Over the Christmas break I traveled to Louisiana to visit my family and to Georgia to visit my wife’s family. In both cases (especially in the Georgia case!) the overarching theme was the consumption of delicious homemade food in quantities which were somewhere between preposterous and superhuman. I’m not the only one with this experience. The very first TV commercial I saw after the stroke of midnight at the dawn of this year was for a weight loss program. So was the second commercial.

“Eh,” I used to scoff, “Calories in < calories out. Simple as that. It’s basic thermodynamics.” But after some reading and further reflection, I’ve decided that this is one of those abuses of thermodynamics that’s true in a trivial way, but potentially so misleading as to be effectively false. Here’s three thermodynamic systems that illustrate the point.

The Automobile:

People and cars run on chemistry. A molecule of fuel enters the system, carrying binding energy with it. The system snaps that molecule, combining it with oxygen, releasing its energy, and generating leftover molecules which are ejected from the system. In a car, gasoline is a hydrocarbon like octane (C₈H₁₈). Two molecules of octane are combined with 25 molecules of atmospheric oxygen (O₂) to form 16 carbon dioxide molecules (CO₂) and 18 water molecules (H₂O), and this process releases energy. In every case, energy and mass are conserved.

So to make a car weigh more, you put more gas in it than it burns. To make a car weigh less, you burn more than you put in. This is what the inexorable laws of thermodynamics say. And yet there’s no such thing as an anorexic or obese car. Ok, that’s obvious. What’s the point? It’s an illustration of the fact that while thermodynamics is true and followed to the millijoule, it’s not the only constraint to the weight of a car. There is a feedback mechanism in that the fuel you can add is limited by the fuel you have burned. Put in too much, and the full tank prevents you from adding more. Put in too little, and the empty tank prevents you from removing more.

Cars, not on a diet.

The Human Body Temperature:

Unless you’re ill, your body is sitting within a degree or two of 98F or 37C. You can do things to generate lots of extra thermal energy. You can run around, you can shiver, you (well, your hypothalamus) can fiddle with your basal metabolic rate as it does when you get a fever, you can behaviorally adapt by putting more clothes on or turning up the heater.

You can also do things to generate less thermal energy or dissipate thermal energy at a higher rate. You can lay down, you can sweat, you can relax your blood vessels near your skin surface. You can adapt behaviorally by turning up the AC, taking off clothes, and drinking cold water.

Just like the car, this energy exchange is rigidly governed by the laws of thermodynamics. The temperature of you body is absolutely and totally controlled by the balance between the thermal energy your body generates and the thermal energy it dissipates. And yet nobody literally counts calories in the sense of “Holy cow, my body temperature is drifting down. Better wiggle my extremities to generate some heat!” The reason we don’t have to do this, in spite of the fact that body temperature is thermodynamically constrained by literal thermal calories in vs. calories out, is because the body adjusts calories generated and calories dissipated so that they match.

The Human Body Water Percentage:

Remember how you drank an extra glass of water a day for a year and gained 180 pounds of water weight? Me neither, in spite of the fact that mass is obviously conserved. You know the drill – change in weight is equal to mass in minus mass out, but feedback keeps the in/out balance very precise. The amount you drink affects the amount you excrete. The amount you excrete affects the amount you drink.

Conclusions for human weight:

I’m not actually going to advocate a fad diet. I am going to say that the simplistic calories in vs calories out model could be true in the simplistic way people usually mean when they say it. But the reality could also be more complicated, because calories out could be related to calories in, and vice versa. It could be true that some foods make the maintenance of proper feedback more difficult than others. In my own anecdotal experience (which you should not take too seriously, because it is anecdotal), when I eat foods which generally fall under the category of “crap”, I can eat bazillions of calories without getting full and still be hungry an hour later. When I eat foods which generally fall under the category of “actual food”, those things don’t happen.

Probably not in the "actual food" category.

Your mileage may vary. But if it doesn’t, don’t worry that your experience is somehow thermodynamically impossible.