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http://en.wikipedia.org/wiki/Leptin apparently regulates
both how much we
eat and our reproduction ability.
http://en.wikipedia.org/wiki/GnRH is
among the factors that may be involved.


Popovic V, Casanueva FF.
Leptin, nutrition and reproduction: new insights.
Hormones (Athens). 2002 Oct-Dec;1(4):204-17.
PMID: 17018449

Recent data suggest that in addition to leptin's role
in conveying
signals of the amount of energy stores to the central
nervous system, this
adipocyte secreted hormone interacts with the endocrine
system to provide
critical information about the size of fat stores, acting as
a permissive
factor that allows the triggering of energy demanding
situations as the
onset of puberty and reproduction. Animal and human data are
concordant with
the concept that leptin plays an important permissive role
in the initiation
of puberty and in maintenance of reproductive function
thereafter. Leptin
regulates the gonadotropin-gonadal axis at a central level.
The hypothalamus
is an important site of leptin's action where a complex
network of
neuropeptides is involved in leptin's effect on GnRH. In
addition, leptin
plays a role during pregnancy and lactation as it is
produced by the
placenta and is present in milk. Plasma leptin levels are
elevated during
pregnancy and this hyperleptinemia is not accompanied by a
reduction in food
intake, suggesting a state of leptin resistance. Leptin is
also detected in
the amniotic fluid and its levels are high in venous cord
blood at delivery
correlating positively with weight at birth which suggests a
potential role
in intrauterine growth. The fact that in females leptin
levels are higher
than in males, even when corrected for body fat, suggests
that the
reproductive system is modulated by leptin in a different
way in males and
females estrogens. In hypoleptinemia resulting from specific
genetic causes,
leptin levels may still be adequate for the function of the
reproductive
system in humans, a phenomenon which differs from the
findings in
leptin-deficient animals which are infertile. Due to species
differences in
the role of leptin, it is difficult to extrapolate data from
rodents to
human physiology. However hypoleptinemia due to non-genetic
causes such as
anorexia nervosa and exercise leads to loss of reproductive
function.
Genetic/developmental factors influence the threshold
required to turn off
the behavioral, metabolic and endocrine responses to
perceived caloric
deprivation.