I went on my first diet at the age of 14, joining my local Weight Watchers group and being astonished to discover that I had a full stone to lose. I, like many other magazine-reading teens, had assumed that I would have an extra 10 lbs to “get rid” of. Ten was the magic number of pounds written about in articles about getting summer-ready. Ten was a stubborn number of pounds, for sure, but it...