Signals that link energy to reproduction: Gastric fill, bulk intake, or caloric intake?

Sense and nonsense in metabolic control of reproduction

An exciting synergistic interaction occurs among researchers working at the interface of reproductive biology and energy homeostasis. Reproductive biologists benefit from the theories, experimental designs, and methodologies used by experts on energy homeostasis while they bring context and meaning to the study of energy homeostasis. There is a growing recognition that identification of candidate genes for obesity is little more than meaningless reductionism unless those genes and their expression are placed in a developmental, environmental, and evolutionary context. Reproductive biology provides this context because metabolic energy is the most important factor that controls reproductive success and gonadal hormones affect energy intake, storage, and expenditure. Reproductive hormone secretion changes during development, and reproductive success is key to evolutionary adaptation, the process that most likely molded the mechanisms that control energy balance. It is likely that by viewing energy intake, storage, and expenditure in the context of reproductive success, we will gain insight into human obesity, eating disorders, diabetes, and other pathologies related to fuel homeostasis. This review emphasizes the metabolic hypothesis: a sensory system monitors the availability of oxidizable metabolic fuels and orchestrates behavioral motivation to optimize reproductive success in environments where energy availability fluctuates or is unpredictable.

Mesenteric infusion of a volatile fatty acid prevents body weight loss and transiently restores luteinising hormone pulse frequency in ovariectomised, food-restricted ewes

Pulsatile luteinising hormone (LH) secretion is suppressed by food restriction and rapidly restored by return to ad lib. feeding concomitant with an increase in the oxidation of free fatty acids, although there is no increase in plasma leptin concentrations or body fat content in ovariectomised ewes. The ingestion of food may stimulate LH secretion by increasing availability of oxidisable metabolic substrates. Ruminal digestion is characterised by the production of volatile fatty acids and, of these, propionate is the major gluconeogenic substrate, and both glucose and propionate are oxidisable in a variety of tissues. To examine whether increases in mesenteric propionate concentrations are sufficient for restoration of pulsatile LH secretion during a period of food restriction, adult, food-restricted, hypogonadotrophic, ovariectomised ewes received mesenteric vein infusions of 5 μmol/min/kg body weight (BW) propionate or saline, whereas normal weight, ad lib.-fed ewes received mesenteric infusions of saline for 10 days. Blood samples were taken every 10 min for 5 h before the start of the 10-day infusion period, and continued throughout the first 5 h of infusion on the afternoon of day 1, and in the morning on days 2, 7 and 10. Propionate-infused, food-restricted and ad lib.-fed, saline-infused ewes showed a significantly higher LH pulse frequency compared to that of food-restricted-saline-infused ewes on postinfusion days 1 and 2 but not on days 7 and 10, and only the saline-infused, food-restricted group lost a significant amount of body weight. These results indicate that the reproductive system can respond acutely to infusion of metabolic fuels such as propionate, although a sustained recovery of pulsatile LH secretion requires more than an increase in this single metabolic substrate.

Metabolic signals in human puberty: effects of over and undernutrition

Puberty in mammals is associated with important physical and psychological changes due to the increase in sex steroids and growth hormone (GH). Indeed, an increase in growth velocity and the attainment of sexual maturity for future reproductive function are the hallmark changes during this stage of life. Both growth and reproduction consume high levels of energy, requiring suitable energy stores to face these physiological functions. During the last two decades our knowledge concerning how peptides produced in the digestive tract (in charge of energy intake) and in adipose tissue (in charge of energy storage) provide information regarding metabolic status to the central nervous system (CNS) has increased dramatically. Moreover, these peptides have been shown to play an important role in modulating the gonadotropic axis with their absence or an imbalance in their secretion being able to disturb pubertal onset or progression. In this article we will review the current knowledge concerning the role played by leptin, the key adipokine in energy homeostasis, and ghrelin, the only orexigenic and growth-promoting peptide produced by the digestive tract, on sexual development. The normal evolutionary pattern of these peripherally produced metabolic signals throughout human puberty will be summarized. The effect of two opposite situations of chronic malnutrition, obesity and anorexia, on these signals and how they influence the course of puberty will also be discussed. Finally, we will briefly mention other peptides derived from the digestive tract (such as PYY) that may be involved in the regulatory link between energy homeostasis and sexual development.

Critical periods of susceptibility to short-term energy challenge during pregnancy: Impact on fertility and offspring development

In female mammals, reproduction is tightly regulated by energy status and food availability. Although acute energetic challenges inhibit female reproductive behavior and gonadotropin secretion, less attention has been given to the effects of short-term energetic challenge on pregnancy and gestation. Furthermore, species differences in pregnancy physiology necessitate more detailed analyses of specific pregnancy models. Here, we studied musk shrews, which display induced ovulation and obligate delayed implantation, and whose reproductive physiology is tightly linked to metabolic status. We tested whether acute energetic challenges of varied degrees given at specific pregnancy stages (including before and after delayed implantation) have different effects on gestational outcome and offspring postnatal development. We found that 48 h of either 40% or 50% food restriction, which reduced body weight and strongly inhibited sexual behavior, had minimal effects on pregnancy success and litter dynamics when administered early in gestation (pre-implantation). However, <30% of females experiencing short-term food restriction later in gestation successfully gave birth (versus > or =70% of ad-libitum fed controls), and the pups of these food-restricted females exhibited a 30% slower postnatal growth trajectory. Interestingly, although pregnancy success and litter dynamics were unaffected by food restriction before implantation, gestation length was increased by metabolic challenges experienced at this time, indicating that energy status may regulate the timing of implantation. We conclude that 1) there are critical periods of pregnancy, particularly after implantation, when short-term, mild energetic challenges have significant impacts on fertility and offspring postnatal development, and 2) delayed implantation may have evolved, in part, as a buffering mechanism to prevent pregnancy failure during impaired energy balance in early gestation.