The company has disputed Ackman's characterizations, meeting with analysts and investors in January to detail how its business operates and who its customers are. Ackman replied that Herbalife "distorted, mischaracterized, and outright ignored large portions" of his presentation.
Representatives from Icahn's firm, Icahn Capital Management and Ackman's firm, Pershing Square Capital Management, did not immediately comment on Icahn's Herbalife stake.
In an emailed response, a spokeswoman for Herbalife said, "We welcome all parties who see the same value in Herbalife that we do."
Herbalife soared $8.18, or 21.4 percent to $46.22 in Thursday after-hours trading following news of Icahn's holdings.