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Examining the fetal programming of obesity.

THE FIGURES AND trends behind the obesity epidemic are alarming: More than one-third of all adults in the United States are obese, as are 34% of women aged 20-39, and 17% of youth aged 2-19, according to data for 2011-2014 from the National Health and Nutrition Examination Survey.

In our ob.gyn. practices, many of us have witnessed the significant climb in national obesity rates over the past several decades. We've seen a continued increase in the prevalence of obesity among childbearing women, and a steady increase in the incidence of high-birth-weight babies. The percentage of women weighing 200 pounds has more than doubled since 1980, and up to 3-4 times as many children and teens in various age subsets are obese today as in the 1970s.

The obesity epidemic is often attributed to a high-fat and / or calorie-dense diet and decreased activity levels. However, this is only part of the picture. There has been growing recognition in recent years that obesity may be programmed by the in utero and newborn environment, particularly as it relates to nutritional permutations. We now have evidence, in fact, that developmental programming is likely a primary cause of the obesity epidemic.

Exposure to maternal obesity and being born with a low birth weight especially a low birth weight paired with rapid catch-up growth--are both associated with a significantly increased risk of childhood and adult obesity.

Research has demonstrated that newborns may be programmed, in both of these scenarios, with an increased appetite and a predisposition to storing calories as fat. In addition, data are accumulating that exposure to bisphenol A and other endocrine-disruptive chemicals, other environmental toxins, and corticosteroids may exert similar programming effects.

This window into the origins of obesity has significant implications for the practice of ob.gyn., where we have the opportunity to address the programming effects of the in utero and early life environment. Most importantly, we must counsel women before pregnancy about the importance of losing weight, guide them during pregnancy to achieve optimal pregnancy nutrition and weight gain, and prepare them to adopt optimal newborn feeding strategies that will guard against overconsumption.

Programming of obesity

The current obesity epidemic is only minimally due to genetics. Although select genetic mutations may be associated with obesity, these mutations account for an exceedingly small proportion of the obese population. Instead, much of the obesity epidemic involves epigenetic change--in this case, largely epigenetic deregulation of gene expression--and more broadly what we call gestational, or developmental, programming.

Developmental programming is a process by which a stress or stimulus at a critical or sensitive period of development has long-term effects. The major part of the developmental process pertaining to cell division occurs during intrauterine life; more than 90% of the cell divisions necessary to make an adult human occur before birth. Although there are important effects of the early newborn period, developmental programming is therefore largely gestational programming. Depending on when an in utero stress or perturbation occurs, it may permanently change cell number and/or cell differentiation, organ structure, metabolic set points, and gene expression.

The late physician David Barker got us thinking about in utero programming when he demonstrated an association between low birth weight, rapid weight gain in early life, and adult cardiovascular mortality. His theory about how nutrition and growth before birth may affect cardiovascular health later on, as well as other adult chronic diseases and conditions, became known as the Barker Hypothesis.

Many studies, both animal research and human epidemiological studies, have since confirmed and expanded our understanding of these phenomena. Research has demonstrated associations, for instance, between low birth weight and later risks of insulin resistance, diabetes, fatty liver, and the often-underlying metabolic syndrome.

Obesity is also central to the development of the metabolic syndrome, and we now have irrefutable evidence to show that low-birth-weight infants have a higher risk of obesity than do healthy weight infants. We also know, as Dr. Barker and his colleagues had surmised, that the greatest risks occur when there is rapid catch-up growth of low-birth-weight infants in the early years of life.

Moreover, we now understand that maternal obesity has programming effects that are similar to those of an in utero environment of under-nutrition and growth restriction. In the past several decades, the marked increase in maternal obesity has resulted in this programming process having an ever-increasing impact.

Both animal and human studies have shown that infants born to obese mothers have the same increased risks for adult chronic disease --including the risk of becoming obese--as those of low-birth-weight infants. This increased risk is often, but not always, associated with high birth weight, and it is independent of whether the mother has gestational diabetes mellitus (GDM). Having a high birth weight is more likely in the setting of maternal obesity and itself raises the risk of eventual obesity (as does GDM), but an infant's exposure to maternal obesity in and of itself is a risk factor.

The mechanisms

The programming mechanisms that predispose offspring to obesity are similar in infants of obese mothers and intrauterine growth restricted newborns, though they involve different epigenetic signals. Both involve dysregulation of appetite/satiety and of adipogenesis.

Appetite is primarily controlled by a complex circuit of neurons in the hypothalamus of the brain called the hypothalamic arcuate nucleus. Some neurons are orexigenic and stimulate or increase appetite, while others are anorexigenic and suppress appetite by promoting satiety.

During fetal development, hypothalamic neural stem cells proliferate and differentiate into various cell types. Neurons destined for the arcuate nucleus then differentiate into these so-called appetite neurons and satiety neurons. Though there is continued neural development and maturation during newborn life, hypothalamic control of appetite and satiety is largely set during this period.

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Differentiation to appetite or satiety neurons is regulated by a complex interplay of pathways that may be significantly altered by the nutrient environment. Research in our laboratory and others has shown that both limited and excess nutrition can program the structure and function of the arcuate nucleus--changing its wiring, in essence--such that there is an increased ratio of appetite to satiety neurons (Clin Obstet Gynecol. 2013 Sep;56[3]:529-36).

There also appears to be a programmed down-regulation in the reward pathway of the brain, and some studies have shown that children of obese mothers and children who were born with low birth weights have a higher preference for sweet and high-calorie foods. This all begins at the neural stem cell level.

With more appetite neurons and fewer satiety neurons, as well as a down-regulation of reward--and an abundance of available food--a newborn is at high risk of becoming obese. Eating for this child will not only be pleasurable; it will be driven by an enhanced appetite, an inability to feel full after reasonable amounts of food, and a down-regulation of reward (potentially requiring greater amounts of food or a shift in preference for high fat/sweet food to achieve the pleasure from eating).

In addition to alterations in appetite / satiety, the nutrition environment in utero can alter adipose tissue development and function.

Like neural development, adipogenesis --the process by which preadipocytes proliferate and differentiate into mature adipocytes--is tightly regulated by a cascade of transcription factors that are expressed in response to stimuli, including nutrients. In animal studies we have found an up-regulation of adipogenic and lipogenic transcription factors in intrauterine growth restricted offspring as well as in offspring of obese mothers (Reprod Sci. 2008 Oct;15[8]:785-96 and Curr Diab Rep. 2013 Feb;13[1]:27-33).

This up-regulation leads to greater proliferation of preadipocytes and greater lipid synthesis and storage in mature adipocytes. Not only will the newborn have an increased number of adipocytes, but he or she will have an increased number of hypertrophic lipid-filled fat cells. The enhanced adipogenesis will contribute to the newborn's programmed propensity for obesity, and the directive to "just eat less" will likely be ineffective throughout childhood and beyond.

Programmed offspring are resistant to both central and peripheral effects of leptin and insulin, resulting in impaired satiety (i.e., overeating) and manifestations of GDM. Responses to an array of additional energy regulatory factors (e.g., ghrelin) demonstrate a similar programmed dysfunction.

In practice

There are several approaches that ob.gyns. can take to prevent child hood and lifelong obesity. Most importantly, we must counsel our obese patients to lose weight before pregnancy. In doing so, it may be meaningful and effective to ask the patient to think about her baby's future as an obese adult.

Patients who have experienced the challenges of trying to lose weight, and who are told about the developmental origins of obesity and how obesity can be programmed, may be more motivated to lose weight to avoid passing on to their children the burden and challenges that they've experienced. We can tell obese patients that their children may well be predisposed through the current in utero environment to have an increased appetite and a propensity to store body fat, and that they subsequently will face higher risks of diabetes and other serious chronic conditions.

We should also appropriately counsel women on healthy weight gain during pregnancy, and urge them not to gain excessive weight.

Newborn feeding strategies are also important for babies exposed to gestational programming of obesity, but especially small babies given the high risk of obesity when there is rapid catch-up growth. We must encourage good growth of both the low-birth-weight and macrosomic infant during the newborn period, but not overgrowth.

The importance of breastfeeding cannot be overestimated, as it has been demonstrated to reduce the occurrence of excessive newborn weight gain and improve long-term infant health. We should encourage breastfeeding for the natural opportunity it provides to avoid excessive feeding, in addition to its other benefits. And for newborns who are bottle fed, we should counsel the new mother on optimal feeding and strategies for comforting a crying baby, which will protect against overfeeding.

Regarding environmental exposures, this area of developmental programming is continuing to evolve at a rapid rate. Both animal research and epidemiological studies support the association of developmental exposure to BPA and other chemicals with obesity.

For the present, we should educate our patients regarding optimal nutrition prior to and during pregnancy, and the avoidance of potentially toxic or metabolically active chemicals or drugs. We look forward to continued research into the mechanisms and preventive/therapeutic strategies for optimization of childhood and adult health.

BY MICHAEL G. ROSS, MD, MPH, AND MINA DESAI, MSc, PHD

Dr. Ross is professor of obstetrics and gynecology at the University of California, Los Angeles. Dr. Desai is assistant professor of ob.gyn, at the university. They reported having no relevant financial disclosures.
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Title Annotation:Master Class
Author:Ross, Michael G.; Desai, Mina
Publication:OB GYN News
Date:May 1, 2016
Words:1782
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