Cyclopropane fatty acids (CPA) are a subgroup of fatty acids that contain a cyclopropane group.^[1] Although they are usually rare, the seed oil from lychee contains nearly 40% CPAs in the form of triglycerides.^[2]

CPAs are derived from unsaturated fatty acids by cyclopropanation. The methylene donor is a methyl group on S-adenosylmethionine (SAM). The conversion is catalyzed by cyclopropane-fatty-acyl-phospholipid synthase.^[3] The mechanism is proposed to involve transfer of a CH₃⁺ group from SAM to the alkene, followed by deprotonation of the newly attached methyl group and ring closure.^[4]

Cyclopropene fatty acids are even rarer than CPAs. The best-known examples are malvalic acid and sterculic acid. Sterculic acid as its triglyceride is present in sterculia oils and at low levels in the seed oil of species of Malvaceae^{[lower-alpha 1]} (~12%), cottonseed oil (~1%), and in the seeds of the tree Sterculia foetida (~65-78%). These acids are highly reactive but the cyclopropene ring is destroyed during refining and hydrogenation of the oils. They have attracted interest because they reduce levels of the enzyme stearoyl-CoA 9-desaturase (SCD), which catalyzes the biodesaturation of stearic acid to oleic acid.^[7]

At least one review indicates that CPFA are carcinogenic, co-carcinogenic, and have medical and other effects on animals;^[8] according to this review, "CPFA in food is dangerous to human health".