The “Calories in, Calories out” idea is such a drastic oversimplification that it is flat out wrong. Foods have complex effects on the brain and hormones.
What a Calorie is
I want to make sure that we understand each other, so let me quickly define what a “calorie” means.
A calorie is a measure of energy:
“1 calorie is the amount of energy required to increase the temperature of 1 gram of water by 1 degree Celsius.”
The official measure of energy is Joule. 1 calorie equals 4.184 joules.
What we usually refer to as “calories” is actually kilocalories (kcal).
One kilocalorie, or one dietary Calorie (with a capital “C”) is the energy required to heat 1 kilogram of water by 1 degree celsius.
One dietary Calorie (kilocalorie) is 4184 joules.
But what does “energy” mean?
“Energy is the capacity of a system to do work.”
The human body requires energy to move, breathe, think, contract the heart, maintain electrical gradients over cell membranes, etc.
On a molecular level, the body functions with an enormously complex set of chemical reactions. These chemical reactions require energy, which is where calories step in.
Bottom Line: A dietary Calorie is the amount of energy required to heat 1 kilogram of water by 1 degree Celsius. The body uses energy (calories) to drive chemical reactions.
A pound of fat is 3500 calories (a kilogram is 7700). If you eat 500 calories less than you burn every day, then after a week (7 * 500 = 3500) you will have lost a pound of fat.
From this comes “a calorie is a calorie” – the idea that all calories are created equal, no matter what foods they come from.
Even though it is true that obesity is caused by excess calories and weight loss caused by a calorie deficit, this is still such a drastic oversimplification that it is downright wrong.
The fact is that different foods can have vastly different effects on our bodies and go through different metabolic pathways before they’re turned into energy (1).
Just focusing on the calorie content of foods and disregarding the metabolic effects they have is a highly flawed way of thinking.
Fructose, when it enters the liver from the digestive tract, can be turned into glucose and stored as glycogen.
But if the liver is full of glycogen, it can be turned into fat… which is then shipped out or lodges in the liver.
Consumed in excess, it can cause insulin resistance, which raises insulin levels all over the body. Insulin drives fat gain (2, 3).
Fructose also doesn’t get registered in the same way as glucose and doesn’t impact satiety in the same way. Fructose doesn’t lower the hunger hormone ghrelin (4, 5).
So… a 100 calories of fructose may increase your insulin over the long term, lead to higher ghrelin levels and increased appetite.
Then you have a 100 calories of protein. About 30% of the calories in the protein will be spent on digesting it, because the metabolic pathway requires energy.
Protein may also increase levels of fullness and boost the metabolic rate (6, 7).
This increased protein may even be used to build muscles, which are metabolically active tissues that burn calories around the clock.
Clearly… a 100 calories of fructose will have completely different effects on the body than a 100 calories of quality protein. A calorie is NOT a calorie.
In this way, the fructose will drive increased energy intake compared to the protein, via its effects on hormones, body and brain.
Drinking a can of soda every day for 5 years will have a vastly different effect on the body and long-term energy balance, compared to eating the same calories from eggs.
Some people say that “any” food can be harmful in excess. Well… I disagree. Try eating broccoli in excess, or eggs. You will feel full very quickly and not want to take another bite.
Compare that to a food like ice cream, which is very easy to consume large amounts of.
Source: Why “Calories in, Calories Out” Doesn’t Tell The Whole Story