Keywords

1 Introduction

Rising obesity rates have been a ballooning problem in the West, and now globally, for several decades, with the prevalence of the disease exceeding 35 % in adults as of 2012 (Ogden et al. 2014). The incidence of obesity and its chronic disease morbidities has plateaued here for over a decade, heavily straining health systems (Jensen et al. 2014; Ogden et al. 2014). Sedentary lifestyles and poor dietary practices have been historically cited as the primary culprits (Dabney 1964) (Beaton 1967), and have been the focus of an entire subculture dedicated to changing the behavioral and psychological approach the epidemic of obesity. Many healthy-living programs have been primarily aimed at weight loss as the means to improving the health of individuals with obesity, and thereby increasing the health of society as a whole (Brown et al. 2009; Flodgren et al. 2010; Lau et al. 2007). However, the modest success of these initiatives leaves much to be desired (Kirk et al. 2012). Like many other general indicators of health, that an individual is obese is not the problem on its own; like many other general indicators, taken in context, obesity can be a useful predictor for other co-morbidities that arise alongside – if not as a result of an individual being obese. The increased risk of developing many other chronic diseases including hypertension and Type 2 diabetes mellitus has made obesity a leading candidate for health research.

Although the undesirable nature of obesity is well recognized for its impacts on the health of the individual, the story does not end here. An emerging body of evidence has begun to shed light on obesity and overweight in a very specific subset of the population; pregnant women. It has been long understood that maternal obesity carries an increased risk of complications in pregnancy, a phenomenon which is well-reviewed in the literature (Stupin and Arabin 2014) (Faucher and Barger 2015). However, in the last 20 years, we have seen a paradigm expansion – if not a shift – towards a greater understanding of the maternal-fetal relationship and how this relationship is modulated by the maternal diet and her metabolic physiology. Indeed, one might consider whether in a state of poor maternal nutrition (high fat, sugar and artificial sweeteners) or maternal metabolic dysfunction (like in the case of gestational diabetes or diabetes) the fetus may be exposed to “toxins” of a natural origin, including glucose, free fatty acids and fructose. The focus of this chapter will be to provide a summary of the identified impacts of maternal obesity and poor nutrient intake to date, and to explore future directions that can help reconcile the major challenges facing offspring born to pregnancies complicated by obesity.

2 Obesity Where for Art Thou?

As a disease, obesity may be characterized by any number of features, the most predominant of which is increased body mass index (BMI; kg/m2). The World Health Organization (WHO) BMI classifications are divided into overweight (BMI 25–29.99); Obese class I (BMI 30–34.99), Obese class II (BMIT 35–39.99) and Obese class III (BMI ≥ 40) (Organization 2015) and although BMI gives no indication of body composition, BMI and waist circumference measurements are useful indicators of obesity and its associated complications (Jensen et al. 2014).

Dietary, lifestyle, and genetic factors constitute an enormous part of the molecular basis of obesity (Pate et al. 2013). The most common culprit is the consumption of an “obesogenic” diet. Prolonged intake of fatty foods in large quantities contributes to fatty acid uptake into circulation and the tissues of the body (Brunner et al. 1979). As circulating fatty acid levels steadily rise over time, internal clearance systems fail to clear them rapidly enough, resulting in widespread ectopic deposition in metabolically important tissues, including the liver, skeletal muscle, pancreas, and adipose; known to be associated with the metabolic syndrome (Rasouli et al. 2007). This accumulation is undesirable for several reasons. Fatty acid by-products have a direct influence on glucose homeostasis. Specifically, at increased concentrations, metabolites of fatty acid oxidation such as diacylglycerides and ceramides can activate inhibitory “stress” serine kinases, as well as decreasing the activity of Akt (de la Monte et al. 2010; Szendroedi et al. 2014). These effects work synergistically to inhibit insulin signalling (Summers et al. 1998; Yu et al. 2002), leading to impaired insulin sensitivity and dysregulation of blood glucose level maintenance. Impairment of this system by these substrates is a potent contributory factor to insulin resistance in obesity, elevating blood glucose levels and increasing the risk of Type 2 Diabetes Mellitus (Li and Zhang 2000).

Compounding the issue, obesity is characterized by a state of chronic, low-grade inflammation (Wellen and Hotamisligil 2005). As a major cytokine producer, adipose tissue plays a contributory role to inflammation throughout the body by increasing production of proinflammatory cytokines as the tissue mass expands (notably TNFα) (Hotamisligil et al. 1993, 1995). Furthermore, saturated free fatty acids demonstrate partial specificity of pattern recognition receptors including Toll-like receptors (TLRs) (Fessler et al. 2009; Shi et al. 2006) where the principal activation of an interleukin-1-pathway, dependent upon the adapter MyD88 through a series of kinases and scaffolding proteins, leads to activation of NF-κB ultimately initiating pro-inflammatory cytokine secretion (Sabroe et al. 2008). The nearly ubiquitous presence of these TLRs in tissues means that inflammation is almost a certainty following lipid accumulation (Ahmad et al. 2012). Obesity related inflammation then, is associated with stress kinase activation, and is an aggravating factor for insulin resistance (Lee et al. 2014). Leptin regulation and responsiveness is perhaps the most important example of this. Cytokine-mediated inflammatory profiles are managed in part through the signalling of Suppressor of Cytokine Signalling (SOCS) proteins (J. Wang and Campbell 2002). Some SOCS proteins can have the undesirable effect of inhibiting the tyrosine kinase activity of the leptin receptor (Bjorbaek et al. 1999). In a healthy state, leptin acts at the arcuate nucleus of the hypothalamus to generate a feeling of satiety and terminate eating (Satoh et al. 1997). However, when leptin signalling becomes dysfunctional, circulating leptin levels increase futilely and leptin resistance develops (Bjorbaek et al. 1998).

Though fairly rare (~5–10 % of obese cases) inborn errors of metabolism as well as predisposing risk factors for obesity can arise from single nucleotide polymorphisms (SNPs ) (Choquet and Meyre 2011). Defects in leptin signalling pathways, encompassing deficiencies in leptin production, deficiencies in the leptin receptor itself, or any number of mutations which block the ability of the leptin receptor to transduce signals (Lubis et al. 2008) are associated with obesity. An obesogenic model of leptin knockout was first demonstrated quite dramatically in mice by removing their ability to produce leptin entirely. These ob/ob mice eat ravenously becoming extremely obese (Pelleymounter et al. 1995). A wide host of defective components in the leptin signalling pathway can manifest with similar effects (Bjorbaek et al. 1999). Other genetically based receptor defects include mutations of the GPR120 receptor, which regulates adipogenesis and appetite. GPR120 activity is linked to insulin desensitization and adipocyte hypertrophy, suggesting further genetic bases for obese conditions (Hirasawa et al. 2005; Ichimura et al. 2012).

The genetic bases for obesity may also be transmitted through more complex inheritance patterns, related to multiple allelic dysfunctions. Because of their often widespread inputs, individual alleles may be partially implicated in a variety of disease states. Accordingly, specific “polygenic” aggravators of obesity risk have been described in the literature, the best characterized of which include the “fatso” gene (FTO), and the melanocortin 4 receptor gene (MC4R).

The FTO gene product is a DNA demethylase (Gerken et al. 2007) with a strong preference for single-stranded DNA (Han et al. 2010). Several SNPs in this gene have been directly linked to increased BMI, increased odds of obesity, and increased fat mass from a young age (Dina et al. 2007; Frayling et al. 2007). Indeed, initial estimates suggested that certain SNPs of Fto might be responsible for as much as 22 % of the attributable risk for obesity (Dina et al. 2007). Evidence from animal models has further demonstrated that increased Fto induces hyperphagia and obesity (Church et al. 2010), while the loss of Fto is protective against the development of obesity (Fischer et al. 2009). The exact mechanisms by which Fto regulates energy homeostasis are not well understood, though its high concentrations in the hypothalamus and ability to modulate the expression of STAT3 (a key factor in leptin signalling) suggest that this as a possible point at which Fto modifies the dietary habits of the organism (Tung et al. 2010). This is further corroborated by evidence of Fto co-localization with the leptin receptor long isoform in the arcuate nucleus (Wang et al. 2011). The demethylase activity of Fto, combined with its proximity to leptin signalling activity, have suggested a role for it as a transcriptional co-activator which may be involved in the regulation of fat tissue (Wu et al. 2010).

The MC4R gene product is a G-protein coupled receptor which is expressed in the brain, and is part of the melanocortin receptor family of proteins (Huszar et al. 1997). In vitro inactivation of MC4R is known to induce the development of obesity, hyperphagia, and disruptions to glucose/insulin regulation (Huszar et al. 1997). More recently, genetic analyses have revealed a sizeable involvement for MC4R in heritable cases of obesity, naming specific mutations as primary culprits (Vaisse et al. 1998; Yeo et al. 1998). Further data from genome-wide studies have linked mutations in MC4R to increased waist circumference, insulin resistance, and fat mass (Chambers et al. 2008; Loos et al. 2008). These findings have established MC4R inactivating mutations as leading candidates for many cases of early onset, genetic obesity (Loos et al. 2008). Interestingly, though the function of MC4R in regulating appetite and obesity is not well understood, it is known that individuals heterozygous for a null mutation in this gene develop obesity, which is suggestive of an expression-level sensitivity to MC4R in this system (Yeo et al. 2000).

Despite great efforts in managing the growing population of individuals that develop obesity, we have been unsuccessful. No doubt a multi-system approach is needed, where multiple causative and indirect associations can be targeted at once. But even with this focused approach, “symptomatic” treatment of the epidemic through dietary and lifestyle has left much to be desired at least in part due to compliance issues (Kirk et al. 2012). New ways to predict and manage obesity demand a new way of understanding where the problem is coming from and how it can be handled.

3 A New Perspective on Obesity Development: The Early Life Environment

Over 20 years ago, the first glimmer of a new approach to understanding disease risk emerged in an unsuspecting area of science: perinatal development . In the late eighties, Dr. David Barker published an observational study of seniors in England whose birth weights had been recorded in the early twentieth century (Barker et al. 1989). The data showed a novel association between birth weight and disease risk in later life. Though birth weight is an imperfect surrogate measure of intrauterine development patterns, these studies provided the first look into the possibility of the continued influence of the in utero environment into later life. In 1992 this would be fleshed out as the hypothesis that disease programmed at birth is a side effect of inaccurate signaling between the fetus and the mother about the extrauterine environment, leading to long term maladaptations (Hales and Barker 1992). This exciting data paved the path for the more modern hypothesis of the Developmental Origins of Health and Disease (Barker 2004), and has provided powerful tools for evaluating the basic problems concerning the prediction and management of obesity.

4 Maternal Metabolic Adaptations, Gestational Weight Gain and Obesity

During fetal development, the maternal environment acts as the portal through which all fetal requirements are channeled. All nutrients that the fetus requires – and toxic substances the fetus may be exposed to – are derived from the mother’s circulation. The growing fetus is in a very vulnerable state, and is extremely susceptible to perturbations, however subtle, that may induce developmental adaptations that have long-term effects. For this reason, the mother undergoes a set of physiological changes to accommodate the growing needs of the fetus in a buffered environment where the balances of nutrient distribution are drastically altered. Nearly all organ systems demonstrate significantly altered profiles during pregnancy; musculoskeletal, neuroendocrine, and cardiovascular (Chapman et al. 1998); herein we will focus on metabolic alterations (Ryan et al. 1985).

Maternal metabolic adaptations are largely the result of endocrine-mediated changes in carbohydrate, protein and fat metabolism. Early in pregnancy, the mother is in an anabolic state, where placental lactogen, progesterone and estrogens alter hepatic glucose production, lipogenesis and gluconeogenesis (Desoye et al. 1987). These early adaptations serve to increase adipose deposition, which later in pregnancy can be used as a fuel supply primarily for the mother, since glucose is preferentially diverted to the placenta and to the growing fetus. As such in the last third of pregnancy the mother is catabolic; defined by a state of insulin and leptin resistance, where elevated circulating glucose, triglycerides and fatty acids (Punnonen 1977), can accommodate the increasing demands placed on the maternal system from both the fetus and the metabolically active placenta. Although changes are largely mediated by placental lactogen, estradiol and progesterone (Beck and Daughaday 1967), new emerging data suggest that maternal – bacterial relationships could also potentially impose changes on maternal metabolic function during pregnancy. Maternal gut bacterial load increases over the course of pregnancy (Collado et al. 2008) and there is a significant change in gut microbiome composition from the first to the third trimester of pregnancy, accompanied by increased bacterial diversity between mothers (Koren et al. 2012). Although the composition and diversity of the gut microbiome during the first trimester is similar to the that of nonpregnant healthy women, over the course of pregnancy, there is an increase in the abundance of Proteobacteria and Actinobacteria (Koren et al. 2012). Since an increase in Proteobacteria has been observed in inflammatory bowel disease, it has been suggested that a similar dysbiosis of the gut occurs during the third trimester of pregnancy. The concept that maternal gut bacterial populations change over the course of pregnancy to influence maternal and fetal metabolic development is novel, and hasn’t yet been proven, but presents an interesting deviation from the current concept that metabolic function is entirely regulated by placental hormones (Gohir et al. 2015).

We can briefly pause here to consider the points upon which we have touched thus far. Obesity is defined as the state of being overweight, typically denoted by a BMI of 30 or greater. In many cases, obesity presents with a phenotypic profile consistent with metabolic syndrome, characterized by insulin and leptin resistance, elevated blood pressure, high blood triglycerides, and low blood HDL cholesterol levels. The comorbidities associated with obesity include Type 2 diabetes mellitus, hypertension, Non-Alcoholic Fatty Liver Disease, and cardiovascular diseases. Adjacently, maternal pregnancy induced metabolic adaptations are oriented to ensure satisfactory growth and development of the fetus, whilst meeting the increased metabolic demands place upon the mother during pregnancy. By adopting a state of relative insulin resistance and increasing the role of fat in supplying the metabolic demands, these adaptations improve glucose delivery to the fetus. Indeed, many of the metabolic adaptations that accompany late pregnancy mimic the characteristics that define the pathophysiology of obesity including hypertriglyceridemia (Punnonen 1977), hypercholesterolemia (Bartels et al. 2012), insulin and leptin resistance (Catalano et al. 1991), and increased adipose depots (Kopp-Hoolihan et al. 1999). Even shifts in the gut microbes that occur with obesity are not inconsistent with the few data on maternal pregnancy induced microbial shifts (Koren et al. 2012). Pregnancy, however, is a normal physiological state of metabolic adaptation largely based upon the secretion of placental hormones, and upon removal of the placenta – the maternal metabolic function returns to normal non-pregnant values. It is not surprising then, that if this fine balance between maternal metabolic adaptation and fetal nutrient demand is disrupted, that long term deleterious impacts on physiological function occur.

5 Maternal Obesity: Is This a Toxic in Utero Environment?

The fetus is less isolated than once thought. After decades of research, we have begun to understand the intrauterine environment as an active responder to the external environment, and the placenta a more forgiving filter. In this spirit, an examination of the fetal interaction with the outside world grows increasingly warranted, and what better place to look than the maternal metabolic environment. The characterization of the maternal obesity phenotype is an important step towards a deeper understanding of the interplay at work here and much of what we know today comes from animal studies: from rodents to sheep and non-human primates.

The use of animal models circumvents many of the ethical issues that would be associated with performing many equivalent experiments on humans. The ones of interest range from the manipulation of diets to the harvesting and analysis of otherwise inaccessible tissues. Further, different animal models offer unique advantages that may favor their use in answering certain research questions. For example, the use of rodent (mostly rat and mouse) models offers the unique advantage of a shorter gestation time and an ease of manipulation compared to higher order animals. However, studies in rodents are limited since their biology is far from human. Conversely, non-human primates are a close evolutionary ancestor to humans, but non-human primate models have long gestation times, ethical constraints and often prohibitive expenses. Independent of the model that is used, the use of animals has been indispensible to the elucidation of many mechanisms underlying the effects of maternal obesity, both in the mother and in the offspring. Where mechanisms currently escape us, animal models further our understanding by offering a convenient inroad to forward thinking and hypothesis generation.

6 Impacts of Maternal Obesity

6.1 Maternal Outcomes

In rodents fed a high fat diet (HFD) during and before pregnancy, consistent effects are observed in maternal physiology and metabolic profiles. Several reviews have repeatedly shown metabolic dysregulation, an aggravated systemic pro-inflammatory profile, and an increased risk of obstetric complications (Athukorala et al. 2010; Dube et al. 2012; Stupin and Arabin 2014).

In both rat and mouse models, HFD-induced obesity both before and during pregnancy demonstrated the generation of a phenotype which was consistent with the metabolic syndrome. In mice, dams fed a HFD during pregnancy demonstrate increased adiposity, hyperleptinemia (Jones et al. 2009), insulin resistance, and serum free fatty acids (Mao et al. 2010). This is similar to observations made in a rat model increased, where a HFD resulted in maternal hyperleptinemia (Mark et al. 2011), as well as increased plasma glucose, insulin, and triglyceride levels (Srinivasan et al. 2006) (Hayes et al. 2012). In mice, increased levels of inflammatory markers such as interleukin 6 (IL6), interleukin 10 (IL10), and interferon gamma (IFNγ) (Kepczynska et al. 2013) are observed. However, there does not appear to be increased macrophage infiltration into adipose tissue by gestational day 18 (E18) (term E21-22) (Ingvorsen et al. 2014).

The data in non-human primate models is somewhat limited in nature, but are nevertheless consistent with rodent data. In Japanese macaques, there is evidence to suggest that maternal HFD – independent of obesity – can generate an increase in circulating pro-inflammatory cytokine levels (notably IL1ß and MCP-1), a condition which was aggravated in animals that developed insulin resistance. In macaques, a HFD decreased uterine blood flow independent of obesity or other morbidities (Frias et al. 2011). Perhaps the greatest importance of this finding lies in that it provides a potential mechanistic link between intrauterine growth restriction (IUGR), a strong indicator for later life complications, and maternal HFD intake through changes in placental function.

Evidence describing an interaction between a HFD and pregnancy in humans is fairly well reported, largely from epidemiological and mechanistic studies, is are consistent with the reports that have been made in animal models. Epidemiological data suggests that obesity increases the likelihood of experiencing obstetric complications in pregnancy (P. M. Catalano and Ehrenberg 2006). Notably, there is an increased risk of pre-eclampsia, the development of Gestational Diabetes Mellitus, delivering large for gestational age babies (LGA), a greater incidence of miscarriage (Metwally et al. 2008), and caesarean section (Athukorala et al. 2010; Lutsiv et al. 2015). In humans, obesity is linked to the downregulation of the leptin receptor (Farley et al. 2010), hyperleptinemia (Farley et al. 2010; Ramsay et al. 2002), hyperinsulemia, increased plasma triglycerides, and decreased vascular responses (Ramsay et al. 2002). A state of chronic, systemic low-grade inflammation has been documented, attributable to the release of greater levels of inflammatory adipokines; of special note is TNFα, which is released by adipose tissue and is a known modulator of insulin resistance (Hotamisligil et al. 1993). However, this observation is not entirely consistent across studies as evidence exists that adiposity is not related to levels of pro-inflammatory cytokines in pregnancy (Friis et al. 2013). This understanding of the changes to maternal physiology is important to set the context of further discussions regarding fetal development and the environment which shape this development.

6.2 Fetal/Offspring Outcomes

The introduction of a high fat and/or obesogenic dietary insult is linked in varying degrees of certainty to adverse outcomes both in first generation offspring, as well as continued deviations from healthy states in subsequent generations (Dabelea and Crume 2011; Vickers 2014). However, our current understanding of the mechanisms underpinning these adverse outcomes and transgenerational transmission are unclear. Early life exposure to an obesogenic environment perturbs a wide range of physiological parameters in offspring, including the promotion of phenotypes consistent with metabolic syndrome, low-grade inflammation, hepatic and endocrine dysregulation, and cardiovascular impairment. We will explore here the current paradigms surrounding these features, and the data that have improved our understanding of them over time.

In mice, a high fat , obesogenic maternal diet is shown to direct the development of metabolic syndrome in offspring (Samuelsson et al. 2008). This finding is supported by the characterization of offspring phenotypes which showcase insulin resistance (Dunn and Bale 2009; Graus-Nunes et al. 2015; Gregorio et al. 2010; Murabayashi et al. 2013; Samuelsson et al. 2008; Volpato et al. 2012; Vuguin et al. 2013), increased blood pressure (Elahi et al. 2009; Masuyama and Hiramatsu 2012; Samuelsson et al. 2008), increased serum triglycerides (Masuyama and Hiramatsu 2012), and increased adiposity and body mass (Samuelsson et al. 2008; Volpato et al. 2012); (Dunn and Bale 2009; Masuyama and Hiramatsu 2012). The concurrent presence of hyperglycemia and increased serum triglycerides is proposed to aggravate the dysregulation of glucose management by promoting “gluco-lipotoxicity ” in the beta cells of the pancreas (Cerf 2010). Other findings include increased serum cholesterol levels (Elahi et al. 2009), and increased serum leptin (Graus-Nunes et al. 2015; Howie et al. 2009) despite the apparent absence of central leptin resistance (Graus-Nunes et al. 2015). An analysis of adipose tissue in these offspring suggests that there is increased macrophage infiltration (as measured by CD68 levels), increased TNFα mRNA levels, and a decrease in GLUT4 expression , suggesting that the insulin resistance is a contributing factor to the observed low-grade inflammation that is present (Murabayashi et al. 2013). Of interest is the fact that these offspring were never fed a high fat diet postnatally, though it is very well established that doing so produces an aggravated form of metabolic syndrome (Elahi et al. 2009).

In many cases, the effects of a maternal HFD are sexually dimorphic; where for example in mice, only male offspring showed increase fat mass and serum insulin (Ashino et al. 2012), while other data show that the magnitude of hypertension in female offspring was more pronounced (Elahi et al. 2009). A notable difference was increased serum TNFα and IL1ß in adult offspring born to dams fed a HFD, suggesting the presence of an inflamed state (Ashino et al. 2012). Using transgenic mice heterozygous for the expression of GLUT4 to induce a model of Type 2 diabetes mellitus, it has been shown that G4+/− offspring born to mothers fed a high fat , obesogenic diet , are hypertensive (Vuguin et al. 2013), and males further demonstrated insulin resistance, glucose intolerance, and increased adiposity (Hartil et al. 2009).

Adverse hepatic outcomes have also been shown in offspring born to mothers fed a HFD and/or obesogenic diet including Non-Alcoholic Fatty Liver Disease (NAFLD) and Non-Alcoholic Steatohepatitis (NASH). NAFLD and NASH are characterized by an accumulation of fat in hepatocytes and have been referred to as “the hepatic manifestation of metabolic syndrome” (Musso et al. 2009). This lipid deposition is the result of an inability to correctly regulate the balance between the deposition and synthesis of fats, and their removal by oxidative processes (Matherly and Puri 2012). Changes in factors known to be involved in this balance are often mechanistically linked to disease development in the offspring. In offspring of mothers fed a HFD, increased liver triglycerides suggest the development of NAFLD (Ashino et al. 2012; Bruce et al. 2009) and decreased phosphorylation of c-Jun terminal kinases (JNK) and I kappa B kinase (IκK) – important regulators of insulin signaling – suggest a decrease in hepatic insulin signaling (Ashino et al. 2012). Decreased hepatic functionality of electron chain complexes has been implicated in the process (Bruce et al. 2009), as well as an upregulation of hepatic genes involved in glycolysis, gluconeogenesis, oxidative stress, and inflammation (Vuguin et al. 2013).

Rat models investigating offspring outcomes in response to maternal HFD largely paint the same picture. Rat models show maternal obesity results in offspring with hyperglycemia (Franco et al. 2012), insulin resistance (Burgueno et al. 2013) (Nivoit et al. 2009), altered pancreatic signalling (Howie et al. 2013) and increased adiposity (Howie et al. 2009; White et al. 2009). In one study, the mortality of offspring from high fat fed obese mothers was three times that of control fed mothers (Hayes et al. 2012).

In male offspring born to high fat fed obese mothers, glucose intolerance, weight gain, insulin resistance and hyperglycemia are observed (Srinivasan et al. 2006). There is conflicting evidence regarding maternal obesity effects on offspring birth weight with some observing increased birth weights (in males) (Srinivasan et al. 2006), while others report growth restriction (Connor et al. 2012; Howie et al. 2009; Mark et al. 2011).

There has also been extensive research on rats regarding leptin as a mediator between early life exposure to an obesogenic environment and long-term obesity risk. Almost ubiquitously, a maternal HFD produces hyperleptinemia in offspring (Mark et al. 2011) (Franco et al. 2012) (White et al. 2009) (Burgueno et al. 2013; Howie et al. 2009b). Leptin resistance is thought to occur centrally where a decrease in STAT3 and SOCS signalling in the arcuate nucleus of the hypothalamus is indicative of impaired leptin signalling, and therefore a state of leptin resistance (Franco et al. 2012). Indeed, in diet-induced obese mice, the ability of leptin to activate signalling in arcuate neurons and promote arcuate neurite outgrowth is significantly reduced (Bouret et al. 2008). Similarly, in offspring born to diabetic mothers (i.e., state of hyperglycemia in utero), decreased hypothalamic leptin signaling in arcuate neurons was also associated with decreased neural projections from the arcuate nucleus to the paraventricular nucleus (Steculorum and Bouret 2011). The impacts of maternal obesity on hypothalamic signalling have been shown to be exacerbated in the presence of a post-weaning HFD where a combination of an in utero + postweaning HFD results in greater levels of hyperphagia, adiposity, hyperlipidemia, and glucose intolerance in offspring (Chen et al. 2009; Vickers et al. 2000). This was linked to increased hypothalamic NPY signaling and leptin resistance in adult offspring (Chen et al. 2009). More recently maternal obesity has been shown to dampen in vivo hypothalamic NPY response to acute hyperglycemia and decrease glucose uptake and lactate release in hypothalamic cell culture models (Chen et al. 2014). Taken together, the stressors of a maternal HFD and/or obesity on offspring in rodent models demonstrate an increased propensity towards the development of a phenotype that is consistent with The Metabolic Syndrome.

The data on physiological outcomes relevant to the fetus in non-human primates (NHP) is limited, though what is known offers valuable insights which are arguably more relevant in informing human studies than rodent models. The major phenotypic finding in the NHPs involves hepatic injury. Several studies demonstrate that a maternal HFD produces offspring with increased liver triglycerides (Aagaard-Tillery et al. 2008; Grant et al. 2011; Grayson et al. 2010) (McCurdy et al. 2009) and predisposition to NAFLD (Aagaard-Tillery et al. 2008; McCurdy et al. 2009; Thorn et al. 2014). Interestingly, offspring did not develop The Metabolic Syndrome unless in utero exposure was combined with a postweaning HFD (Fan et al. 2013; Thorn et al. 2014). Further, despite no change in birth weight, offspring showed increased adiposity (Grant et al. 2011). In the livers of these offspring, there is also evidence of irregularities in key genes that regulate circadian rhythmicity ( Suter et al. 2011).

It has been shown that in NHP, a maternal HFD alone is sufficient to significantly reduce uterine blood flow. This condition is aggravated in the presence of a maternal hyperinsulinemic state, which also restricts the blood flow to the placenta. This drives, in part, a higher incidence of stillbirths among these offspring (Frias et al. 2011). Other cardiovascular insults include endothelial artery damage, measured by the presence of inflammatory cytokines, which may play a role in the development of later life cardiovascular complications (Fan et al. 2013), though more investigation is warranted. Maternal HFD produces (postnatally) impaired thyroid function in offspring, where decreased free thyroxine levels, as well as expression of thyroid releasing hormone, thyroid stimulating hormone, and the precursor thyroglobulin in the hypothalamus and thyroid suggest slowed metabolic processes and lower energy expenditure (Suter et al. 2012). An increase in thyroid hormone receptor ß may be compensatory in these animals, consistent with a deficiency in thyroid hormone signalling. This is potentially an aggravating factor in the discussion of altered substrate utilization, resulting in increased fatty circulation and deposition (Suter et al. 2012).

Owing to the earlier discussed difficulties in conducting experiments on humans, the data regarding fetal outcomes in humans is, on the whole, less than satisfactory. The evidence is largely epidemiological, often drawn retrospectively from observational studies. Multi-factor analyses of the factors predisposing human fetuses to later life disease have largely established a role for obesity, and glucose/insulin dysregulation. It is clear that obesity has an impact in later life outcomes, demonstrated by an increase in offspring obesity (Rooney and Ozanne 2011; Yang and Huffman 2013) and weight gain (Jansson et al. 2013). Maternal obesity is associated with childhood obesity (Schack-Nielsen et al. 2010; Starling et al. 2015) although in some studies the exact role of genetic and lifestyle associations in not clear (Lau et al. 2014). It is clear however, that maternal obesity and excessive weight gain during pregnancy is significantly associated with fetal macrosomia (overgrowth, LGA) (Gaudet et al. 2014; Mamun et al. 2014) and that LGA babies are at a greater risk of metabolic complications (Boney et al. 2005). It is interesting to note that a subset of obese women also deliver small for gestational age (SGA) babies (Leung et al. 2008; Rajasingam et al. 2009). Further, the risk of offspring obesity is positively correlated to socioeconomic status – a useful consideration for identifying at-risk populations (Whitaker 2004). Indeed, maternal hyperglycemia is a well-established insult predicting adverse fetal and later life outcomes (Tenenbaum-Gavish and Hod 2013) such as offspring obesity and diabetes (Dabelea et al. 2000).

In humans , maternal obesity is associated with premature death in the adult offspring (Reynolds et al. 2013), indeed birth weight shows a “u-shaped” association with mortality, where both low and high birth weight is correlated with premature death (Baker et al. 2008). Maternal obesity is highly correlated with fatty liver in offspring (Brumbaugh et al. 2013), increased incidence of asthma (Forno et al. 2014), and cardiovascular disease (Fraser et al. 2010). Recently, maternal obesity has been associated with changes in immune cell profiling in umbilical cord blood (Wilson et al. 2015), and although more investigation into immune development of offspring is needed, these data could suggest one mechanism linking risk of immune diseases including asthma and atopy (Harpsoe et al. 2013; Lowe et al. 2011) to the intrauterine environment.

A serious issue with human investigations is that outcomes can become difficult to track over time, requiring staggering sample sizes and careful measurements to accurately and validly capture them (Yang and Huffman 2013). Though human data is the most relevant, it is also difficult to come by in good quality thus the most pressing matter in understanding the maternal-f etal interplay is improving the quality and quantity of human data in this field.

6.3 Placental Outcomes

As the interface between the mother and fetus, the placenta plays a pivotal role in fetal growth and development. All of the nutrients and blood that reach the fetus pass through by way of the placenta (Harding 2001). Acting as a first-line filter, the placenta plays a major role in the composition of the fetal environment, and is therefore of great interest in discussions concerning the impacts of diet on fetal outcomes. Many of the notable findings in the placenta arising from high fat and obesogenic diets concern changes in nutrient transporters, as well as a tendency towards an inflammatory state. Though conclusions thus far are largely inferential, there is an incontrovertible link between the placental changes and offspring outcomes.

In mice, placental responses to a HFD diet are marked by altered nutrient transporter activity levels, as well as sexually dimorphic changes to gene expression levels. In mothers fed a HFD, the transport of glucose and amino acids are increased, as well as an accompanying increase in the levels of their transporters. At E18.5 (near term), glucose transport across the placenta is increased fivefold, and amino acid transport increased tenfold (Mark et al. 2011; Sferruzzi-Perri et al. 2013). This is accompanied by a fivefold increase in the expression of the insulin-independent glucose transporter 1 (GLUT1), and a ninefold increase in the expression of sodium-coupled neutral amino acid transporter 2 (SNAT2) (Jones et al. 2009; Sferruzzi-Perri et al. 2013). In cases of a maternal obesogenic diet, which incorporates both high fat and high sugar, expression of fatty acid transport protein (FATP) was also increased in the placenta (Sferruzzi-Perri et al. 2013). Furthermore, overall gene expression has been demonstrated to be sex-dependent, showing greater differences in the placentas of female fetuses (Mao et al. 2010). Maternal diabetes and nutrient intake modulate placental gene expression differentially and interact where dietary changes in placental growth were modulated by maternal diabetes (Kappen et al. 2012). These data support the hypothesis that the maternal metabolic state sets the stage for placental growth and development and that these processes are modulated by dietary intake of fat.

Rat models of HFD/obesity in pregnancy produce a phenotype of altered placental growth, endocrine signaling, and inflammation. Early trophoblast invasion is increased twofold, accompanied by an increase in metalloprotease 9 – a key protein in placental remodeling, in a model of maternal obesity (Hayes et al. 2014). This is relevant because altered placental establishment can lead to complications including (but not limited to) pre-eclampsia (Goldman-Wohl and Yagel 2002). While there is evidence to suggest that in a rat model, a HFD does not change placental VEGFα and PPARγ (markers of vascular growth) (Mark et al. 2011), the vasculature may nevertheless be altered, producing a hypoxic environment in the labyrinth zone (Hayes et al. 2012) and restricting junctional zone growth (Mark et al. 2011). Evidence of increased cyclooxygenase 2 (COX2) expression in the placenta of rat dams fed a HFD is suggestive of an inflammatory environment, possible compensating for impaired blood flow (Saben et al. 2014a).

Though the data in non-human primates is somewhat limited, a HFD model of pregnancy in Japanese macaques has provided valuable information regarding placental alterations. Thirteen inflammatory cytokines were elevated in the placentas of animals, which demonstrated sensitivity to a HFD (by developing obesity and hyperinsulemia). However, even where particular sensitivity was not observed, proinflammatory cytokines IL1ß and MCP-1 were upregulated. These animals also display increased placental triglycerides, and altered uteroplacental perfusion. Taken together, the data from non-human primates is consistent with those data in rodents, in painting a picture of an inflamed and hemodynamically challenged placenta (Frias et al. 2011; Thorn et al. 2014).

Human placenta data is consistent with those observed in animals, in terms of hypoxic states, and alterations in endocrine signaling and nutrient transporter expression patterns. However, it is notable that in human studies, changes to nutrient transport protein levels are opposite in directionality to what was seen in animals. Obese women exhibiting hyperleptinemia and leptin resistance were also observed to have decreased SNAT, SNAT4, and leptin receptor expression in the syncytiotrophoblast of their placenta (Farley et al. 2010). Placental GLUT4 expression is decreased (Colomiere et al. 2009), while GLUT3 levels on the maternal side of the placenta (trophoblast) increased (Janzen et al. 2013). There is evidence of impaired insulin signaling, both at the level of the receptor and downstream molecules; insulin receptor (IRß) and PI3Kp85α expression (downstream mediator) were decreased in placenta from obese women – a state that was aggravated with existing maternal diabetes (Colomiere et al. 2009). Further to this, there is evidence of increased SNAT2 activity in the placental sodium-dependent amino acid transport “system A” of obese women. Placental nutrient sensor, mTOR has been linked to obesity where increased mTOR and IGF/insulin signaling suggests a role for these molecules in determining placental nutrient transport expression, which is relevant to the predisposition of offspring to adverse states in adulthood (Jansson et al. 2013).

It appears that placental vascular development is also impaired in placenta of obese women; where increased levels of hypoxia induced factor 1 alpha (HIF-1α), accompanied by increased NFκB and JNK signaling, and the promotion of proinflammatory cytokine expression (Saben et al. 2014b). The transcription factor FOXO4 is downregulated, which may be important on account of its activation in oxidative environments, demonstrating anti-hypoxic and anti-inflammatory activity (Saben et al. 2014b). This potentially suggests a compensatory role for FOXO4 in combatting the state of inflammation in the placenta. Implicit to this idea, placental inflammation appears to play a pivotal role in human pregnancies associated with HFD and obesity.

Taken together, these data support the role the placenta plays in mediating fetal outcomes, and many indeed modulate the exposure of the fetus to these “toxic” metabolic environments and metabolites. An improved understanding of the mechanistic signaling pathways involved in these changes will be pivotal to an improved description of the dietary impact on the placenta, and correspondingly the fetus.

7 Concluding Remarks

There is no doubt that the intrauterine environment modulates fetal growth and development and that key fetal adaptations as a result of exposure to a multitude of agents will change offspring phenotype. This chapter has expanded the use of the term “toxin” to include changes in the metabolic profile of the mother; this may include high circulating levels of glucose, free fatty acids and/or pro-inflammatory cytokines. Indeed, phenotypic outcomes in offspring born to mothers exposed to environmental toxicants including pthlalates (Lee et al. 2015), BPA (Alonso-Magdalena et al. 2010), and cigarette smoke (Behl et al. 2013) are consistent with what this chapter presents in cases of maternal obesity/overweight. This may be suggestive of a common driver of phenotype, and that the developmentally plastic fetus responds to varying cues with similar outcomes. Whether these are adaptive in nature however is difficult to imagine currently, as exposure to these “toxicants” is a relatively recent event in our evolutionary past. However, what is clear is that the fetus is amenable to a number of environmental factors including those that one normally does not term “toxic”. As this field continues to evolve and grow, future investigations should be aimed at discerning the fine details of the signaling pathways involved, placing a larger emphasis on human studies.