Changes in reproductive hormone secretion in response to short-term changes in nutrient intake Responses to a single day of fasting

In primates, as in nonprimates, periods of chronic or severe undernutrition often result in a suppression of reproductive hormone secretion with an accompanying decrease in fertility (Zubiran and Gomez-Mont, 1953; Warren and Vande Wiele, 1973; Smith et al., 1975; Vigersky et al., 1977; Dubey et al., 1986). This sensitivity of the reproductive axis to severe undernutrition would be advantageous, ensuring that fertility is impaired in times when energy resources are limiting for the high energyrequiring tasks of carrying a pregnancy to term and rearing a growing infant. Such a sensitivity to chronic energy availability would be most advantageous for large animals with relatively large energy stores, who could subsist on internal energy stores in times of famine and then reproduce when food is more readily available. In contrast, much less research has examined the effects of mild to moderate changes in food availability on the activity of the reproductive axis. This review will focus on whether mild changes in nutritional intake can regulate reproductive hormone secretion, and will assess our current understanding of the mechanisms by which subtle changes in metabolic status regulate the central neural drive to the reproductive axis. Specifically, the effects of mild changes in food intake on reproductive hormone secretion in primate species, both humans and nonhumans, will be considered, and hypotheses regarding the utility of a reproductive system that is responsive to subtle metabolic cues are proposed. The physiological regulation of the reproductive axis in Old World macaques, such as rhesus monkeys (Macaca mulatta), is similar to that of humans, with specific similarities in the neuroanatomy of GnRH neurones, the responsiveness of the hypothalamic–pituitary axis to gonadal steroid hormones, and hormonal fluctuations across the menstrual cycle in females (Hotchkiss and Knobil, 1994; Silverman et al., 1994). Of particular relevance here is that the responsiveness of humans and nonhuman primates to mild changes in nutritional status appears to be similar.