Castor oil may have a bad rap among people who were force-fed spoonfuls as children, but it’s no myth that the tonic has health effects. Now, scientists have elucidated the molecular mechanism of the active ingredient in castor oil, which has been used for thousands of years as a laxative and labor-inducer. Ricinoleic acid, the fatty acid that makes up about 90% of the oil, binds to one particular receptor in the intestines and uterus, the researchers discovered. The discovery explains how castor oil works and could lead to the development of less unpleasant drugs.
Although taking a daily spoonful of diluted castor oil as a general health aid is no longer in vogue, alternative health stores still sell the foul-tasting liquid as a laxative. The Food and Drug Administration has categorized castor oil as “generally recognized as safe and effective,” but researchers don’t understand its mechanism.
“When you study classic, old drugs, you almost always learn something from them,” says first author of the new study Stefan Offermanns, a biologist at the Max Planck Institute for Heart and Lung Research in Germany. “The major surprise here was how specifically castor oil worked.”
Offermanns and his colleagues were screening different fatty acids for their ability to bind to certain cellular receptors when they got a hit with ricinoleic acid. Knowing castor oil’s broad use in traditional and alternative medicine, the team decided to take a closer look at the compound. Using a large library of molecules that block the cellular receptors they were studying, they were able to home in on two that ricinoleic acid connects to: EP 3 and EP4. Both are prostaglandin receptors, which have varied roles in the body, from changing the structure of neurons to controlling how blood clots. In experiments on mice, the researchers showed that ricinoleic acid induces its laxative and labor-inducing effects by interacting with EP3. When someone swallows castor oil, ricinoleic acid latches onto EP3 molecules in smooth muscle cells on the walls of the small intestine and causes contractions, explaining castor oil’s effectiveness as a laxative. Similarly, the researchers showed that ricinoleic acid binds to EP3 in the uterus and causes contractions. The team published its results today in the Proceedings of the National Academy of Sciences.
“There were many theories as to how castor oil worked, including broad toxicity to intestine cells and effects on water and electrolytes,” says Offermanns. But ricinoleic acid is much more specific than those theories suggest, acting through just one receptor. How the receptor causes contractions, however, is still not known. But the new link between EP3 and intestine and uterine muscle cells could inspire work to find out.
“They did these experiments quite elegantly and comprehensively,” says biologist Phillip Bennett of Imperial College London. “And at one level, this finding is sort of a quaint little curiosity, but there’s more to it than that.”
The knowledge that ricinoleic acid binds to EP3 could be used to design drugs that target the receptor, says Bennett. Such drugs could serve as laxatives or labor-inducers without the side effects, like nausea, of castor oil.
A daily dose of castor oil won’t keep the doctor away, and modern medicine still hasn’t backed up claims that it also treats skin conditions, eases pain, and cures infections. So take castor oil with a grain of salt, or a spoonful of sugar.