This sentence is an appeal to Ethos.
Ethos is a literary device used to convince or persuade the audience by the character´s traits involved. In this particular sentence the First Lady can be considered a role model and renowned character for many people. Consequently, It is normal to recognize the credibility of important, eminent, intelligent or famous people whom people admire. For this reason, this statement carries out a very convincing argument about the type of diet this notorious person recommended to the audience.
On the other hand, It can be mentioned others type of appeal:
Pathos is a method of convincing the audience with an argument that appeal to the reader´s emotions and feelings.
Logos is used to persuade the audience by presenting a statement and its logical consequences.