Never mind the calories, feel the hydrogenation.© Getty

Eating some fats could make you fatter than others, even if their calorie count is the same.

That's the finding from researchers who fed trans-fatty acids, commonly found in fast food, to monkeys. Those that ate a daily dose of the trans-fatty acids gained 30% more lard around their bellies than those who ate different fats containing exactly the same amount of calories.

'Trans-fats' are already considered to be a dietary villain because they boost levels of 'bad' cholesterol and promote heart disease. But when it comes to obesity, it is generally assumed that trans, saturated and unsaturated fats are equally problematic, because they are loaded with the same amount of energy.

This study says otherwise. It suggests that trans-fats could promote obesity more than other types of fat. People who eat them could be "walking down the road to disaster", says lead author Kylie Kavanagh at Wake Forest University Baptist Medical Center in Winston-Salem, North Carolina.

"It's consistent with the idea that fatty acids are not just calories," says Dariush Mozaffarian who studies dietary fats at Harvard School of Public Health in Boston, Massachusetts, and was not involved in the study.

Mounting evidence shows that the different types of fatty acids, which are incorporated into cell membranes, have wide-ranging effects on the body aside from those on cholesterol. "It'll be the direction of research for the coming years," Mozaffarian says.

Popular choice

Trans-fats are made by pumping hydrogen into food oils. Because of their long shelf-life and useful consistency, they have become popular in the food industry and are common in commercial baked, deep-fried and snack foods.

Kavanagh's team set out to test whether trans-fats fur up the arteries of monkeys. She fed one group of monkeys 8% of their calories from partly hydrogenated soy-bean oil, a common source of trans-fats. The dose was equivalent to about one human meal of a cheeseburger and fries a day. A second group of monkeys ate an identical diet, except their trans-fats were replaced with a slightly different chemical form of fat.

At the end of six years, Kavanagh scanned the monkey's arteries, but the thing that really stood out was their bellies. The trans-fat eating monkeys had gained around 7% in weight, while their healthier counterparts had put on just 2%. They also had about a third more flab around their abdomen. "You can see white globs of fat in these guys," Kavanagh says. She presented the results of the study on 12 June at the Annual Scientific Sessions of the American Diabetes Association in Washington, D.C.

Insulin danger

The researchers also found that the trans-fat monkeys had higher blood glucose and were much more insulin resistant, suggesting that they are headed toward becoming diabetic.

The finding echoes studies in people, which have hinted that a diet rich in trans-fats could contribute to diabetes and weight gain. But it has been difficult to pin down the effects of trans-fats compared with other elements of our diets.

The trans-fats may directly stimulate the pancreas to make more insulin, which in turn makes the body resistant to the excess of this hormone. Another possibility is that trans-fats, when inserted into cell membranes, may somehow prevent cells from reacting to insulin normally. But Kavanagh says it is not clear how this leads to a spare tyre around the middle.

Whatever the mechanism, the writing is already on the wall for trans-fats. Denmark has banned them from processed foods, and some US food companies are phasing them out since the Food and Drug Administration required that trans-fat content be listed on food labels. But "they are alive and kicking in fast foods", Mozaffarian says.

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