How often have you heard someone say: Being fat is in my genes. My grandma was fat, my mum is fat and I’m fat. Eating breakfast won’t make a blind bit of difference.

Indeed they would have a point, because obesity and being overweight does appear to run in families. But then you have to ask yourself another question. How active is this family? What kind of a diet do they eat? Do they eat enough fruits and vegetables? Is it all down to genetics or is something else playing a role?

It’s true that some people are genetically predisposed to be larger than others, but actual body weight has more to do with environmental factors, such as what and how much you eat and how much or little exercise you take, than the genetic pool on which you are drawing.

Researchers have recently managed to per­suade a group of lean and overweight men to take part in a seven-month experiment where daily food intake and energy expenditure were accu­rately measured. During part of the stay they were intention­ally overfed by 50 per cent of their required calories. The scientists watched and measured with abated breath. Would the natural skinniest miraculously and spontaneously burn off the excess calories into thin air and thus not gain weight?

No, they didn’t. In spite of claiming that they could eat as much as they wished, the lean men put on as much as the fat ones (over 8 kilos in forty-two days). So next time you look enviously at that slim girl tucking into a cream cake, remember – she is compensating for it elsewhere in her diet or is doing loads of exercise to burn it off.

How about an excuse like this? It’s my metabolism; I burn fewer calories than slim people, that is why I put on weight.

Well, then you must be able to defy the laws of physics. Unless you are one of the minuscule number of people born with a metabolic disease such as Prader Willi syndrome, in which children have insatiable appetites and grow to the most enormous sizes, then the larger you are the higher your metabolic rate. It’s a tricky one to grasp, this, and people carrying excess weight will argue until they drop that they are the exception. They are almost certainly not. Studies have shown that the heavier you are, the more calories it takes to keep you alive. And if you stop and think about it, you’ll see it must be true. The more there is of you, the more calories are needed to fuel all the cells, and the more calories you need to move your greater weight around. Just as a big Jaguar guzzles far more petrol than a Mini, a 16-stone woman has been shown to burn up around 1,900 calories a day before she has even got out of bed, whereas a 9-stone woman burns up about 1,350.