Most of us put weight on very slowly, just a few calories too much every day. So if you eat a bag of crisps (130cal) and a Flake (170cal) or their equivalent every day you can expect to put on a pound in under a fortnight. To use up 300 cals, you need half an hour of high impact aerobics so walking to the shop won't burn it off.

So it means that if you want to lose weight, exercise is the second thing to do. The first is to eat less, and the indulgent foods are easier to give up than meals.

Ravussin has found that a sustained 10% excess caloric intake results in a 7% weight gain. His calculations clearly imply (and are consistent with the observational evidence from population studies) that the obesity epidemic is primarily driven by increased food intake and not by decreased physical activity.

Exercise is important for general health reasons. It doesn't work as the prime weight reducer for most of us, not least, because after jogging for half an hour, we might reward ourselves with a bar of chocolate. Instant weight gain.