The long reign of trans fats as worst dietary substance may be nearing an end. Trans fats are still very, very bad, mind you. But lately, researchers have pointed to a new culprit in the country's battle with obesity: fructose. A study published in the new issue of the Journal of Nutrition suggests that one of the reasons people on low-carbohydrate diets lose weight is that they reduce their intake of fructose. Fructose, a type of sugar found in fruit, can be made into body fat with stunning speed.

The researchers, from the University of Texas Southwestern Medical Center, asked healthy, normal-weight volunteers to consume three different drinks in three separate tests. The drinks varied in their composition of fructose and glucose. After a drink, the subjects ate a regular lunch an hour later. The study found that the process by which sugars are turned into body fat increased when as little as half of the glucose was replaced with fructose. While glucose passes through the liver, which decides whether to burn it for energy or turn it into fat, fructose hits the ground running.

"It's basically sneaking into the rock concert through the fence," says Dr. Elizabeth Parks, the lead investigator. "It's a less-controlled movement of fructose through these pathways that causes it to contribute to greater triglyceride synthesis. The bottom line of this study is that fructose very quickly gets made into fat in the body."

Fructose consumed in the morning changed the way the body handled food eaten at lunch. Primed by the earlier fructose, the lunch fat was more likely to be stored than burned, the study showed.

This doesn't mean fruit is bad for you. Fruit has some redeeming values. But beware of the high-fructose corn syrup added to many processed foods, Parks says.

A study released last month also fingered fructose. A paper presented at the Endocrine Society meeting by a UC Davis scientist showed that people who ate a diet in which 25% of their energy came from fructose gained more intra-abdominal fat (the pot-belly effect) and had higher triglycerides than people who ate a similar diet in which 25% of their energy came from glucose. The author of that study, So Peter Havel, concluded that people with metabolic syndrome should avoid drinking too many fructose-containing beverages.

-- Shari Roan

High-fructose corn syrup is found in many sodas, snacks and processed foods.

Photo: Hal Wells/Los Angeles Times