As the argument goes, the Law of Thermodynamics proves that if you burn more calories than you consume you will lose weight. I blogged about this in a general, non-science-y way here. Today I’ll give you the exciting science to prove my point. Stick with me though, I promise to make this interesting.

That’s the argument that I hear most often when I tell people that I eat well and workout a lot and am still fat. They call me a liar because the science behind the law of thermodynamics, they say, is simple, complete and irrefutable.

First things first, there isn’t a “Law of Thermodynamics”. There are four of them (the first one is actually called the zeroth law which you gotta love). Since these people aren’t concerning themselves with the entropy of a crystal at absolute zero, I’m going to assume they are referring to the first law. Actually I assume that they are phonetically parroting something they heard from someone else and that they wouldn’t know one of the laws of thermodynamics if it bit them in the ass but I digress.

The first law states that in a thermodynamic process the increment in the internal energy of a system is equal to the increment ofheat supplied to the system, minus the increment of work done by the system on its surroundings. It is often simplified to “energy can neither be created nor destroyed”.

I do not disagree with the law. But, note the first four words “In a thermodynamic process..” What I disagree with the gross misrepresentation of the human body as a perfect thermodynamic process. I have four issues with this:

Issue 1: It assumes that there is no option for calories other than to be burned or stored

Carbohydrates work in basically that manner. Fats and proteins don’t because they have other things to do in the body. Protein does everything from building muscle cells to repairing cell membranes. Fat transports non-water-soluble nutrients around, insulates neurons, and can also be used to repair cell membranes. Fat and protein can also be broken down and recombined into whatever the body needs. Eat a steak and your body might use it to make a cell membrane. That delicious alfredo sauce might be used to produce insulate neurons in your brain. I don’t know why I remember this, but my physics teacher told us that fat and protein are to the body what wood is to us. You can burn it for fuel, but you can also build a house with it or make it into a piece of paper. At any rate, the calories in the proteins and fats used in this manner are neither burned nor stored. So we have our first hole in the calories in/calories out equation.

Issue 2: It asserts that Basal Metabolic Rate (BMR) is Easily Predictable and Stable

Your BMR is the number of calories that you would need to stay alive for 24 hours if you spent the day still and in bed – just what would be required for breathing, heart rate, etc.

It is typically calculated using the Harris-Benedict equation. James Harris was a botanist who freely admitted that his equation failed to take into account large amounts of muscle mass or the additional calories provided by excess body fat and that it was thus much more effective for individuals at, or very close to, an ideal body weight.

It is well documented that BMR responds to decreased caloric intake by slowing down. So if two friends have the exact same Harris-Benedict score, but one has been dieting; then the dieter will actually burn less calories at rest than his friend. Said another way, if these two friends eat the same number of calories in excess of their BMR score, the dieter will gain more weight than the non-dieter. Again, in this case calories in/calories out just doesn’t hold up.

Issue 3: It requires that the body be a perfectly efficient calorie burning machine

Not so much. The Second Law of Thermodynamics explains about entropy and how systems are never 100% efficient. [Edit: As a commenter pointed out entropy only applies in a closed system which the human body is not for the reasons mentioned below. What I meant to say and didn’t was that the first law also applies to a closed system and as such people who think that the first law applies to the human body would have a difficult time weaseling out of saying that the second law applies.] Regardless, the human body is even more complicated because it’s ability to burn/store/use nutrients affected by things like genetics, environment, hormones, previous caloric restriction and subsequent reduction in BMR, hydration level, environment, chemicals etc.

Issue 4: If we wanted long term weight loss using this theory then we’d have to eventually turn to starvation

If we wanted to use calories in/calories out effectively, knowing what we now know about entropy and how decrease in caloric intake causes decreased basal metabolic rate, we would just have to keep decreasing our calories and increasing our exercise until eventually we would be following disordered eating patterns. It would be the only way to stay under our ever plummeting BMR and eat less than we burn. Perhaps this is why 95% of intentional weight loss efforts fail. The body releases weight at first but then the damn science catches up to us and not matter how hard the people who’ve attended a physics amateur hour try to yell to the contrary, they will never be able to out-science the actual laws of thermodynamics and the complicated human body which is, I swear to god, not a lawnmower.

Bonus Issue: Your friend who eats everything in sight and never gains weight.

Everyone knows somebody like this. I have a number of friends who eat way more than I do, exercise less and stay rail thin. Why do these people get to credit their metabolism but I’m just fat lazy excuse-making slob if I suggest that my metabolism may be as slow as theirs is fast?

So next time somebody tells you that it’s just calories in/calories out, consider telling them that you’ll be happy to talk to them about it once they have recited the laws of thermodynamics, defined the Harris-Benedict Equation and discussed its specific limitations, and explained your friend who eats a ton and doesn’t gain weight.