone levels, [47] and central nervous system sensitivity to the leptin hormone that controls feelings of hunger [48]. Germ-free mice exhibit differences in taste receptors that recognize fats when compared to conventionally colonized mice, suggesting that the microbiome influences taste [49]. Similarly, mice lacking the Toll-like receptor 5 that detects bacterial flagellin have reduce statiety and develop obesity [50]. Thus, the obese microbiota may indirectly impact feeding behaviors and then directly manipulate nutrients and metabolism to further contribute to the development of obesity. Again, the majority of these studies have been conducted in adult or sexually mature animals. It will be important to conduct more mechanistic studies in adolescents and preadolescents—both humans and mice—to better understand the relationship between microbial metabolites and the development of obesity.

Future Directions Current evidence strongly implicates the intestinal microbiome in the development of obesity and associates specific microbes with weight status during adult stages. Considering the developmental origins of obesity and the compositional differences between pediatric and adult gut microbiomes, increased research investment is needed to understand the impact of microbiome on pediatric obesity. One important goal is to determine if specific configurations of microbes are consistently associated with health or obesity at different life stages. If microbial communities are indeed structured around such keystone species, then it would be important to determine the impact of removing or replacing that member. Furthermore,

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studies to date have almost exclusively focused on bacteria, ignoring the contribution of fungi, among other microbial species. If specific microbes are not consistently linked with obesity, then specific microbial physiologies or products derived from diverse microbial taxa may contribute to obesity and would be worthy of further study. As the number of candidate mechanisms by which microbiome contributes to adult obesity increases, it will be important to determine if similar or distinct mechanisms contribute to pediatric obesity in humans and animal models. Further, we anticipate it may be useful to determine how the pediatric microbiome correlates with specific weight loss intervention strategies and outcomes in children in order to identify configurations that are predictive of subsequent therapeutic efficacy and cooperate in the therapeutic response.

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References Papers of particular interest, published recently, have been highlighted as: • Of importance •• Of major importance 1.

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Conclusions

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Pediatric obesity continues to threaten the health of future generations. To date, treatment efforts are stymied by the human counter-balance mechanisms that resist weight loss. These mechanisms remain unclear; however, the intestinal microbiome may play a role in predisposition to obesity, resistance to weight loss and propensity to regain weight. The microbiota likely influence weight through differential nutrient absorption and regulation of local inflammation; however, other novel mechanisms are under investigation including influences of the microbiota on taste and appetite. It is clear from the existing evidence that the intestinal microbiome is influenced by interactions between the individual and the environment. Specific factors, including maternal diet and BMI, delivery method, early feeding, antibiotic use, pre- or probiotic use, and early life environment are currently under investigation; however, no one factor has emerged as a “smoking gun.” Many more questions exist than answers; however, the intestinal microbiome is now established as a key factor in the development—and 1 day perhaps, the treatment—of pediatric obesity.

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Compliance with Ethical Standards 12. Conflict of Interest Jessica McCann, Patrick Seed, and Sarah Armstrong declare that they have no conflicts of interest. John Rawls reports grants from National Institutes of Health during the conduct of the study. In addition, Dr. Rawls has a patent PCT/US16/ 22958 pending, and a patent US11/080,755 issued. Human and Animal Rights and Informed Consent This article does not contain any studies with human or animal subjects performed by any of the authors.

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