Obesity And Infertility Linked

For a long time, researchers have known that there was a connection between infertility and obesity in women, but a new study tells us they may have been wrong about the reasons they posited for the linkage.

Chronic Exposure

Until now, scientists and doctors had thought infertility in such cases was caused by a resistance to the insulin hormone. Obesity is often accompanied by constant high levels of insulin. Such chronic exposure to high levels of insulin was thought to cause a resistance of the muscles and other tissues to the signals given off by the hormone.

A new study performed on mice showed that the pituitary gland, which is responsible in part for releasing the hormones associated with fertility, is still sensitive to the effects of insulin. But in obese mice, the story is different. The constant signaling of the insulin to the body, telling it to release the hormones connected with fertility causes too much of those hormones to be produced. It is an overabundance of the reproductive hormones that brings on the apparent infertility, says the report of this study which appeared in the September 8, 2010 issue of Cell Metabolism. 

This discovery makes a firm tie from the metabolism to a woman's fertility in a heretofore unexpected manner and may have important ramifications for the treatment of women suffering from polycystic ovarian syndrome (PCOS). PCOS is characterized by abnormal menstrual cycles and is often linked to obesity and type 2 diabetes.

Pituitary Gland

The research team led by study author Andrew Wolfe of Johns Hopkins University came upon their discovery as they were studying mice that had been genetically engineered to lack the proteins known as insulin receptors. These receptors rest upon the cell surface, grab onto insulin, and transmit the message of the hormone to the remainder of the cell. The mice had been engineered so that only the cells in the pituitary gland were missing these insulin receptors. It is the pituitary gland that is responsible for, among other things, regulating fertility.

It has long been known that female mice fed a high-fat diet will become obese and subsequently have difficulty reproducing. However, to the surprise of the researchers in this study, the fat female mice that lacked the insulin receptors in their pituitary glands stayed fertile. This was an unexpected result. Researchers had supposed that removing the insulin receptors would cause insulin resistance.

Instead, the researchers found that the pituitary continues to respond to insulin. As the levels of the hormone rise along with weight gain, the pituitary gland begins to release ever-increasing amounts of the fertility regulating luteinizing hormone. When the levels of this hormone remain high for a long time, the natural cycle of the fertility hormones is disrupted so that egg release is not accomplished and the mice are no longer fertile.   

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