Δweight = calories in - caloriesout
Of course, the somewhat more enlightened ones will be a bit more precise:
Δweight = calories consumed - caloriesburned
They'll then go on to explain, in a very authoritative tone, how this is an irrefutable law of thermodynamics, and that a person can't lose weight unless he eats fewer calories than he burns, how this is all very well understood and accepted by all the top scientists, and so forth. And the onlookers all nod in agreement; I mean, after all, this seems so obvious - how can anyone disagree?
At least one part of that proclamation is correct - it is an irrefutable law of thermodynamics (the first one, to be exact) that energy can neither be created nor destroyed, so any "excess" calories we eat must go somewhere. They can't just vanish into fat air, for crying out loud. If we don't burn them during our 30 minutes of cardio three times a week, or... ahem... expel them somehow, then they must be stored in our bodies. Otherwise, we could all pig out on donuts nonstop and remain thin, thereby causing the entire universe to collapse into a microscopic yet delicious piece of bacon.
Our experts are also correct that the equation and accompanying explanation are unquestioningly accepted. In fact, this mantra is so pervasive in our culture that its evidence is difficult to miss. Go into any restaurant or grocery store and just try to swing a tofu wrap without hitting some foodlike product adorned with label "only 100 calories per serving," "33% fewer calories," or of course the holy grail of thermo-dieting, "zero calories." In New York City, some chain restaurants are now required by law to display the total calorie content of the items on their menus. Over the past several years, our water cooler chatter has shifted from the amiable enumeration of a wide receiver's merits to a pissing contest to determine who has the lowest calorie low calorie Lean Cuisine® frozen brick for lunch.
To express even a shade of doubt, to suggest that this common wisdom is in any way flawed or just oversimplified, is to absolve the obese of their sins of willpower and invite admonishment from an army of pseudo-scientists and experts on nutrition and health. In some circles such an utterance is tantamount to questioning the moon landing or the spherical shape of terra firma herself.
I respectfully submit that these individuals' physics grades be retroactively changed to Fs for ignoring half of the course material. It turns out there is more than one law of thermodynamics1, and that Carnot fellow's heat cycle was not his most important contribution to science. He formalized the equally inviolable second law, and its most relevant interpretation for our discussion here essentially says that no chemical reaction is perfectly efficient - some energy is always "lost" in the process.
How is that relevant to a discussion about calories, metabolism, and fat? Well, even if our bodies were perfect theoretical Carnot engines (which they're not, as we'll discuss in later parts of this series), they would still have to obey all the laws of physics (the first and second laws of thermodynamics included). And the chain of reactions our bodies use to convert dietary fat into usable energy is a lot longer than the chain used to convert carbohydrates into energy. In fact, the chain for fats is a superset2 of the one used for carbohydrates. In other words, to metabolize a calorie of dietary fat, our bodies must do some additional steps beyond those required to metabolize a calorie of carbohydrate.
So already we encounter a problem with the idea that a calorie is a calorie, period, end of story. Namely, our bodies can absorb more energy from one calorie of dietary carbohydrate than one calorie of dietary fat, because less of it is wasted in the reactions along the metabolic pathway.
Lamentably, my explanation of metabolic pathways is itself oversimplified and incomplete. But the materials are out there for anyone with an open mind willing to do a bit of research. In particular, the Nutrition Journal has one much more detailed explanation, by someone far more qualified than myself3. And for a very detailed treatment of the subject, as well as a good simluation of taking the red pill4, check out Good Calories, Bad Calories by Gary Taubes5.
Still, the small matter of the second law of thermodynamics isn't the only, or even the strongest, evidence against the calories in minus calories out hypothesis. What about the experimental evidence - what does it actually show when we put this theory to the test? While they're re-enrolling in physics, why do these nutritionists also need to sign up for a probability course? And just what the heck do hormones have to do with anything not related to baby making?! All this, and much more, to come.
So until next time, whenever you hear the faint but unmistakable cry of "remember Carnot!" undulating softly through the warm night air, well... chances are it's me.
- Feinman, Richard, and Eugene Fine. ""A calorie is a calorie" violates the second law of thermodynamics." Nutrition Journal 3.1 (2004): 9.
- Taubes, Gary. Good Calories, Bad Calories. New York: Knopf, 2007.