Despite its wide acceptance, it explains just one part of a more complex picture, as a new study demonstrates.
Researchers accepted the standard view that our children are getting obese because they sit in front of the television or in the classroom all day.
But when they put a group of pre-school children through a series of exercise programmes for six months, to their surprise none of the children lost any weight.
A group of 545 children from nurseries in Glasgow were either put on the exercise programme, which consisted of three, 30-minute sessions every week for 24 weeks together with an education pack that encouraged more activity in and around the home, or they carried on with their usual sedentary lives.
But there was no difference in weight loss between the two groups. The puzzled researchers believe that the exercise programme was perhaps not strenuous enough, but did admit that diet may also play a part.
Now there's a thought - provided, of course, it's one that leaves out all processed foods.
This was demonstrated in a separate study of the Mediterranean diet, which is rich in fruits, vegetables, cereals and olive oil, and which avoids processed foods. The researchers found that overweight people who stuck to the diet were very unlikely to become obese, while people of standard weight didn't become overweight.
So - it's not just how much you eat, it's what you eat that really counts. So put that in your calorie counter.
(Source: British Medical Journal, 2006; 333: 1041-3 (children's exercise study); Journal of Nutrition, 2006; 136: 2934-8 (Mediterranean diet study)).
E-news broadcast 23 November 2006 No.312 [Subscribe]