The debate around "A calorie is a calorie" relates to the whole "calories in vs calories out" concept for weight loss/gain. Examples like "500 calories of twin

The debate around "A calorie is a calorie" relates to the whole "calories in vs calories out" concept for weight loss/gain. Examples like "500 calories of twinkies and 500 calories of lettuce are equivalent" focus heavily on the "calories in", but forget to consider the "calories out" aspect. What I mean there is, when someone says that the body processes different macronutriets differently, a lot of that has to do with how it affects "calories out" If you eat 500 calories of a food that your body will only process a small amount of, and expel the other 450 calories as waste, that 450 is part of the "calories out". That's only a net "calories in" of 50. On the other hand, if you eat 500 calories of a food that your body chooses to process/store most of, that's 400 calories in, and only 100 calories out. To clarify, I'm not talking as much about absorption, more-so about things like how a macronutrient affects insulin sensitivity. Carbohydrates decrease insulin sensitivity more than protein and unsaturated fats would. Decreased insulin sensitivity means increased insulin resistance. "Having insulin resistance means that the walls of the cells within the body that need to take in glucose for energy, have a much-lowered sensitivity to insulin. This makes it harder for the cells to receive the necessary glucose through the cell wall. That glucose then remains in the bloodstream, resulting in high blood sugar levels, which the liver then has to manage. When this excess sugar reaches the liver, it is converted to fat. The fat cells leave the liver via the bloodstream and cause weight gain. If left unmanaged this can lead to obesity." The calories out part of the equation isn't just the energy you expend per day through movement/metabolic processes/etc, it's also the calories that are expelled as waste because the body didn't store that macronutrient.