Studies of developmental programming of offspring phenotype by suboptimal maternal nutrition initially focused on the adverse outcomes that occur when maternal nutrition is reduced. This emphasis was driven by interest worldwide on the adverse outcomes resulting from cohorts of intrauterine growth-restricted (IUGR) babies, e.g., the Dutch Hunger Winter, the Leningrad famine, and the Chinese famine.[16, 17] These studies also provide valuable information on programming challenges that result in offspring obesity since the offspring of nutrient-restricted mothers are predisposed to obesity if their postnatal diets are rich in calories and fat, as is typically the case in the modern, westernized diet. Thus, developmental programming of offspring obesity is one of the adverse outcomes of reduced maternal nutrient availability, which is inevitably accompanied by reduced fetal and neonatal nutrient availability. The mechanisms involved in the predisposition of offspring of nutrient-restricted mothers to later-life obesity have been reviewed extensively, and much can be learned from these studies in relation to the mechanisms and outcomes. However, this article focuses mainly on evidence from carefully controlled animal studies that throw light on the extent to which, and the mechanisms by which, maternal obesity alters offspring phenotype, predisposing the next generation to obesity.
Any analysis of the effects of maternal obesity on the developing fetus and neonate must assess the complex, interacting mechanisms involved. First, it is crucial to consider the independent and interactive effects of the obese maternal phenotype per se and the diet inevitably associated with that phenotype. Second, it is now clear that many different mechanisms play a role in a developmental-stage-related fashion. Some mechanisms that have a significant effect on offspring phenotype appear to occur even before pregnancy, acting through altered gamete function; others occur during pregnancy, acting through the impaired placental development and function that occurs in response to poor maternal nutrition; yet others occur in the neonatal period, acting by affecting lactation and maternal care. Thus, variations in outcomes show temporal specificity, since the developing organism passes through multiple critical periods of development at different stages of fetal and neonatal life, and the vulnerability of various organ systems to a specific challenge differs according to its stage of development at the time of the insult. These critical stages of vulnerability usually represent periods at which the maturing organ is undergoing key stages of proliferation or differentiation.
When considering developmental programming, it is useful to separate and consider the interrelationships of the challenges and outcomes followed by the mechanisms involved. In addition, controlled animal studies allow investigators to introduce an intervention designed to address a causal hypothesis, as described more fully elsewhere in this supplement. In the human setting, epidemiological studies based on carefully collected cohorts of offspring for whom much developmental data are available have given much impetus to the study of the relationship between maternal obesity and programming of offspring for obesity, but unless an intervention is introduced, epidemiological studies can only indicate associations. To demonstrate causality, interventions with changes in maternal diet, exercise, or pharmacological agents are required. Of importance, animal studies allow the investigator a much greater degree of control over potential confounding variables, such as lifestyle differences, and can throw light on potential interventions that may have benefit before or during pregnancy and lactation in women. Understanding the challenges and outcomes and their related mechanisms is essential for the translation of this exciting new area of developmental biology to obstetric, neonatal, adult, and even geriatric medicine.
Comparative physiology of precocial and atricial species in relation to developmental programming
The millennia of evolution have produced a very broad spectrum of pregnancy strategies, trajectories of fetal development, and neonatal offspring developmental patterns that optimize each mammalian species' ability to survive and reproduce in its particular environmental niche. Rodents are, in general, altricial species born after relatively short pregnancies and requiring considerable maternal care in the immediate postnatal period to regulate basic neonatal functions, such as maintenance of offspring body temperature, while they develop the ability to thermoregulate. However, this generalization does not apply universally to rodents. There are exceptions, such as the guinea pig, which is born at a very mature stage and able to eat solid food from birth. Precocial species such as humans, nonhuman primates, and sheep are more mature at birth. However, even in precocial species, not all physiological systems are equally mature at birth. Humans are considered to be precocial in many respects, but with regard to organized locomotion or ability to control their body temperature, human newborns are far less advanced than newborn lambs or monkeys. All these species provide valuable data, since comparative physiology provides the opportunity to observe the different ways in which a particular developing system, adrenal steroid production for example, responds to a challenge during development, what growth factors are recruited or inhibited, what neural connections are impaired, and, most importantly, what gene changes occur and which of the epigenetic marks placed on the genes will persist into postnatal life and lead to a persistent change in phenotype.
Although the separation into altricial and precocial species is not all-embracing, it is of value because many maturational changes that occur postnatally in altricial rodents occur during gestation in precocial species, i.e., when the levels of key regulatory and metabolic factors present in the offspring's blood are very different from the levels in the postnatal period. The prenatal environment differs greatly from the postnatal environment. Before birth, developing tissues are exposed to an arterial P02 of about 40 mmHg. After birth, arterial P02 is 100 mmHg. This difference potentially has consequences for programming mechanisms, such as oxidative metabolism and production of oxidative stress, which is discussed below as a potential mechanism for both normative and programmed development. Similarly, before birth, fetal glucose concentration is half that in postnatal life, and the fetus is exposed to a very different endocrine milieu before versus after delivery. In addition, the fetus is exposed to hormones and metabolites produced by the placenta while the neonate is not. As a result of all these differences, challenges during the key windows of vulnerability referred to above, occurring at different times in relation to birth and in the presence of very different environments, may result in perturbation of different cellular mechanisms with different outcomes in precocial and altricial species.
There are also major differences among species in the nutritional burden the mother bears during pregnancy and lactation. Humans are monotocous species – meaning that women generally bear only one fetus, though 1 in 80 natural pregnancies results in twins and experience with assisted reproductive techniques clearly indicates the ability of women to conceive and carry more fetuses to term. In contrast, rodents are polytocous species bearing large litters. Thus, even under optimal feeding conditions, the nutritional demands of pregnancy and lactation on the litter-bearing rodent mother are much greater than on mothers in monotocous species. The feto-placental biomass nurtured by a pregnant rat is equivalent to a pregnant woman bearing a 25–30 kg baby. In addition, there are qualitative differences in the metabolism of key nutrients in rodents compared with primates. Two key examples can be given of differences in micronutrient metabolism between rodents and primates that have potential significant effects on programming mechanisms: 1) the methionine cycle, which is important in gene methylation, and 2) vitamin C, which is a powerful antioxidant. The rodent methionine cycle, which is central to epigenetic transformation resulting from altered methylation, differs from that of primates. Folic acid reduction to active tetrahydrofolate by dihydrofolatereductase in human liver is <2% of rat liver. In addition, rodents can synthesize their own vitamin C. The biochemical differences between these species present several interesting issues that will clearly produce differences in mechanisms and outcomes between rodents and primates in response to nutritional challenges, such as maternal overnutrition or low-protein diets lacking methionine.
Differences also occur in key metabolic systems. There are two forms of the key gluconeogenic enzyme phosphoenolpyruvatecarboxykinase, one mitochondrial (PEPCKM) and the other cytosolic (PEPCKC). Hepatic PEPCKC comprises 90% of PEPCKC in rats and mouse livers and, as a result, PEPCKM only represents 5–10% of the total PEPCK activity. In human liver, 50% of the PEPCK activity is PEPCKM, and this balance is seen in most mammalian species. The mRNA, or protein, level for PEPCKC is often used as a good index of the rate of gluconeogenesis, which is an important fetal response to nutrient deficiency. However, the impact of specific nutritional challenges on both of these isoforms requires studies in species that are similar to humans and have both forms of the enzyme. An enormous amount of very valuable metabolic information has been generated over the past 25 years using the techniques of mouse genetics combined with metabolic analysis, but there is also a need for studies in precocial species, such as sheep and nonhuman primates, for comparative evaluation and translation to the human situation.
With respect to lipid metabolism, differences have also been demonstrated in key hepatic enzymes between rodents and humans. Acetyl-CoA carboxylase (ACC) exists as two isoforms, ACC1 and ACC2. In rats, ACC1 is mainly expressed in lipogenic and ACC2 in oxidative tissues. In humans, ACC2 is the major enzyme in both oxidative and lipogenic tissues.