The double bonds in unsaturated fatty acids are predominantly cis-configured. That means, hydrogen atoms, which are combined to carbon atoms by a double bond, are on the same side of that double bond. This causes the fatty acid chain to bend.

With trans fatty acids, the hydrogen atoms, which are combined to carbon atoms by double bonds, are on opposite sides of the double bond. The fatty acid chain is, therefore, straight.

Trans fatty acids particularly occur in partly hardened fats. They are naturally found in beef and mutton and other ruminants. They are also found, to small extent, in the milk and milk products of these animals. They are formed by microorganisms in animals’ stomachs. Because of this, they end up in meat and milk.

The ingestion of more than 10-20g per day, the exact quantity is still under discussion, leads trans fatty acids to cause an increase in LDL and entire cholesterol levels, as well as to lower HDL cholesterol levels.

In Germany, for instance, an average of approximately 4-6g of trans fatty acids are ingested with food. Trans fatty acids are, in current thinking, counted as a possible factor in cardiovascular diseases.