This paper discusses the phenomenon of nutritional
flushing in ewes whereby increased nutrition stimulates folliculogenesis and ovulation rate. In addition the paper reviews recent findings on the effects of increased levels of nutrition on the blood concentrations of reproductive and metabolic
hormones in the ewe and some of the intraovarian changes that take place in response to nutritional stimulation. Finally, in the paper, we propose a model of the physiological mechanism for the nutritional stimulation of folliculogenesis and we review how closely the model fits recent published and unpublished evidence examining the mechanism of
flushing. Nutritional stimulation alters the blood concentrations of some metabolic
hormones. By using short-term models of nutritional
flushing, we have shown that as the blood concentrations of
insulin and
leptin increase that of
growth hormone decreases while that of
IGF-I appears unaffected by the nutritional
flushing. Nutritional
flushing also alters the blood concentrations of some reproductive
hormones. Again, using the same model, we have shown that there is a transient increase in FSH and a decrease in
oestradiol concentrations in the blood. The changes in
oestradiol are particularly evident in the follicular phase of the oestrous cycle. In the ovary, the effect of nutrition is to stimulate folliculogenesis. These changes are associated with intra-follicular alterations in the
insulin-
glucose, IGF and
leptin metabolic systems. The stimulation of these intra-follicular systems leads to a suppression in follicular
oestradiol production. The consequence of these direct actions on the follicle is a reduced negative feedback to the hypothalamic-pituitary system and increased FSH secretion that leads to a stimulation of folliculogenesis.