The Thermodynamic Argument For Fad Diets

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:


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:



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:


Conclusions for human weight:


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.

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By Eric Lund (not verified) on 03 Jan 2013 #permalink


Change in energy = (energy in) - (work done in exercise) - (energy metabolised and radiated) - (energy excreted)


