April 18 (HealthDay News) -- Overeating, not the obesity it causes, is the actual cause of metabolic syndrome, suggests a study with mice by researchers at the University of Texas Southwestern Medical Center at Dallas.

Metabolic syndrome is a collection of health factors that increase the risk of developing insulin resistance, fatty liver, heart disease and type 2 diabetes.

This study was among the first to propose that weight gain is an early symptom, not a direct cause, of metabolic syndrome, the researchers said.

"Most people today think that obesity itself causes metabolic syndrome," senior author Dr. Roger Unger, professor of internal medicine, said in a prepared statement. "We're ingrained to think obesity is the cause of all health problems, when, in fact, it is the spillover of fat into organs other than fat cells that damages these organs, such as the heart and the liver. Depositing fatty molecules in fat cells where they belong actually delays that harmful spillover."

In this study, Under and his colleagues compared normal mice to mice that were genetically altered to prevent their fat cells from expanding. Both groups of mice were overfed.

The normal mice got fat but didn't develop signs of metabolic syndrome until after about seven weeks of overeating. The genetically altered mice stayed slim but became seriously ill within a few weeks and displayed evidence of severe heart problems and major increases in blood sugar levels eight weeks before minimal heart problems developed in the normal mice, the researchers said.

The genetically altered mice showed significant damage to heart cells and to the insulin-secreting cells in the pancreas. They also got sick quicker, because the extra calories they consumed weren't stored in fat cells, but rather in other tissues, the researchers said.

The study was published online in the journal Proceedings of the National Academy of Sciences.

The exact cause of metabolic syndrome, which affects about 50 million Americans, is unknown, but lack of exercise and obesity have been tagged as the primary underlying contributors to the development of the condition, according to background information in the study.

SOURCE: University of Texas Southwestern Medical Center at Dallas, news release, April 16, 2008