When you find the label of a food showing that there are 0g of trans fats in a serving, does this really mean zero? Well as the FDA has written the regulation, you can “round down to zero” any amount of trans fat of 0.5g per serving or less. Of course, if there are no partially hydrogenated oils in the food, there really may not be any trans fats, but how much is of serious concern?
Trans fats are known to increase the risk of coronary heart disease by raising the levels of LDL (“bad”) cholesterol and lowering the levels of HDL (“good”) cholesterol.
Trans fats are created when a vegetable oil is subjected to hydrogenation to make a shortening with a higher melting point. Margarine and baking fats like Crisco are good examples. Hydrogenation amounts to treating the oil with hydrogen gas under high pressure, in the presence of a catalyst. If hydrogenation is carried to completion, you end up with a solid saturated fat. If hydrogenation is only partial, you get a soft spreadable oil.
And while saturated fats raise LDL, trans fats not only raise LDL but lower HDL cholesterol.
In the original vegetable oils, there are double bonds in the fatty acid molecules that make up the oil. These are always cis and therefore of no health concern. (The terms cis and trans refer to whether the two hydrogens are on the same or opposite sides of the double bond, as shown in the illustration.) However, during hydrogenation some of the double bonds convert to trans and these fats can be significantly less healthy for you.
Trans fats also occur naturally in meat and dairy products, but there has been some question as to whether the somewhat different fats involved are as harmful.
The question is, following the thinking of Paracelsus, what level of trans fats are actually bad for you, and is there a level of trans fats which are relatively harmless? It turns out that this work has been done. In an interview with NPR in 2009, Penn State Food Science professor J. Lynne Brown explained that experiment showed that if 3.8% of the calorie intake is in trans fats, that is enough to raise the levels of LDL. Amounts below that do not seem to have any effect. In a 2000-calorie diet, this would amount to about 8 grams of trans fats per day. If a serving contains 0.5 g of trans fats, then you would need to eat 16 such servings during the day to be at any risk.
We contacted Professor Brown to get the original reference to this work (it’s here), but more to the point, she pointed us to some even more interesting work by Bendsen, Christensen, Bartels and Astrup in the European Journal of Chemical Nutrition.
These workers distinguished the trans fats created by hydrogenation (“industrial”) from those found naturally in beef and dairy products (“ruminants”), and looked at the levels of each which are needed to raise LDL cholesterol.
They note that case studies from 1994 suggest that there is an effect above 3.3 g per day. However when ruminant trans fats were studied, consumption in moderate amounts (4.2 g per day) there was no effect on blood lipids. And recently, a study found no difference between the effect of industrial and ruminant trans fats at up to 5 g per day in overweight women.
They summarize a number of such studies to conclude that ruminant trans fats do not affect the risk of heart disease and may even be slightly protective. They qualify this by noting that ruminant trans fats are usually consumed in lower quantities, and that the dairy products themselves may be somewhat protective.
So, to conclude, manufacturers have done a pretty good job in recent years of reducing trans fats from most products. However, there are likely traces of 0.5g or less per serving if the products use partially hydrogenated vegetable oil. While consumption of modest amounts of these products is most likely safe, you still need to avoid multiple doses in the same day. Consumption of larger amounts of trans fats from beef and dairy seem to have much less effect.