> “Calorie is a calorie” is thermodynamically correct, but not true when it comes to diet. [insert alcohol analogy here] In case this doesn't quite compute for

> “Calorie is a calorie” is thermodynamically correct, but not true when it comes to diet. [insert alcohol analogy here] In case this doesn't quite compute for some, I have a very simple thought experiment to apply to the CICO fallacy. If you fill your gasoline car with diesel, will the engine perform correctly? Of course we would not expect the engine to perform as well (or at all) on diesel. It doesn't matter if diesel contains more than enough energy per volume for the engine to run. It doesn't matter if the engine runs with some diesel mixed in with gasoline because it will not run as well and suffer other problems that can damage the engine. The car simply is not optimized to run on a fuel it wasn't designed for. If you run a human body solely on sugar, it will eventually crash and burn, figuratively speaking. The body can run on some exogenous sugar, but it's likely not optimal; in fact, it's not even indicated for when protein and fat are available. Likewise, the human body will fail solely on protein or solely fat, and it may suffer problems when eating fat and sugar simultaneously. However, it operates quite well on some mixture of protein and fat, and can tolerate a relatively small amount of carbs without much consequence. This is because the human body was designed by ~4 million years of evolution to move away from fruit-eating to hunting (and the occasional eating of tubers when game is scarce). It is hardly more "omnivorous" than a cow is, given that cows will eat meat (and may other things) when you put it in front of them. Most large mammals can tolerate eating a wide variety of things, but that doesn't mean their species didn't develop to specialize in a specific diet. CICO also encounters an issue when it comes to using exercise as a means of maintaining or losing body fat. When you engage in exercise, your glycogen stores are prioritized over fat because it can be oxidized more rapidly. Although you are always relying on some amount of fat and carbs, neither being absolutely exclusive, exercise prioritizes carb metabolism. What happens when someone relies heavily on exercise for weight loss is that they may lose some weight because they've expended a lot of glycogen from their muscles and liver, but that doesn't really equate to losing an appreciable amount of body fat, which is what most people are interested in when they want to lose weight. It's not that exercise doesn't work, but it's not as efficient as people realize – that is unless you exercise hard and long enough that your body is forced to start relying much more heavily on fat oxidation.