This is probably more a general knowledge than a language knowledge question, but can anybody tell me what the phrase 'empty calories' is supposed to mean? And does it, at all, have any substantive meaning?

I ask this because, as far as I am aware, a calorie is a calorie - a capacity to do work, a unit of energy. If it is empty, what is it empty of? Not energy, for sure. If it is not empty, what is it that we expect to fill it? Water? Crystals? The Holy Spirirt?

Nonplussed-and-perplexed-about-'diets' in London