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human
CMV: To lose weight, eating less calories than you use always works

I'm not an overweight guy, but I have recently decided to lose 6 kg of weight to get myself from the upper 'heatlhy' BMI range to the mid range number (I'm tall). Now, first I decided to check what I was eating and how much (which was a lot). Then I started exercising more (I have a tracker which estimates calorie burn) and making sure that I eat less than what I burn. I also switched some sugars for more protein but overall I still eat fries, pizza, etc. It's working for now (only been 9 days and lost .8 kg).  Since I started this and some people made note of why I was checking what I was eating and noting it down, some people said that it didn't matter what they tried, they couldn't lose weight by 'moving more and eating less'. I don't buy those arguments, and am of the opinion that they might be fooling themselves (maybe unconsciously) by forgetting some foods/drinks or overestimating their exercise. 

Now, there are arguments stating that a calorie isn't always a calorie. And there is science in support of that. However, most of that science is saying that some macronutrients use more energy to be digested (like protein uses up to 30% of its calorific value for its own digestion, while fat only uses up to 5%). This does not invalidate the Calorie In Calorie Out (CICO) principle, but in fact support its even more. Let's compare to (imaginary) diets, one consisting solely of protein, other consisting solely of fat. As long as both remain below the calorie burnt, both diets would cause someone to lose weight. However, the protein diet is expected to lose much more weight. Let's put some numbers to it. I'll use myself for reference. On average, I burn 3500 kcal/day, so intake will be 3000 kcal in this imaginary example for a weight loss of approx .5kg/week. Eating only protein, this intake of 3000kcal would result in an 'effective' intake of 3000 x ( 1 - 0.3) = 2100 kcal, while the fat would be 3000 x ( 1 - 0.05) = 2850 kcal. So both would still be, obviously, below the 3000kcal intake. 

Of course, there are other effects at play too. Such as sugar levels, hormones, etc, causing someone to feel hungry and wanting to eat more than they should. These are separate, but connected, issues. Similarly, someone that fidgets and bounces might burn more than they realise. Metabolism might vary slightly, or someone with more muscle mass might use more calories to maintain them. All connected to weight gain/loss and eating habits. 

However, that does not take away that CICO still holds. 

You can change my view by: 

* Showing that it is possible to gain weight while taking in less calories than being used, or the opposite: losing weight while taking in more 'effective' (correcting for the calories used to digest the food) calories.
* Or perhaps giving a good reason as to why certain people promote the idea that CICO doesn't hold (that isn't an excuse as for why they can't)
gpt
You posted the basic principle of weight gain / loss, it is not debatable that it works like that. It is the truth. Now on effectiveness of just using this model, a lot can be said. You asked why people promote models that don't include CICO. The model doesn't necessarily have to include it, as long as it results in net calorie therefore weight loss. As long as the model does that, it can focus all it wants on other factors eg positive behaviour reinforcement, stressing the psychological benefits of healthy eating etc. You are approaching the problem from a mechanism based point of view, whereas you may also approach it from an evidence based point of view. That is how modern medicine works really, ideally you both understand the mechanism and know your therapy is effective. However, if we don't know exactly why something works but is proven to work, we do usually implement it.