"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, Ph.D., professor of pathology and biochemistry and head of the Lipid Sciences Research Program.
"What it says is that trans fat is worse than anticipated," Rudel said. "I was surprised."
According to the U.S. Food and Drug Administration (FDA), 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.
Kylie Kavanagh, D.V.M., presented the findings today at the 66th annual Scientific Sessions of the American Diabetes Association in Washington, D.C. She said that over six years, male monkeys fed a western-style diet that contains trans fat 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.
All that extra weight went to the abdomen, and some other body fat was redistributed to the abdomen. Computed tomography (CT) scans showed that the monkeys on the diet containing trans fats had dramatically more abdominal fat than the monkeys on the monounsaturated fat. "We measured the volume of fat using CT," Kavanagh said. "They deposited 30 percent more fat in their abdomen."
The monkeys all were given 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 said. "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 researchers said that this amount of trans fat is comparable to people who eat a lot of fried food.
"We conclude that in equivalent diets, trans fatty acid consumption increases weight gain," said Kavanagh.
Over the entire course of the study, there was a small but significant difference in weight between the two groups. "In the world of diabetes, everybody knows that just 5 percent weight loss makes enormous difference," Kavanagh said. "This little difference was biologically quite significant."
Rudel said, "The study was specifically funded to look at the role of trans fatty acids in atherosclerosis."
He said that at the time he got a 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, though there are data showing it was a risk factor for atherosclerosis.
Kavanagh said the six-year length of the study was equivalent to 20 years in people.
According to the FDA, trans fat is found in vegetable shortenings, some margarines, crackers, cookies, snack foods, and other foods made with or fried in partially hydrogenated oils. Unlike other fats, the majority of trans fat is formed when food manufacturers turn liquid oils into solid fats like shortening and hard margarine by adding hydrogen.
Since Jan. 1, the FDA has required the amount of trans fat to be listed in the nutrition facts panel on all foods. But the restaurant industry is exempt.