There are direct connections between nutrition and the endocrine system. Nutritional status and body energy reserves are important to the hypothalamic-hypophysis-gonadal axis integrity in cattle [
18]. Many hormones that are influenced by digestive physiology, including secretin, growth hormone (GH), insulin, and insulin-like growth factor I and II (IGF-I and II), carry out important roles in animal reproduction. Thus, idiosyncrasies in nutrition will impact the endocrine systems and vice versa. Leptin is a peptide hormone produced primarily by adipose tissue, and is the putative link between nutritional history, environmental stressors and systems physiology in mammals. Leptin is essential for puberty and postpartum reproduction, and is positively correlated with body condition in ruminants. Interestingly, leptinemia and expression of leptin in tissue are also affected by stage of pregnancy and lactation, colostrum intake, circulating levels of insulin, glucose, glucocorticoids and GH, ingestion of fatty acids, especially linoleic acid, and photoperiod [
19]. Dietary restrictions affect the onset of puberty in heifers, weight at calving, mammary growth, milk production and postpartum anestrus in cows [
4,
20,
21]. Environmental estrogens consumed with the diet can act as endocrine disruptors, directly interfering with reproductive function. For example, phytoestrogens and xenoestrogens stimulate endothelin-1(ET-1) synthesis in the oviduct cells. Considering that ET-1regulates tubal contractility, environmental estrogens may have deleterious effects on embryo transportation and implantation [
22]. Heifers born from dams receiving protein supplement during gestation tended to be younger at puberty and had a greater pregnancy rate [
20].
A critical time period exists during which neuroendocrine functions can positively be manipulated through nutrition. For example, female calves fed a high energy/high protein milk replacer were younger and weighed less at the onset of puberty and two weeks younger at conception and calving than those on a low energy diet [
23]. Dietary treatment during early calfhood changes gene expression in higher brain centers, creating a link between feed intake, energy expenditure and reproduction. Heifers fed a high concentrate diet had a decreased gene expression of neuropeptide Y in the arcuate nucleus, subsequently affecting GnRH secretion and negative effects on GH [
4,
21]. Previously, researchers suggested pushing heifers to reach 60-65% of the adult body weight before the breeding season, through intensive feeding, but recent research decreased the target body weight to 50-55% [
24]. High energy diets caused greater circulating concentrations of leptin, insulin and IGF-I in heifer calves and in cows [
4,
21,
25,
26]. IGF-I plasmatic concentration has been positively related to follicular growth and to shorter calf-to-conception intervals [
25,
26]. Liver and adipose tissue IGF-I response to GH administration is lower in cows with negative energy balance, as during early lactation, likely caused by down-regulation of GH receptors [
27,
28]. IGF-I decreased the blood fatty acid profile during early lactation after the administration of bovine somatotropin [
28], which is in contrast to the previous research that adipose tissue stores were minimal as a result of the negative energy balance. Furthermore, protozoal infection in calves has been shown to cause prolonged thyroid deficiency dictated by reduced feed consumption, subsequent to impaired secretion of thyroid hormones and metabolism [
29]. Because triiodothyronine (T3) is essential for GH-dependent IGF-I synthesis in the liver, decreased thyroid activity as a consequence of parasitism can have direct repercussions on growth and reproduction. In summary, nutrition mostly affects reproductive functions such as age at puberty, fertility, calving, mammary growth and milk production. In general, a high concentrate/high protein diet is recommended in heifer calves and cows during states of negative energy balance. Further research is needed to better understand the mechanisms by which specific nutrients influence hormones affecting systems physiology. This may also enhance biotechnology both in cattle and other livestock.