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What are trans fatty acids?

Unsaturated fatty acids in foods can exist in two differently-shaped forms, scientifically described as the cis and trans forms. Some trans fatty acids are naturally found in milk and butter. When oils are hydrogenated, the unsaturated fatty acids become partially-saturated though retaining a degree of unsaturation. In the course of this, these still partially-unsaturated fatty acids have, to some extent, become converted to the trans form. While some research suggests that trans fatty acids may be harmful, the evidence is somewhat conflicting. As yet there is no official guidance on the subject, other than that the amount currently consumed should not be increased.