March 7, 2008 -- Trans fats have long been the bane of dietitians because they raise levels of artery-clogging LDL cholesterol and lower "good" HDL cholesterol levels -- which increase the risk of heart disease. Yet two new studies show that not all trans fats are created equal, and natural trans fats don't appear as harmful to cholesterol levels as artificial trans fats.
Trans fats, or "trans fatty acids," come in two forms:
- Industrial trans fats are artificially created by manufacturers by adding hydrogen to liquid vegetable oils; these are called "partially hydrogenated oils." This makes the oils more solid to give foods like cookies, pies, and french fries a rich, crispy texture.
- Natural trans fats are found in meat (cow, sheep, goat) and dairy products. These trans fats are made naturally in the stomach of these animals.
Most trans fats from the diet are industrial trans fat. Studies have shown that industrial trans fats contribute to heart disease by raising bad LDL cholesterol -- the kind that can lead to hardening of the arteries -- and by lowering good HDL cholesterol -- the kind that can reduce heart disease risk. However, studies haven't distinguished whether natural trans fats are as harmful to cholesterol levels as their industrial counterparts.
To answer that question, two studies in the March issue of The American Journal of Clinical Nutrition set out for the first time to directly compare the effects of industrial trans fats to those of natural trans fats on heart disease risk.
Natural vs. Industrial Trans Fats
The first study included 38 healthy Canadian men (average age, 33) who were randomly assigned to eat each of four different diets:
- High in natural trans fats (10.2 grams per day)
- Moderate in natural trans fats (4.2 grams per day)
- High in industrial trans fats (10.2 grams per day)
- Low in any type of trans fats (2.2 grams per day)
Aside from the differences in trans-fat content, the diets were nutritionally similar. After four weeks, the diets high in both industrial trans fats and natural trans fats raised overall cholesterol levels. The diet high in industrial trans fats also increased bad LDL cholesterol levels. Yet the diet with moderate natural trans fats had no negative effects on cholesterol.
In the second study, 40 men and women alternated between three weeks of a diet heavy in either natural trans fats (butter and cheese), or industrial trans fats (cookies). The researchers found that eating industrial trans fats reduced good HDL levels and increased bad LDL levels much more than natural trans fats -- but only in women. Men's cholesterol levels didn't significantly change from either source. The reason for the difference in the sexes is unclear, but it might have something to do with sex hormones, the researchers say.
These studies suggest that the amount of butter, meat, and other sources of natural trans fats in people's diets shouldn't significantly affect their cholesterol levels, especially considering that participants in both studies ate far more natural trans fats than most Americans typically include in their diets.
However, the news isn't as good for artificial trans fats. These studies offer further evidence of their artery-clogging dangers.
Several of the authors of both studies have ties to the food and dairy industries.