Dr Mercola wrote the above sentence in a recent article, and it really got me thinking about how context, convention and background knowledge play such a large part in our understanding and parsing of a sentence, over and above simple semantics and logic.
On the face of it, 'A calorie is not a calorie' is at best a meaningless sentence: a somewhat nonsensical contradiction. 'a is not a' is not an informative utterance straight off. And yet, not only did I understand what Dr Mercola was saying (although it tripped my wordstalk radar like this): I also agreed with him that the underlying proposition is true.
How it gets to mean what it does
How does this work? Well, I think that Dr Mercola used an elliptical allusion here. He was relying on the assumption that his readership is familiar with the cliched expression among dieters: 'a calorie is a calorie,' and that they understood that this expression (itself at least tautological on the face of it) means that when it comes down to it, it doesn't matter whether you are eating cream puffs or broccoli: 100 calories of either one boils down to the same contribution (or detriment) to your weight loss efforts. And then, he denies the truth of that expression.
So, for his apparently nonsensical sentence to make sense, real-world knowledge about another sentence is required. And that cliched sentence is itself an abstraction. What it claims, in an elliptical fashion, is that all calories, from regardless what source, are equivalent and interchangeable. The abstraction is something like this:
We start with the statement 'cream puffs are (the same as) broccoli:' - obviously false, right? But then, we zero in on one single attribute common to both: the calories that they contain, and claim equivalence in that regard. Kind of like the old 'pound of feathers is the same as a pound of lead.' And so 'a calorie is a calorie' is short for saying 'a calorie (in a cream puff) is (the same as) a calorie (in broccoli).
So much real world knowledge and so much knowledge of logic lies in the background of Dr Mercola's negation of that sentence, then! (I do think, however, that 'All calories are not equal' would have been a more elegant way for him to have said it. But then I wouldn't have gotten to play like this!)
...and why I agree with it
As for the truth of this bizarre-looking sentence, then, well, c'mon now! If you believe that all calories are equivalent, try eating mountain ash bark like the moose do up here this time of year. It's full of carbohydrate calories, but lacking a ruminant's stomachs, you're not going to get much benefit out of it. Besides, calories from different macronutrient sources are digested in completely different places in the body, requiring different amounts of energy to process them, different pH environments, and they carry different levels of micronutrients - and antinutrients - with them, which also makes them non-equivalent. On the experiential level, even people who are not especially in touch with their bodies; even people who do not believe that food is medicine or has a great effect on us, recognize that different foods affect them in different ways. Unfortunately, sentences like 'a calorie is a calorie' justify the recognition of many people that, on first thought, they'd rather eat cream puffs than broccoli: allow them to think that if they measure out caloric equivalents, it really will come to the same thing, and prevent them from moving on to the second thought, that broccoli really is so much more satisfying than cream puffs.
As much as I work to suspend judgment and avoid dogma, I find myself agreeing so strongly with Dr Mercola (having adumbrated his obscure sentence!) and feel that the cliche he is negating is dangerous and irresponsible.