In a March 20 letter, the Federal Trade Commission put on notice the manufacturer of the popular Muscle Milk line of protein shakes: If a celebrity gets paid for a promotional post on Instagram, that business relationship would need to be disclosed.

The FTC highlighted an Instagram post by Nina Agdal, in which the model poses in the corner of a boxing ring with boxing gloves and two cartons of Muscle Milk at her feet. The caption of the Instagram post: “Spent my morning in the ring with @MuscleMilk @BoxingNapoleon,” followed by emoticons of a trophy and a flexed bicep.

“If your company has a business relationship with Ms. Agdal, that relationship should be clearly and conspicuously disclosed in her endorsements,” wrote Mary Engle, associate director of the FTC's division of advertising practices, framing the letter as a reminder of the agency's disclosure standards.