I've always been fascinated by the staying power of "a calorie is a calorie" and the way people enjoy deploying this phrase.
In some literal sense, this statement is true -- but that sense is the one in which it is also a tautology, which means that it is true but empty of content. It may be true by definition, but it doesn't tell you anything about the way the world is.
When people say "a calorie is a calorie," though, they definitely mean to say something about the way the world is. They mean to say that what foods you eat doesn't matter for your weight, as long as you eat the same number of calories. Or they mean that your weight can be calculated through finding the difference between the calories you consume in food and the calories you burn in exercise and living. Or they mean that if two people eat the same way and move the same way, they will have the same experience in terms of weight gain or weight loss.
Stated this way, these things seem obviously false. I have an acquaintance who was in a bad car accident years ago. His digestive system was damaged, and he had to have a lot of surgeries. Now he is fine, but for a long time he struggled to keep his weight up, even when eating a lot of food, because his system didn't work well. Obviously, his condition contradicts the previous statements.
Maybe you're thinking, Well, those are exceptional circumstances. But there are many ordinary examples. Regular readers know that I'm a fan of the work of Dr. Jason Fung, whose book The Obesity Code proposes that weight is a matter of hormones and biology, not thermodynamics, so that you have to look at how your body is reacting to food and not just at its caloric content. Roughly speaking, frequent small meals mean your body produces a lot of insulin; this can cause insulin resistance, which causes the body to produce more insulin, which changes how your body responds to food. Even short periods of fasting can counter these effects.
Right at the start of his book Dr. Fung mentions several obvious examples of the how the calories-in-calories-out model obviously fails. Prior to puberty, boys and girls have the same body fat percentage. After puberty, women have almost 50 percent more body fat, despite eating less. Pregnancy induces weight gain, beyond the effect of eating more. Various drugs are known to cause weight gain, regardless of food intake. If you give people insulin, they gain weight; in fact there's a thing called "diabulemia" where people with Type 1 diabetes deliberately give themselves less insulin than they need, in order to lose weight.
Given all of these complicated factors, what is the point of "a calorie is a calorie"? I wrote before about why this strange tautology might be so attractive to people. But there's also an interesting kind of metaphysical way to look at it. I was recently reading Philip Mirowski's More Heat Than Light, about the relationship of economics and physics. In an early chapter, he discusses the work of Emile Meyerson, who Mirowski says proposed that "a sweeping postulate of the identity of things in time" was "central to all human thought." Things are always changing, but we can't understand things unless we take them as, in some sense, fixed.
In a passage I really like, Mirowski says that the story goes like this: "Someone proposes some hypothesis, and then a mathematical savant constructs an "equivalent" statement H*(x) of the hypothesis, highlighting some mathematical quantity x. The Meyersonian tendency then exerts its sway, and x begins to be treated analogously to the general philosophical category of substance: Namely, it is thought to obey some conservation laws [e. g., -- "a calorie is a calorie!"]... These conservation laws, in turn, provide the accounting framework that enables quantitative manipulation. Somewhere along the line, entity x gets conflated with object x', which becomes associated with all sorts of metaphysical overtones, such as the permanence of natural law, the bedrock of phenomenological reality, the identity of mind and body, and so forth."
That is, the whole concept of a calorie might just be a reifying projection of the fact that the science of thermodynamics works well with one mathematical formalism rather than another. We then make the leap, possibly unjustified, to the idea of reality that is conserved and unchanging, so that a calorie is a calorie.
Of course, statements like "a calorie is a calorie" also take on a life of their own because they fit into people's social and political commitments and allow them to blame "individual responsibility" for things that, as Dr. Fung elegantly explains, have nothing to do with choices and everything to do with the social and political aspects of food and nutrition science -- for example, the fact that "eat small frequent meals" and "a calorie is a calorie" both allow for nutrition "advice" that doesn't harm the bottom line of large food corporation.
In fact, in "The Foreign Policy of the Calorie," historian Nick Cullather traces the emergence of the calorie framework through its use as a social, political and cultural tool, arguing that the calorie framework "popularized and factualized a set of assumptions that allowed Americans to see food as an instrument of power, and to envisage a 'world food problem' amenable to political and scientific intervention."
Well. After decades of calorie-is-a-calorie bullshit, the New York Times just ran an article saying it might not be just "what" you eat, but also "when" you eat that matters. In my opinion, the focus of the article on circadian rhythms makes no sense -- as commenters pointed out, many cultures have late night dining habits and excellent health. But at least they will willing to break away from the ridiculous metaphysics and get into the fact that there are, in fact, many variables in play, and they may well interact in complicated ways that we just don't understand.