My mum used to complain that she couldn't lose weight. A size 18 and a couple of stone heavier than ideal, she tried in vain for years to shed the extra. Every week she headed to the gym, where she pounded the treadmill like a paratrooper, often three times a week. Most days she took the dog for a brisk, hour-long walk. She didn't eat unhealthily – the rest of the family ate exactly the same meals, and did a fraction of the exercise she did. She ought to have been the slimmest of the bunch: that she remained overweight was a frustration to her, and a mystery to all of us.