The study, by Ronald P. Mensink and Martijn Katan of the Agricultural University in Wageningen, found that such hydrogenated oils increased the amount of "bad" cholesterol (known as LDL) and reduced the amount of "good" cholesterol (HDL) in the blood.

"The amount in most foods is very small," said Herbert. So whether consumers should be concerned "is a very open question."

Trans fatty acids are not found in nature. Their natural form is a long, kinky molecule whose twisted shape keeps vegetable oils fluid. "They kink and hook back on themselves so they can't be packed and can't be solid at room temperature," said Schaefer.