The problem with CICO is that it ignores the roles that hormones play in calorie consumption and fat burning.

By ignoring this CICO concept is not only oversimplifies what is happening in your body, but it is extremely misleading.

When you consume foods you get an insulin response. Insulin response is part of an elaborate chain of reactions that controls how your body regulates the usage of glucose and storage of fat.

Basically:

Your body will metabolize carbohydrates and fructose/sucrose sugars immediately into glucose. It will try to use this glucose as much as possible for fuel for your cells.

However too much glucose can be dangerous. So once your glucose levels in your bloodstream reach a certain level it will trigger your liver to start storing glucose in the liver.

Once the liver is near capacity then it will trigger the glucose to be turned into stored fat.

During that entire process YOU CANNOT BURN STORED FAT.

THAT is the part that CICO ignores.

One of the things that insulin does, along with associated processes and hormones, is inhibit the usage of stored fat for energy. The goal is to regulate glucose in the blood stream and store excess glucose as fat. That can't work if your body is busy consuming fat as energy.

SOo...

If you eat in a continuous manner and keep your insulin levels high then it makes it physically impossible for your body to "burn fat".

In order to lose weight using CICO you need to use up all the calories you consumed, wait long enough for the effects of insulin to wear off, and then start burning through your fat reserves.

It seems like this transition period takes times.

This is what bicyclists often see during races and such things. They load up on carbohydrates for the high energy they provide, but when glucose levels get low they "Bonk Out". That is they lose significant energy.

Where as if your glucose levels were never high to begin with then that can avoid that phenomena.

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One of the major problems with Americans diet is the high concentrated of sugars and complete lack of fiber.

When people consume natural fruits and vegetables high in carbs and sugars then that is mixed in with a lot of fibers. Long chain carbohydrates locked together and difficult to digest.

Which means that in order to get all the sugars out of the mass of the food it needs to be carried into intestines and broken down with the assistance of your gut biome.

Which means that high carb/high sugar foods in their natural state do not release all their calories instantly. It take several hours of digestion to extract everything.

Where as modern diets are full of extremely concentrated forms of carbs and sugars and no vegetable mass to go with it. Corn syrup drinks, concentrated juices, pasta, bread, candy, chips, etc.

These high energy foods are cheap to produce and provide a lot of "filler" in foods and are easily stored and consumed for snaking.

So as a result this style of diet produces the glucose spikes and the resulting "bonking out". Which is experienced by people as periods of acute tiredness, inability to concentrate, and lack of energy.

The easiest way to solve this is by having a quick high carb snack, which then restores the energy levels.

Which means that Americans, in order to maintain glucose levels, tend to eat 8 or 9 or 10 times a day (including snacks).

This not only promotes high calorie intake it also destroys the ability to consume stored fat as energy AND probably contributes heavily to problems like insulin insensitivity.

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This is were one of the major misconceptions of fasting comes from:

During WW2 the USA observed the effects of famine among Europeans. To examine the effect of starvation diets on people the USA Military ran a number of controlled experiments involving volunteer service men.

Some of the effects observed involved intense prolonged hunger, lethargy, etc. Lots of bad effects and negative impacts to organs, various nutritional deficiencies, among other things.

It was then assumed that fasting would result in the same effects as extended starvation diets. And as been treated that way by the medical community for decades.

The trouble is that starvation diets involve regular meals, just very very limited ones. Where as fasting is no food at all.

The way the body reacts to fasting is different.

Why this is happens is theorized to be a survival technique.

Imagine you are a hunter gatherer. It takes significant physical effort to gather food.

If when you cease to eat your body sort of "hunkers down" down to conserve energy then it would potentially lead to a sort of death spiral were less and less food means less and less energy and, thus, less and less ability to get more food. You gradually become weaker and slower as time goes on.

Where what typically happens is after an extended period of not eating your hunger ceases and your energy increases. The idea being that the increase in energy allows to you work harder to hunt/harvest foods.

This is probably why it is becoming more popular for athletes to combine exercise and fasting.

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And all of this, I think, heavily relates to why things like intermittent fasting, paleo diets, and keto diets are popular.

Now I don't believe that a diet of mostly steak and butter is particularly healthy, but I also don't think that a diet of cheap high-carb fiber-free "factory foods" is particularly healthy either.

I think it's likely that a high carb diet combined with moderate fats and proteins can probably be very healthy, provided those carbs come in a more "natural" format combined with significant amount of fiber.