According to a study conducted by the US Agricultural Research Service (ARS), peanut butters do not contain trans fats. Recurring rumours that commercial peanut butters contain trans fats are not founded.
The rumours first started because small amounts (1-2 per cent of total weight) of hydrogenated vegetable oils are added to commercial peanut butters to prevent the peanut oil from separating out. According to M. Sanders, lead researcher at the ARS's Market Quality and Handling Research Unit, the hydrogenation process can generate the formation of trans fatty acids in oils.
For the study, Sanders prepared 11 brands of peanut butter, including major store brands and "natural" brands, for analysis by a commercial laboratory. He also sent paste freshly prepared from roasted peanuts for comparison. The laboratory found no detectable trans fats in any of the samples, with a detection limit of 0.01 per cent of the sample weight.
This means that a 32-gram serving of the studied peanut butters contain zero to about 0.0032 gram of trans fats without being detected. However, peanut butter has plenty of unsaturated fatty acids. The most abundant is oleic acid, the monounsaturated fat believed to be good for the cardiovascular system. In this analysis, oleic acid levels ranged from 19 per cent of total weight in one private-label brand to 27 per cent in one "natural" type.
Source: Agricultural Research Service