Unhealthy fats

Part 2: Trans fats = toxic fats

There is one class of fats that everyone agrees is harmful: they are called trans -fats.

To make a solid margarine from the liquid vegetable oils they must be hydrogenated to change the double bond(s) from bent to straight. This process involves heating the fat with hydrogen and a catalyst under pressure. Unfortunately the conversion in this process is not complete. A small proportion of the unsaturated fatty acid is not hydrogenated but converted to a trans-fatty acid (TFA).

For example, the TFA arising from oleic acid is still unsaturated, with a double bond in the same place, but with a different spatial arrangement around the double bond. This change gives the new molecule a different shape and property. Partially hydrogenated vegetable fat is used primarily in fast food and other commercially manufactured fried and baked foods to boost their shelf life; TFAs are found in thousands of products on supermarket shelves. [1] A diet high in frozen meals, pies, crackers, biscuits, chips, muffins, doughnuts, snack bars, butter or margarine, is likely to contain high levels of TFAs.

There is no argument by either side of the fat debate that fats containing such TFAs are unhealthy. TFAs inflame the arteries and accelerate heart disease.[2] Despite this, margarine manufacturers claim that margarine is healthier than butter because they class saturated fatty acids and TFAs in margarine together, and then show that the two combined in margarine are less than the total saturated fatty acid content of butter. This, of course, is based on the totally untrue assumption that saturated fatty acids are harmful.

If we were wise, we would ban this harmful ingredient from our food supply. Denmark banned these commercial fats in 2004 with no adverse effect on taste or price of affected food, including fast food and even their famous Danish pastries.[3]

But even then the message can still go wrong. In a recent New York Times there were samples of new FDA food labels that included TFA levels in foods. But conjugated linoleic acid (CLA), which is entirely healthy and beneficial as it has been shown to reduce cancer and heart disease risk and help with overweight, is a trans-fat. So now butter, which contains about 0.5 g per serving of CLA, has to be labelled as containing trans-fat. This makes butter look unhealthy when it is actually exactly the opposite.

So let me put the record straight: trans-fats in vegetable margarines and cooking oils are harmful; trans-fats in animal fats are good for you.

References

1. Stender S, Dyerberg J, Astrup A. High levels of industrially produced trans fat in popular fast foods. N Engl J Med 2006; 354: 1650-1652.

2. Mozaffrian D, Katan MB, Ascherio A, et al. Trans fatty acids and cardiovascular disease. N Engl J Med 2006; 354: 1601-1613.

3. Astrup A. The trans fatty acid story in Denmark. Atheroscler Suppl 2006; 7: 43-46.