NEW YORK - The artificial trans fats that once abounded in processed foods have become notorious for their undesirable effects on cholesterol levels. But a small clinical trial suggests that natural trans fats may not do the same damage.
When 61 healthy women followed a diet with a hefty dose of natural trans fats for four weeks, researchers found there were no changes in the women's LDL ("bad") cholesterol and only small changes in HDL, or "good," cholesterol, in some women.
So-called industrial trans fat was once widely used in crackers, chips and other baked or fried processed foods, but the other kind of trans fat occurs naturally in meat and dairy products.
It's known that industrial trans fats tend to raise people's levels of LDL cholesterol, while also lowering HDL cholesterol -- a double whammy against heart health.
But much less research has gone into the possible effects of natural trans fat.
And since food manufacturers have been removing the artificial kind from their products, the natural variety is becoming our main source of dietary trans fat, said Benoit Lamarche, a professor of food sciences and nutrition at Laval University in Quebec, Canada.
"The question is, 'is this a problem?'" said Lamarche, the senior researcher on the new paper. "This study suggests it's not."
Among overweight women in the study, however, HDL cholesterol declined -- by an average of five percent, though the average HDL level remained in the desirable range.
Since HDL cholesterol is heart-healthy, that's a potential concern, Lamarche told Reuters Health.
But on balance, he said, "we don't see what we see with industrial trans fats."
"The effects seem to be different, particularly with LDL," Lamarche said.
So does that mean a healthy, normal-weight woman can eat all the meat and butter she wants?
No, according to Lamarche, whose study was funded by Dairy Farmers of Canada, Dairy Australia, Agriculture and Agri-Food Canada and the Canadian Dairy Commission.
"This doesn't change the nutritional guidelines," he said, noting that the conventional advice is to avoid trans fats and limit saturated fat.
Saturated fat, found mainly in meat and dairy, can boost LDL cholesterol. But unlike trans fat, it does not lower HDL. The American Heart Association says people should keep saturated fat to less than seven percent of their total daily calories. (That's 140 calories if you eat 2,000 calories in a day.)
It's also always tough to know exactly how any single nutrient might affect a person's health in real life.
In studies like the current one, diets are carefully controlled to try to pinpoint a nutrient's effects.
In this case, Lamarche's team used a butter enriched with natural trans fats to substantially boost the women's intake over four weeks -- equivalent to what you'd get if you downed eight servings of dairy products in a day.
The women spent another four weeks using a "control" butter with about one-third the amount of trans fat. All of the other diet components -- from calories to protein, to fiber and other types of fat -- were kept the same between the two diets.
Studies like that are important for understanding the specific effects of natural trans fats, said David J. Baer, a researcher at the US Department of Agriculture who wrote an editorial published with the study.
But there are still questions, Baer told Reuters Health. Only a handful of studies have tested the potential short-term effects of consuming natural trans fats. And, Baer said, those studies have involved different "doses" of the fats, different approaches to adding them to the diet and different groups of people.
The only previous study that included women found that natural trans fats did boost women's LDL (but not men's). However, Baer pointed out, it used a higher daily allotment of the fats than the current study did.
So for now, Baer said, "it's hard to make a blanket statement" about natural trans fats.
He noted that some researchers believe the point is moot. If you follow conventional wisdom and limit saturated fat, you'll end up with little natural trans fat in your diet.
On the other hand, some researchers are looking at ways to boost the concentration of one natural trans fat in dairy products -- known as conjugated linoleic acid, or CLA. Animal research has suggested CLA can be heart-healthy and promote fat loss.
That could be done by changing how dairy cows are fed.
"If you want to produce a high-CLA product," Baer said, "it will probably have more of the other trans fats too."
So that, he noted, is one additional reason for studying the potential health effects of natural trans fats.