A processed food product demonised because of its allegedly toxic effects may be safe in small amounts and one version could even protect against heart deaths, research has shown.
Trans fatty acids (TFAs), also known as trans or hydrogenated fats, have previously been linked to high cholesterol, heart conditions, strokes, diabetes and some cancers.
High intakes have even been associated with an increased risk of infertility and Alzheimer's disease.
But new research now suggests that small amounts of artificially-produced trans fat in food are nothing to worry about and a naturally occurring TFA found in milk and meat may actually be beneficial to health.
Lead scientist Dr Marcus Kleber, from Heidelberg University in Germany, said: "Our results show that the low levels of industrially produced TFA we found ... did not pose a health risk, and therefore could be regarded as safe.
"We also found that trans-palmitoleic acid (a naturally occurring TFA found in milk and meat from ruminant animals) is associated with better blood glucose levels and fewer deaths from any cause, but especially a lower risk of sudden cardiac death."
Other experts warned members of the public not to be misled by findings which may have been influenced by confounding factors, and reinforced the message that trans fats are not a healthy addition to diet.
Artificial trans fats are created when oil is treated with hydrogen - hydrogenated - to make it more solid.
They were widely used in the past as ingredients in processed foods, such as cakes, biscuits and pies, and for frying. Consumption of artificial trans fats has been greatly reduced in Europe, where there have been calls to limit or outlaw the ingredients.
In June US regulator the Food and Drug Administration (FDA) clamped down on artificial trans fats by revoking their "generally recognised as safe" (GRAS) status and effectively banning them from food products.
Scientists conducting the new research measured concentrations of trans fats in the red blood cells of 3,259 people in south-west Germany undergoing a diagnostic procedure for investigating heart disease between 1997 and 2000.
Participants were then monitored for around 10 years, during which time 975 (30%) of them died. Their trans fat levels were linked to information about deaths, medical history, lifestyle factors such as smoking and physical activity, and Body Mass Index (BMI).
Although higher concentrations of TFAs in red blood cell membranes were associated with raised levels of the harmful form of cholesterol, low-density lipoprotein (LDL), they were also linked to reduced BMI, lower triglyceride blood fats, and less risk of diabetes.
Higher levels of industrial trans fats did not lead to a greater likelihood of dying, in contrast to other research in the US.
However, trans fats made up a much smaller proportion of total fatty acids in the blood of the German patients, the scientists pointed out - just under 1% on average compared with more than 2.6% for the American study participants.
Dr Kleber said the team was surprised to find that naturally occurring TFAs were associated with lower death rates, chiefly because of a reduced risk of dying from sudden cardiac arrest.
Writing in the European Heart Journal, the researchers said most studies highlighting health risks associated with trans fats had recruited patients decades ago when TFA levels in food were higher than they are today.
They added: "We observed rather low concentrations of TFA in our patients and found no association with mortality."
Dr Tim Chico, reader in cardiovascular medicine at the University of Sheffield, said: "This study inadvertently runs the risk of misinforming the public about the risk of heart disease and trans fats."
He pointed out that trans fats had been strongly linked to narrowing of the heart arteries, which could lead to heart attacks.
It was also important to bear in mind that none of the study participants were healthy, and 80% were confirmed as having heart disease.
"The fact that trans fat levels did not predict mortality in the study may be because once the patients were known to have heart disease they were treated appropriately with statins and other medications to lower the risk of further heart problems," said Dr Chico.
"My advice remains the same; whether or not you are healthy or already have heart disease you should eat a healthy diet (rich in vegetables, nuts, seeds, and olive oil) and avoid processed foods that may contain trans fats and other unhealthy ingredients."
Professor Tom Sanders, a nutrition expert from King's College London, highlighted limitations of the study including the possible confounding effect of poorer participants having a higher industrial trans fat intake and richer people consuming more dairy products.
Victoria Taylor, senior dietician at the British Heart Foundation, said the study supported the link between trans fats and raised LDL cholesterol levels.
She added: "Although it may be the case that not all trans fats behave in the same way, this does not mean that we should be eating more than we are now or that our recommendations necessarily need to change. Additional research is needed, as well as consideration of any changes in the context of the diet as a whole to ensure a benefit to our overall risk of heart and circulatory disease."