A fairly popular mantra on the subject of weight loss is that “it’s just calories in vs. calories out”. There’s a sense in which this is obviously true but uninteresting and also a sense in which it is obviously false but uninteresting. I’ll explain, then get to why this is so tiring.
Calories-in-vs-calories-out is obviously true in the sense that through mere chemical reactions matter is neither created nor destroyed, so if a person’s body is going to be made up of less matter, the matter no longer a part of them had to go somewhere. Since the human body is highly efficient, calories are a reasonable approximation of this. That is, since calories are precious to the human body it will not merely throw them away uselessly. Thus if one wants to get rid of fat matter without using a scalpel to cut it away, all of the options involve convincing the body to burn the calories for what it perceives as a useful purpose. This is quite true. It is uninteresting because it merely describes what goes on during fat loss. It says nothing whatever about how to convince one’s body that burning fat for energy is a good idea. That’s the thing we actually want to do and the part that’s not so easy to figure out how to do while still being a functioning member of society. (Observationally, the people who say that this is easy are unmarried and/or have no children.)
Calories-in-vs-calories-out is obviously false in the sense that this formula does not in fact compute fat loss. All it tells you is a uni-dimensional constraint on a complex system. It’s quite possible for your body to burn protein (harvested from muscles) in order to make up a caloric deficit while conserving fat. In fact, the body somewhat prefers to do this because it’s a much better strategy in a famine than conserving muscle and spending all of its fat. Plus, in extremes like starving to death, one will die before there is zero fat in the body (even excluding the brain). Very little fat, but more than zero. It’s entirely theoretically possible that if the body were to become disregulated, the body could think it’s got no fat to burn while it’s got tons. You can induce this with certain kinds of brain lesions in rodents, where they swell up to astonishing obesity on tiny amounts of food and you can starve them to death without them getting meaningfully thinner. Most of us, fortunately, are not in the unhappy position of lesion-induced-fatmice, but the point is that calories-in-vs-calories-out will only tell you that in a calorie deficit you will burn something. If your house is warmer in winter than the outdoors, we know you’re burning something. The problem is, it might be your furniture, or worse, your floor joists. This is uninteresting because you don’t even have a good way of measuring calories in and you barely have a terrible way of measuring calories out. Measuring food works to like +/- 10%. The sort of calorie deficits that people try to achieve are often in the 15-20% range. The other end is even worse, though. The only really reliable way of knowing how many calories you burn is to have a device which measures your CO2 output—this can be a thing strapped to your head or an airtight room. After that, guesses about how many calories you are burning might be accurate to +/-50%. (This is exascerbated by the fact that the body will, in a calorie deficit, down-regulate your metabolism.) This is why the advice from non-idiots for achieving a calorie deficit is to keep reducing food until you start losing weight, at which point you know you’re in a calorie deficit (assuming you’re not merely losing water weight).
The reason that all of this is so tiring is that it’s all an exercise in missing the point. It is true that there are laws which govern the human body, but we operate within all of them all the time. Merely picking one and ignoring the rest is not being insightful. Sometimes it’s not even getting that limited relationship correct.
To give an example, suppose you came across a strength coach who told you that f=ma. (That is, force is equal to mass times acceleration.) Thus if you want to get stronger, that is, to produce more force, you need to increase your acceleration. The more you accelerate the same mass, the stronger you will get. It’s basic science!
The problem is that this is treating an instantaneous relationship as if it was a causal relationship. f=ma is describing what happens in the moment when force is being applied to mass. It’s not training advice for the long term. It says nothing about what stimulates muscles to be able to apply more force.
In like manner, calories in vs calories out only describes what happens in the moment. The second law of thermodynamics only tells you the momentary relationship between various things. It’s not dieting advice.
If you want to know what to eat, you need to consider how the body behaves in response to various stimuli. Bodies do not all behave in the same way, otherwise there would be no such thing as diabetics. Moreover, you also need to consider how much exercise the body is getting; exercise induces all sorts of changes in the body such as increasing hunger, basal metabolism, insulin sensitivity, and many, many other things. All of this means that a person who has too much fat on their body and wants to get rid of some of it will have to do a lot of work to find out what works for them. A person with a particular dysregulation in their body will have an enormously difficult time losing fat until they figure out how to fix this dysregulation. For someone who dysregulates because of insulin problems, they will need to solve this differently than someone who is dysregulating because of thyroid problems.
To give an example, about 6 years ago I lost close to 40 pounds eating an extremely low-carb diet, eating when I was hungry until I was full. This diet clearly fixed a disregulation in my metabolism because after about a week on it I simply became a lot less hungry. I didn’t eat as much, or as often, and felt full and had plenty of energy. I wasn’t stressed, I just felt like everything was fine with less food. (I subsequently gained much of the weight back through some poor choices involving candy; I have discovered that I will significantly dis-regulate if I eat a lot of fructose. As long as I limit candy, cookies, etc. to Christmas and Easter, my weight is very stable while eating when I’m hungry until I’m full. I plan to go back to eating strictly low-carb to see if that will get rid of the weight again, but I need to get some things in my life in order before I do because of dealing with antagonistic family members.)