Conception to embryogenesis

Changes in maternal food intake before pregnancy can have marked effects on the conceptus and these effects appear to be dependent on the magnitude of change in maternal body weight⁽Reference Gopalakrishnan, Gardner and Dandrea⁷⁾. In sheep, therefore, a sudden and maintained reduction in food availability that is sufficient to cause a substantial decline in maternal body weight can result in accelerated maturation of the hypothalamic–pituitary axis of the fetus to the extent that premature birth can occur⁽Reference Bloomfield, Oliver and Hawkins²²⁾. Interestingly, this outcome only occurs in approximately 30–40% of mothers exposed to this nutritional intervention⁽Reference Bloomfield, Oliver and Hawkins^8, Reference Bloomfield, Oliver and Hawkins²²⁾ and may reflect differences in the maternal catabolic response to chronic food deprivation and/or difference in body composition at the start of the study. For example, in one such study a subgroup of the animals exposed to periconceptional nutrient restriction had a baseline doubling in plasma cortisol⁽Reference Jaquiery, Oliver and Bloomfield²³⁾.

The long-term outcomes of being born to mothers exposed to periconceptional nutrient restriction are very modest and those effects, for example on glucose homeostasis, are inconsistent⁽Reference Bloomfield, Oliver and Harding^24, Reference Todd, Oliver and Jaquiery²⁵⁾. This outcome contrasts with the effect of maternal nutrient restriction commencing from the time of mating, which has no effect on gestational length, birth weight or glucose homeostasis or on the hypothalamic–pituitary axis of the offspring⁽Reference Gardner, Pearce and Dandrea^26–Reference Gardner, Van Bon and Dandrea²⁸⁾. It does, however, result in a marked leftward shift of the baroreceptor curve relating blood pressure to heart rate⁽Reference Gardner, Pearce and Dandrea²⁶⁾. The extent to which this type of adaptation may be exacerbated when these individuals are exposed to an obesogenic environment and become hypertensive⁽Reference Williams, Kurlak and Perkins²⁹⁾ clearly warrants further investigation.

Placental development and maturation of the kidney

Adaptations within placental structure, as well as in its endocrine sensitivity, have a pronounced influence on the extent to which changes in the maternal diet may adversely affect fetal growth⁽Reference Heasman, Clarke and Firth^30, Reference Heasman, Clarke and Stephenson³¹⁾. A reduction in maternal food intake, coincident with uterine attachment and invasion of the fetal trophoblast into the syncitium, reduces placental growth as a consequence of reduced cell proliferation⁽Reference Gnanalingham, Williams and Wilson³²⁾. This outcome is accompanied by raised glucocorticoid action⁽Reference Gnanalingham, Williams and Wilson³²⁾, which appears to be a direct response to the reduction in maternal plasma cortisol⁽Reference Bispham, Gopalakrishnan and Dandrea¹⁶⁾. At the same time, maternal plasma insulin, thyroid hormones, leptin and insulin-like growth factor-1 are reduced (see Table 1). These adaptations accompany the modest reduction in maternal body weight and the increased plasma NEFA⁽Reference Bispham, Gopalakrishnan and Dandrea¹⁶⁾ that may be important, in conjunction with the reduction in maternal plasma insulin, in maintaining the concentration of glucose⁽Reference Bispham, Gopalakrishnan and Dandrea^16, Reference Hay³³⁾, which is the main metabolic substrate required by the fetus. Conversely, increasing the amount of feed consumed by the mother from calculated requirements to ad libitum has very little effect on fetal growth through gestation⁽Reference Clarke, Heasman and Juniper^34, Reference Budge, Bispham and Dandrea³⁵⁾, and at term gestation fetal fat mass can even be reduced⁽Reference Budge, Bispham and Dandrea³⁵⁾.

Table 1. Summary of the major maternal endocrine changes following exposure to nutrient restriction between 28 d and 80 d of gestation in the sheep (adapted from Bispham et al.⁽Reference Bispham, Gopalakrishnan and Dandrea¹⁶⁾ and Symonds et al.⁽Reference Symonds, Budge, Mostyn and Watkins⁶⁴⁾)

(Values are means with their standard errors for seven sheep per group)

Mean values were significantly different from those for the control diet:

* P<0·05, ***P<0·001.

To date, all organs and mechanisms that have been examined in the adult offspring born to nutrient-restricted mothers appear to be re-set following exposure to this targeted nutritional intervention. However, adaptations that may place in the offspring at increased risk of later adult disease are not seen until individuals are subsequently exposed to an obesogenic environment⁽Reference Sharkey, Gardner and Fainberg^18, Reference Chan, Sebert and Hyatt^21, Reference Sebert, Hyatt and Chan³⁶⁾. This outcome is most notable in the kidney in which there is a marked attenuation of both cellular stress and inflammatory responses⁽Reference Sharkey, Gardner and Fainberg¹⁸⁾. These responses contrast with other pathways, including apoptosis and glucocorticoid action, that may initially protect the kidney from glomerulersclerosis⁽Reference Williams, Kurlak and Perkins²⁹⁾. It is likely that with age the cardiovascular health of these offspring will deteriorate more rapidly⁽Reference Chan, Sebert and Hyatt²¹⁾ and that they will therefore be at increased risk of CVD⁽Reference Symonds, Stephenson and Budge³⁷⁾. Indeed, there is further indirect evidence to support this outcome, as if these offspring become obese they show an exacerbated increase in blood pressure with age compared with those born to control-fed mothers⁽Reference Symonds, Stephenson and Budge³⁷⁾. Ultimately, these offspring exhibit all the symptoms of the metabolic syndrome including raised plasma TAG, hyperinsulinaemia, hyperleptinaemia and leptin resistance together with ectopic lipid deposition⁽Reference Chan, Sebert and Hyatt^21, Reference Williams, Kurlak and Perkins^29, Reference Sebert, Hyatt and Chan³⁶⁾.

Fetal and neonatal growth, leptin and maturation of the hypothalamic–pituitary axis

There are substantial differences between both species and nutritional models in the development of a number of organ systems that relate to their critical windows of development. Such differences are also found in the hypothalamic–pituitary axis, which primarily matures after birth in rodents compared with the final third of gestation in sheep and human subjects⁽Reference Symonds and Budge¹¹⁾. The importance of this profound difference between species, primarily linked to maturation at birth, has a substantial impact on the magnitude of leptin surge at birth and therefore its relative influence on postnatal growth and development, as summarised in Fig. 1. In small mammals leptin has a primary role in regulating brain development after birth⁽Reference Bouret, Draper and Simerly³⁸⁾; there is currently no evidence of comparable effects in large mammals.

Fig. 1. Comparison of the changes in plasma leptin after birth in mice ()⁽Reference Ahima, Prabakaran and Flier⁶⁵⁾, rats (· · ·▪· · ·)⁽Reference Cottrell, Cripps and Duncan⁴⁵⁾ and sheep ()⁽Reference Bispham, Budge and Mostyn⁴³⁾. Values are means with their standard errors represented by vertical bars for six to eight animals per time point.

In the sheep leptin administration to either the preterm fetus (10 d before birth)⁽Reference Yuen, Owens and Muhlhausler³⁹⁾ or to the neonate⁽Reference Mostyn, Bispham and Pearce⁴⁰⁾ has very limited effects on metabolism, which are confined to brown adipose tissue. Interestingly, in the sheep the amount of leptin required to induce the effects that follow an approximately 5–10-fold rise in plasma leptin is 10-fold greater in the fetus⁽Reference Yuen, Owens and Muhlhausler³⁹⁾ compared with the neonate⁽Reference Mostyn, Bispham and Pearce⁴⁰⁾ (i.e. 0·48 mg/kg body weight v. 0·04 mg/kg body weight). In the fetal sheep the administration of leptin results in a stability in uncoupling protein 1⁽Reference Yuen, Owens and Muhlhausler³⁹⁾, whereas after birth an increase in body temperature is accompanied by an accelerated loss of uncoupling protein 1⁽Reference Mostyn, Bispham and Pearce⁴⁰⁾. Leptin also promotes the loss of uncoupling protein 2 from the lung⁽Reference Gnanalingham, Mostyn and Webb⁴¹⁾ and pancreas⁽Reference Gnanalingham, Mostyn and Wang⁴²⁾ but not adipose tissue⁽Reference Gnanalingham, Mostyn and Webb⁴¹⁾, although whether this response is a direct effect or is mediated by the concomitant reduction in plasma cortisol remains to be established. These findings emphasise the comparatively modest effects of administering a physiological dose of leptin to a species with a mature hypothalamic–pituitary axis and in which there is an immediate reduction in plasma leptin⁽Reference Bispham, Budge and Mostyn⁴³⁾ at birth, as is also seen in the human neonate⁽Reference Masuzaki, Ogawa and Sagawa⁴⁴⁾. It is in marked contrast to the rat in which plasma leptin increases 1 week after birth⁽Reference Cottrell, Cripps and Duncan⁴⁵⁾ and parallels the postnatal recruitment of non-shivering thermogenesis⁽Reference Giralt, Martin and Iglesias⁴⁶⁾. In neonatal rats a much higher amount of leptin has been used, i.e. 2·5 mg/kg body weight, and a pronounced divergence in effect is observed between males⁽Reference Vickers, Gluckman and Coveny⁴⁷⁾ and females⁽Reference Vickers, Gluckman and Coveny⁴⁸⁾. This difference is not unexpected given the greater amount of fat present in females⁽Reference Flanagan, Vaile and Petley⁴⁹⁾ across species⁽Reference Waterland, Travisano and Tahiliani⁵⁰⁾, which is usually reflected in higher plasma leptin in females compared with males⁽Reference Flanagan, Vaile and Petley^49, Reference Wong, DePaoli and Lee⁵¹⁾; a difference that is present from birth in both sheep⁽Reference Mostyn, Bispham and Pearce⁴⁰⁾ and human subjects⁽Reference Laml, Preyer and Schulz-Lobmeyr⁵²⁾.

When a comparatively high dose of leptin is administered to neonatal rats a substantial loss of body weight only results in female pups exhibiting pronounced intrauterine growth retardation and also exposed to the additional stress of cross-fostering (not adopted for controls)⁽Reference Vickers, Gluckman and Coveny⁴⁸⁾. Indeed, selective cross-fostering could be a major factor in determining the apparent opposite effects of leptin between nutritional groups in a number of outcomes, including changes in gene expression and methylation relating to glucocorticoid action in the adult liver⁽Reference Gluckman, Lillycrop and Vickers⁵³⁾. In males leptin has little effect on body weight⁽Reference Vickers, Gluckman and Coveny⁴⁷⁾, possibly because these offspring were still growing at the end of the study (i.e. 170 d after birth) when no ‘protective effect’ of early leptin administration was apparent. In females the change in total fat mass is not however accompanied by a reduction in plasma leptin⁽Reference Vickers, Gluckman and Coveny⁴⁸⁾. Furthermore, whether adaptations in the amount of ectopic fat accumulation occur is currently unknown and is clearly important because it is critical in determining the magnitude of adverse metabolic outcomes⁽Reference Despres and Lemieux⁵⁴⁾. Given the pronounced differences in maturation at birth and the interaction between fat mass, leptin and gender outlined earlier, the extent to which such effects from using pharmacological amounts of leptin to prevent adverse body composition in the neonate are relevant to the clinical situation remains in doubt.

Increased maternal fat intake and compromised fetal growth

One current concern is the rise in obesity⁽Reference Hedley, Ogden and Johnson⁵⁵⁾, particularly its increased incidence in women of child-bearing age⁽Reference Catalano and Ehrenberg⁵⁶⁾. Macrosomic newborns of obese women have an increased amount of body fat⁽Reference Sewell, Huston-Presley and Super⁵⁷⁾. However, consistent observations have yet to be produced, as many confounding effects, particularly gender, have yet to be clarified. Indeed, in a recent study of nulliparous obese women in the UK 19% of women had small-for-dates infants compared with 13% producing large-for-dates offspring⁽Reference Rajasingam, Seed and Briley⁵⁸⁾. In mice increasing the amount of dietary fat alone or in combination with other carbohydrates has no effect on birth weight. However, if this dietary intervention is commenced several weeks before pregnancy and is accompanied by increased energy intake, then maternal body weight is raised and subsequent changes in offspring feeding behaviour are observed⁽Reference Samuelsson, Matthews and Argenton⁵⁹⁾. In the case of non-human primates doubling fat availability in the diet, primarily at the expense of protein content, actually reduces fetal weight⁽Reference McCurdy, Bishop and Williams⁶⁰⁾, with approximately 50% of the subjects being resistant to the effects of the high-fat diet on insulin secretion (and the accompanying increase in maternal body weight). Irrespective of the maternal response, in the fetus there is a consistent effect on hepatic gene expression of enzymes involved in gluconeogenesis, in conjunction with raised hepatic TAG content. This adaptation persists up to 6 months after birth, although the difference is greatly reduced after birth, and as the hepatic TAG content remains lower than in the fetus is not indicative of a disease-like state. Interestingly, the effects seen on hepatic genes are not translated to changes in protein abundance. This outcome is not unexpected as fetal glucose production is only normally activated after birth⁽Reference Warnes, Seamark and Ballard⁶¹⁾. It may be that raised hepatic gene expression reflects a non-permanent response to the nutritional stress⁽Reference Budge, Stephenson and Symonds⁶²⁾ imposed on the mother. This finding is also in accord with the effect of feeding a low-protein high-carbohydrate diet to rodents through pregnancy, which promotes glucocorticoid receptor abundance in a range of fetal tissues including the liver⁽Reference Bertram, Trowern and Copin⁶³⁾.

In the non-human primate study described earlier it was found that placing the mothers back onto the control diet after 4 years alleviates the effects of maternal consumption of a high-fat low-protein diet⁽Reference McCurdy, Bishop and Williams⁶⁰⁾. However, hepatic TAG content in the fetus remain above control values in those mothers no longer consuming the high-fat low-protein diet. Furthermore, non-human primates have very low body fat, which explains the near undetectable plasma leptin in the fetus (approximately 0·5 ng/ml). As their maximal fat mass, recorded at 3 months of age, is only approximately 5% body weight they do not exhibit signs of obesity or metabolic compromise. It would, of course, be highly informative to know how food intake changed with age and through each pregnancy in these studies⁽Reference McCurdy, Bishop and Williams⁶⁰⁾.