The absurdities of diets are endless, be they for wellness or for weight. John Harvey Kellogg, inventor of the cornflake, made you slow-chew your food like a cow. Andy Warhol ordered the most unappealing dishes in the fanciest restaurants so he didn’t have to eat them. Others eat vats of cabbage soup or mountains of grapefruit, or even eggs (Charles Saatchi, despite living with Domestic Goddess Nigella).

However you try to shed the pounds, the basic arithmetic is that a pound of body fat is 3,500 calories. The average man needs 2,800 calories a day; the average woman 2,000. If you eat 500 calories less than you need per day, you will lose a pound of weight a week.

Sounds far too simple? Diet theories are far more diverting. Some believe metabolism and types of calories matter. The low-carbohydrate Atkins diet is now moderately mainstream, although critics say the weight loss is ultimately down to calories and the initial water loss involved in a low-carb diet. As for low-fat diets, some experts believe you miss the satiety of fats (plus your body needs them) and now stuff yourself with carbs. Fast dieting is worst of all. Your body goes into a state of emergency and conserves the food it gets into storage — in the form of fat.

While exercise burns calories and makes you fitter, most energy is spent on just being alive. Your brain alone requires 20 per cent of your calories. No wonder most dieting conversations are so fatuous: you are starving your grey matter as well as thinning your blubber.