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A protein bar loved by fitness fans has gone mainstream. Marks & Spencer is going to stock the Grenade Carb-Killer Bar in nearly 1000 stores. It’s part of a trend which has seen Mars, Snickers and Milky Way all released in versions with added protein. But what is the appeal of a snack bar full of protein? Melanie Abbott finds out more. First broadcast on You & Yours, 17 September 2018.