HOUSTON - Though most people think obesity is a problem of hips, thighs and bellies, a new study by US scientists shows that obesity begins in the brain where the female hormone estrogen plays a role.

The study by a group of researchers, led by Baylor College of Medicine and The University of Texas Southwestern Medical School, shows that the hormone works through two populations of neurons, steroidogenic factor-1 (SF-1) neurons and the pro-opiomelanocortin (POMC) neurons, in a particular part of the brain called the hypothalamus.

The researchers found that the two kinds of neurons play different but important roles in metabolism, fat distribution and appetite control, according to the February issue of the Baylor College of Medicine news.

"Before menopause, women are protected from obesity and associated disorders by estrogen," said Dr. Yong Xu, the first author of the study and assistant professor of the USDA Agricultural Research Service Children's Nutrition Research Center at BCM and Texas Children's Hospital.

After menopause, the risk of obesity in women rises, while the risks of estrogen replacement therapy outweigh its effect on reducing obesity, Xu said.

Researchers studied four different kinds of mice to determine the effect of estrogen and focused their work on neurons in the hypothalamus that carry an estrogen receptor alpha. The hypothalamus links the nervous system to the endocrine system to regulate multiple physiological processes, including those controlling bodyweight.

"These hypothalamic populations are among the key sites where metabolic and reproductive cues are integrated. Changes in food availability and energy storage of the body may be reflected, at least partially, by estrogen signals in these neurons, which ultimately tells the animals to or not to reproduce."