Mustard oil is rich in unsaturated fatty acids, but it also contains a special type of fatty acid called erucic acid, which lies at the center of the controversy surrounding the oil. Seeds from the brassica family of plants, which includes rapeseed and mustard, in addition to cabbage and kale, all contain varying amounts of erucic acid. Early experimental studies on animals in the 1950s suggested that erucic acid possibly had a role in the development of heart disease.
There were two notable consequences to this research. The first was that, in response to it, Canadian scientists created canola oil (a combination of the words "Canada" and "ola," the old English word for oil—although some say the "ola" is an acronym for "oil, low acid") by carefully breeding rapeseed plants that produced seeds with extremely low levels of erucic acid. (They also refined the oil through a series of industrial processes to strip it of any of its natural mustardy heat to create a more versatile neutral product.)
The other thing that happened was that mustard oil, thanks to its high levels of erucic acid, was restricted for sale for human consumption in America.
The Truth About Mustard Oil: Behind the "For External Use Only" Label