#8 The worrying return of this workout trend that's a lot of hot (exhaled) air

It's a £90 billion global industry, but the fitness marketing machine can be all talk and no tone if you're not careful...

It’s 6am one weekday in the summer of 2016. The streets of Knightsbridge are delightfully silent, bar for the bin men, as I hotfoot it to my destination. I walk past Harrods, where the designer handbags are placed on literal pedestals in beautifully executed displays, behind windows that glisten in the dawn light. Soon, I turn into a mews full of grand townhouses that you would absolutely not know existed except for Google Maps, and make my way up the path to one that I later learn is across the road from where Karren Brady lives. I ring the bell, just like I had on several other occasions in the past fortnight, and the door opens.