A side effect of incomplete hydrogenation, which has implications for human health, is the isomerization of unsaturated carbon bonds. The cis configuration of these double bonds predominates in the unprocessed fats of most foods. But hydrogenation often converts these molecules to trans isomers, which in fats have been implicated in heart disease ( see trans fats ).
Hydrogenation typically uses hydrogen gas as a reactant and an undissolved (or "heterogeneous") metal catalyst, such as nickel, palladium or platinum. Otherwise, the "homogeneous" rhodium-based catalyst known as Wilkinson's catalyst is often used. Such reactions belong to organic chemistry.