Metabolic syndrome -- a group of symptoms including diabetes, pre-diabetes, high blood pressure and high cholesterol that increases the risk of heart disease -- is no longer thought to be caused primarily by abdominal fat.

Instead, researchers at Yale University School of Medicine have determined, via new imaging technologies, that insulin resistance in skeletal muscle leads to changes in energy storage, leading to metabolic syndrome.

Insulin resistance, which occurs when the body becomes resistant to the hormone insulin, occurs in skeletal muscle when the muscles are no longer able to make glycogen, a form of stored carbohydrate, from food energy. In turn, insulin resistance in skeletal muscle promotes an increase in fats in the bloodstream, which leads to metabolic syndrome.

Using magnetic resonance imaging techniques, the researchers were able to determine that insulin-sensitive individuals in their study converted carbohydrate energy (from eating a high-carb meal) into glycogen that was stored in the liver and muscle.

Among insulin-resistant individuals, however, the carbohydrate energy was rerouted to liver fat production. The process elevated the participants’ triglycerides in the blood by as much as 60 percent while lowering HDL (good) cholesterol by 20 percent. This occurred even though the participants were young and lean, with no excess of abdominal fat.

More than 50 million Americans suffer from metabolic syndrome, and half of the population is predisposed to it.

The researchers pointed out that there is good news to their findings: insulin resistance in skeletal muscle can be treated with a simple method, exercise.

Proceedings of the National Academy of Sciences July 18, 2007

Science Blog July 16, 2007