Although trans fats raise your levels of "bad" cholesterol, they don't appear to have lasting impacts on your blood sugar, according to a new review of the medical evidence.
Researchers found that both blood sugar and insulin, the hormone that keeps blood sugar levels in check, were similar regardless of how much trans fat people ate.
The link between trans fats and high cholesterol levels is widely accepted, but there have been conflicting results on the effect on blood sugar control, which is involved in diabetes.
Trans fats, technically known as trans fatty acids, are found in animal products and chemically processed vegetable oils.
In response to studies linking high consumption of the substances to an increased risk of heart disease, the U.S. Food and Drug Administration has required food makers to disclose trans fats on nutrition labels, and some cities and states have banned them in restaurants or schools.
To get a better sense of trans fats' influence on blood sugar and insulin, Dr. Christos Mantzoros of Harvard Medical School in Boston and colleagues pooled the results from seven experiments including 208 people.
In five of the studies, the participants' blood sugar, insulin and cholesterol levels were monitored for several weeks under a diet of high trans fat consumption, and again for a few weeks when the trans fats were substituted for other fats, such as palm or soybean oil.
Two of the studies compared people who ate a diet that included trans fats to others who ate a diet without trans fats.
There were no changes in blood sugar or insulin levels during the times when people ate trans fats, compared to when they ate the other fats, Mantzoros's group reported in The American Journal of Clinical Nutrition.
However, the researchers found that during the trans fat-eating weeks, "good," or HDL, cholesterol went down and "bad," or LDL, cholesterol went up.
"They saw what you would expect to see" regarding cholesterol, which shows that the studies were well done, said Mark Pereira, an expert in public health and nutrition at the University of Minnesota in Minneapolis.
Pereira, who was not involved in the study, said it isn't definitive proof that trans fats can't influence blood sugar levels.
Although several weeks is enough time to see an effect on cholesterol, he said, a potential impact on metabolism might not show up until later.
"If you're going to control weight and switch around fats in the diet it might take a lot longer, because these fatty acids are being gradually incorporated over time into tissues in to the body," Pereira told Reuters Health.
One of the studies in Mantzoros's analysis lasted for 16 weeks, but it too found no difference in blood sugar and insulin changes between people who ate trans fats and those who ate other fats.
Even if trans fats do have an effect on blood sugar control, Pereira said, it's becoming a moot point as the amount of trans fats that people eat in the U.S. has diminished considerably.
According to the Centers for Disease Control and Prevention, trans fat levels in the blood of white adults dropped by 58 percent from 2000 to 2009.
A large national study found that cholesterol levels have also been declining since the 1980s.
Part of this is due to the popularity of cholesterol-lowering drugs, but researchers suspect that cutting back on trans fats has also made a difference.