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Glucose could be nature's way of telling the body to grow up

ATLANTA" University of Michigan scientists have discovered that glucose, a common sugar absorbed from food during digestion, could play a key role in triggering the onset of puberty in humans and other mammals.

" Scientists have long known there is a relationship between reproduction and nutrition," said Douglas L. Foster, a U-M neuroendocrinologist who specializes in sexual maturation. " When animals live in the wild, the timing of puberty varies with food availability. Experiments with laboratory animals show that short-term fasting interrupts the reproductive cycle.

" What we don't understand is how does the brain know when the body has enough nourishment to support reproduction?" Foster added. " What signals the brain to begin secreting the master hormone that triggers sexual maturation?"

Although still preliminary, Foster's experiments with young sheep show that varying the level of glucose in the animal's bloodstream produces immediate changes in the amount and pattern of hormones secreted by the brain. Foster presented results from his experiments at the Experimental Biology '95 meeting held here April 9-13.

The master hormone which controls sexual development in mammals is called gonadotropin-releasing hormone (GnRH), which is produced in a part of the brain called the hypothalamus. Rapid bursts of GnRH from neurons in the brain stimulate the pituitary gland to produce luteinizing hormone (LH), which in turn stimulates the testes or ovaries to produce sperm or eggs, according to Foster, research scientist in the U-M Reproductive Sciences Program, professor of biology, and professor of obstetrics and gynecology.

" We monitor hormone production in sheep at a time when they are just beginning to mature sexually, and compare changing hormone levels with fluctuations in blood glucose levels," Foster said. By controlling the animal's diet, Foster can control blood glucose levels and the speed at which the sheep grow and develop. Results of the U-M experiments to date include:

In slowly-growing sheep with low fat reserves, fasting decreases blood glucose and production of GnRH and LH. When the fast is over, glucose and reproductive hormone levels rise rapidly within 20 minutes of feeding.

When sheep are given a small amount of a substance that temporarily prevents part of the brain from utilizing glucose, production of GnRH and LH stops almost immediately.

" Our current research focuses on finding the brain's glucose sensor and identifying the nerve pathways between that sensor and the hypothalamic area, which controls secretion of reproductive hormones," Foster said. " The most likely candidate so far is an organ in the brainstem called the area postrema, which may play a key role in the nutritional regulation of reproductive processes. "

Since sheep are in many ways similar to humans, Foster hopes his research could someday help scientists understand why the average age of puberty in young women has declined over the last 100 years from 17 to 13 and why menstruation often stops in young women who participate in strenuous athletic training programs or diet excessively.

The U-M research project is funded by the National Institutes of Health. David C. Bucholtz, a veterinarian and U-M graduate student in physiology, and Keith Schillo of the University of Kentucky are collaborators in the research project.