The Calorie Con
Often you hear people say that weight gain or loss is completely determined by the amount of calories you ingest versus the amount of calories you burn. “Calories in” minus “calories out” equals your weight gain. The second law of thermodynamics is said to prove this. Well, let me disprove this for you.
Climb up a high ladder with a very precise scale and weigh yourself. Lower yourself, and weigh yourself again. Now you’ve gained weight. As you’re get closer to earth, the earth exerts a stronger gravitational force on you, and your weight will increase. In an airplane at altitude, that effect amounts to about 0.3% compared to sea level.
Of course when people say weight, they really mean mass. But one should be precise when invoking the name of the Second Law! Anyway, I’ll refute that too, and I’ll refute it by eating a cookie and taking a piss. “Calories in” > “calories out”, and yet I’ve lost mass.
So let me rephrase that rule in a way that does not clearly violate the Second Law of Thermodynamics: The difference in calories in and calories out is equal to the difference in total caloric content in one’s body. (In this definition, if a shark takes a bite out of you, that counts as calories out. And if your body switches energy sources from potatoes to nuclear fusion, the rule will need some additional tweaking.)
Now we have a correct formulation. Unfortunately, it is quite useless. Unless you’re in a boot camp, you have very little control over calories out, and by “you” I mean the conscious you that makes New Year’s resolutions. Usually, the rest of “you” is in charge. Let’s steal the term “id” from Freud and misuse it here. You may decide to go for a run, but your id determines how that feels. It can make you feel absolutely lethargic if it feels that you should conserve energy. Your id can make you slow down to a crawl, and feel thoroughly miserable. Of course, when you spot a gang of thugs coming toward you, it can take away that lethargic feeling just as easily. A nasty bootcamp instructor can have the same effect. But lacking those, your id has enough avenues to sabotage any exercise plans you might have.
Even at home, your id can make you save energy and decrease “calories out”, for instance by setting your body temperature quite low. You’ll feel miserable and cold, and very much tempted to set the thermostat higher, wear a warm pull-over indoors, or even get under a blanket, all of which will significantly reduces your “calories out”.
Another thing your id can do is to burn muscle tissue. Muscles continually use energy, and having less muscle will mean a lower energy bill for your body. Nothing in the Second Law of Thermodynamics prevents your body from turning some of your heart muscle into fuel, as bad of a plan as that may be.
Your id has plenty influence on the other side of the equation too. You may decide to only eat a light salad today, but your id can inject continuous thoughts about chocolate, pizza, and hamburgers into your mind. You can try to distract yourself, but your id can make that pretty difficult. It can make you feel as if you are literally starving, even though you’ve got enough bodyfat to last a year. Admittedly, it can’t make you walk to the cupboard and grab a bite. But when you’re over with friends and there are nachos on the table, it can play the “grab nachos” action when you’re distracted by conversation.
While you’re deep in thought, or in conversation, any snacks that are within reach are unsafe. So is a fridge stocked with snacks or a cupboard with cookies. While you’re on guard, your willpower is generally strong enough to withstand temptation, but you can’t be on guard all the time — one has to live as well.
In short, “calories in” isn’t something mostly under your control either. And while human bodies certainly obey the laws of thermodynamics, that knowledge is not very useful to understand how people can lose weight. A fructose molecule and a glucose molecule do not get identical treatment by our bodies; biochemistry can tell you what those difference are. Insulin plays an hugely important role in how fat is stored and retrieved from fat cells, and endocrinology is the study of hormones like insulin. Finally, studying psychology will help you understand which behavior is easy to change, and which hard.
Obesity is a hard problem. Solving it will require a lot of research in these and other areas, but this is where solutions will eventually be found. The solutions will not be found in a formula from physics, and trying to do so does not advance human knowledge; rather the opposite.