"You Are What You Eat." MGM/UA. $29.95. Salesmanship thrives in the home-video market. This is a "video companion" to "Dr. Berger's Immune Power Diet," a pop-medicine book by psychiatrist Stuart M. Berger. The book was a 1985 best seller, despite denunciation in the medical world for Berger's promise of simultaneous weight loss and revitalization of the body's immune system. The video (where AIDS is never mentioned, but "immune/power" becomes a recurring motif) is a clumsily produced assortment of familiar exercises, quizzes, recipes and stress-reduction hints, narrated rather leeringly by Berger.