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Every time I hear the 'basic physics' or 'first law of thermodynamics' argument in favor of CICO I can't help but remember the joke about 'spherical cows':

"Milk production at a dairy farm was low, so a farmer wrote to the local university to ask for help. A multidisciplinary team of professors was assembled, headed by a theoretical physicist, and two weeks of intensive on-site investigation took place. The scholars then returned to the university, notebooks crammed with data, where the task of writing the report was left to the team leader. Shortly thereafter the physicist returned to the farm, and advised the farmer, “I have the solution, but it only works in the case of spherical cows in a vacuum.”

Human beings aren't black boxes, we are living, breathing organisms, and as complex organisms we can process food differently depending on what it is. For example, cows eat grass, and can survive on a diet of 100% grass, so they must get some calories out of it. Otherwise they wouldn't be able to sustain the energy expenditure necessary to power a ~2000 pound beast, and would die early. Humans, on the other hand, cannot digest cellulose, so it will land in your stomach as indigestible fiber after which you will promptly poop it out. If you were to give a human an all grass diet, he/she would die within weeks.

In a similar vein, a calorie of carbs is not digested the same way as a calorie of protein, which is also not digested in the same way as a calorie of fat. For the studies that you mention, I'm certain that the macronutrient composition made very little difference when it comes to weight loss in the short term, but I am far more skeptical about whether a high carb diet is actually sustainable after your body has adjusted to a lower metabolic rate. Most studies of this type show a bounce back in weight after the initial weight loss. Furthermore, it's not and shouldn't be all about weight loss; if a calorie is just a calorie why don't you just eat 1500 calories of pure sucrose per day? Assuming you were able to stick to it, you'll quickly get fatty liver disease, diabetes, and a ton of visceral fat. The idea that the macronutrient composition is irrelevant as long as calorie restriction is maintained is too simplistic a view, and a dangerous one to spread.

I'm not saying eating 1500 calories of sucrose is not dangerous, but from an energy point of view yes it's the same as eating 1500 calories of cheese. At the end of the day if your energy expenditure is 2000cal your body will have to burn 500cal. Macronutrients in this case make a small but negligible difference.

It's anecdotic but as an hobbyist bodybuilder tracking calories everyday I can confirm that my weight follows exactly my caloric input (there can be some latency but it's water retention mostly), in line with the experience of a lot of people like in /r/bodybuilding or /r/ketogains. Playing with macronutrients and insulin is interesting for muscle gain or breakdown prevention and energy and hunger management but not really for pure weight loss.

I agree with your last paragraph, macronutrients are very important for health and body composition, and sure I would recommend a low carb diet for losing weight in the long term, in part because fat and proteins are more dense and keep hunger low

From an energy point of view, it's true that 1500 calories of sugar is the same as 1500 calories of cheese. It's also the same as 1500 calories of sawdust. In a lab, you can burn each of these samples under a bomb calorimeter and they'll all read the same thing. There's no debating the scientific definition of a calorie as a unit of energy. My argument is that it's extremely disingenous to just say a calorie is a calorie when it comes to a living organism, because the metabolic pathways for each different macronutrient are so different. It doesn't really matter how much energy a food has, if your body can't use the energy contained therein.

The moment that you accept that macronutrient composition is very important for health and body composition, you acknowledge that in biological system a calorie is not a calorie. If I want to lose weight, I don't want someone to tell me that CICO so I can just eat whatever as long as I stay under x calories, because as you rightfully pointed out some foods are more energy dense than others, cause fewer cravings, and don't spike your blood sugar. With CICO, I can lose weight but get fatty liver disease, or I can lose weight but lose muscle while increasing visceral fat, since fat weighs less than muscle. These are terrible outcomes. People want to lose weight to be healthier and look better; what's the point of losing the weight if your risk factors for metabolic syndrome go up and you still look fat in the mirror?

For the anecdotal evidence you provided, I think it's not as simple as how you've portrayed it, because your body can be in two energy burning modes, glucose burning or fat burning. If you eat more fat while in glucose burning mode (which is what most people are in), it will just get stored as fat and yes you will gain as much weight as the amount of calories you ate. In so called 'fat burning' mode, your body will increase its metabolism to burn off the excess fat. Likewise, if you're in fat burning mode and you eat more carbs, they'll just get stored as fat. This is also anecdotal evidence (sources: https://intensivedietarymanagement.com/who-needs-to-avoid-fa..., https://intensivedietarymanagement.com/smash-the-fat-calorie...), because unfortunately we don't really have good nutritional studies about this.

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