This conclusion, based on animal studies by researchers at Wake Forest University School of Medicine, was presented Monday at the 66th annual Scientific Sessions of the American Diabetes Association (ADA) in Washington, DC.

Trans fats are found in vegetable shortenings, margarines, crackers, cookies, snack foods, and other foods made with, or fried in, partially hydrogenated oils, according to the Food and Drug Administration (FDA). Trans fat is formed when food manufacturers add hydrogen to liquid oils, which turns them into solid fats like shortening and hard margarine.

Trans Fats Trigger Weight Redistribution

Consumption of saturated fat, trans fat, and dietary cholesterol raises low-density lipoprotein (LDL), or "bad" cholesterol, levels, which increases the risk of coronary artery disease, the FDA warns.

"Diets rich in trans fat cause a redistribution of fat tissue into the abdomen and lead to a higher body weight even when the total dietary calories are controlled," said Lawrence L. Rudel, PhD, professor of pathology and biochemistry and head of the Lipid Sciences Research Program at Wake Forest.

Over the six-year course of the study, male monkeys fed a western-style diet that contained trans fats had a 7.2 percent increase in body weight, compared to a 1.8 percent increase in monkeys that ate monounsaturated fats such as olive oil, said Kylie Kavanagh, DVM, in presenting the findings to the ADA.

All that extra weight went to the abdomen -- and some other body fat was redistributed to the abdomen. The monkeys on the diet containing trans fats had dramatically more abdominal fat than the monkeys on the monounsaturated fat, computed tomography (CT) scans showed.

The monkeys on the trans fat diet "deposited 30 percent more fat in their abdomen," said Kavanagh.

Same Calories, Greater Weight Gain

All of the monkeys consumed the same amount of daily calories, with 35 percent of the calories coming from fat. The amount of calories they got should only have been enough to maintain their weight, not increase it, Rudel pointed out. "We believed they couldn't get obese because we did not give them enough calories to get fat."

One group of monkeys got 8 percent of their calories from trans fat while the other group received those calories as monounsaturated fat. The amount of trans fat iin the monkeys' diet was comparable to the amount consumed by people who eat a lot of fried food, the researchers said.

“We conclude that in equivalent diets, trans fatty acid consumption increases weight gain,” said Kavanagh.

The difference in weight between the two groups over the entire course of the study was small but significant, the researchers noted.

"In the world of diabetes, everybody knows that just 5 percent weight loss makes enormous difference," Kavanagh remarked. "This little difference was biologically quite significant."

Be Wary of Restaurant Food

Rudel noted that the study was funded to look specifically at the role of trans fatty acids in atherosclerosis. At the time he received the grant from the National Heart, Lung and Blood Institute, there was not much evidence in the literature and no animal models that documented the hazards of trans fats, he said, though there already was data showing it to be a risk factor for atherosclerosis.

The six-year length of the study was equivalent to 20 years in people, Kavanagh said.

"What it says is that trans fat is worse than anticipated," Rudel commented. "I was surprised."

The amount of trans fats must be listed in the nutrition facts panel on all foods -- a regulation the FDA put into effect as of Jan. 1, 2006. However, the restaurant industry is exempt from the requirement.