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.
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.
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.
Your mileage may vary. But if it doesn't, don't worry that your experience is somehow thermodynamically impossible.
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Digestion is a complicated process, individually different, and depending on many factors (including core temperature, biorythm, and genetic predisposition). Which is another way of saying that, unlike cars, humans do take fuel in and never use it. Generically, you get better at using "fuel" the more often your body gets it (production of enzymes depends on demand to some extent). My guess is that many weird diets work temporarily because the body needs time to adjust to "new stuff". Eg, the average person who eats an apple every other day and who skips to a fruit-only diet will probably have diarrhea for a few days, want to bet they lose weight?
Where you live is also a significant factor: in colder climates you have to burn more calories to maintain body temperature. If you are in Fairbanks and are frequently outdoors (especially this time of year), you can eat at holiday levels year round without gaining weight. Try that in Miami or Houston (or, for that matter, working a desk job in Fairbanks), and you'll get fat in short order. People who work in the field in Antarctica (I know several people who have been there) frequently eat 7000 calories or more per day just to maintain body weight, compared with the 2500 that is considered adequate for an adult male in a milder climate.
The problem surely is this. You have a statement of physics something like
Change in energy = (energy in) - (work done in exercise) - (energy metabolised and radiated) - (energy excreted)
but then one makes the unwarranted leap of concluding that this is a statement of causality - ie that the left side of the equation is caused by the right hand side. But you could, with equal invalidity, rearrange things so that the (energy metabolised) term is apparently caused by the difference between consumption and energy-gain, or vice versa. To really understand the chain of cause and effect you have to know how things like metabolism, appetite and lethargy/energy are affected by consumption and exercise.
(It's worth remembering that at one time it was widely believed that exercise was a cause of overweight because it stimulates the appetite. )
Speaking of thermodynamics, creationists often tell me that evolution violates the 2nd (or whatever number) law of thermodynamics, because it violates entropy. My response is that entropy only applies to a closed system like the universe. We get energy from the Sun. I'm curious does that make sense?