I think the notion of "calories in vs. calories out" is ridiculous.
Foods affect our bodies in different ways and go through different metabolic pathways.
Not only that, but the foods we eat can directly affect the hormones that regulate when and how much we eat.
Therefore, the types of foods we base our diet around are just as important as the amount of calories we are eating.
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.
According to the "calories in, calories out" (CICO) way of thinking, obesity is simply a matter of eating too many calories.
Proponents of this often say that the types of foods you eat aren't very important, that the caloric contribution of foods is the key.
They say that the only way to lose weight is to eat less, move more and that it is any individual's responsibility to keep calories balanced.
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.
Bottom Line: Proponents of the "Calories in, Calories out" way of thinking say that the only thing that matters when it comes to weight loss is calories, disregarding completely the metabolic and hormonal impact of foods.
How much energy we eat and how much energy we expend matters. It is of utmost importance.
The first law of thermodynamics tells us that energy can not be destroyed, it can only change form. So if the energy that is entering the body is greater than the energy leaving the body, then the body will store the energy, usually as body fat.
If we take in more energy (calories) than we expend, we gain weight. If we expend more energy than we take in, we lose weight. This is an unbreakable law of physics and isn't even debatable.
However... this fact, tells us nothing about WHY this is happening.
A person who has gained weight is the same as a person who eats more than they burn.
Let me explain this with a simple analogy...
Imagine that the entrance hall of a movie theater is full of people. These people are all there because they're going to watch a very popular movie that just came out.
If you would ask... "why is this entrance hall full of people?" and someone were to answer with "because more people are entering it than leaving it" - then you'd think it was a fairly ridiculous answer, right?
It tells you nothing about the cause of the entrance hall being full, it is simply stating the obvious.
Saying that weight gain is caused by excess calories is just as ridiculous as saying that the entrance hall is so crowded because more people are entering than leaving.
The next logical question to ask would be... why are people eating more?
Is it a consequence of a series of logical decisions to eat a bit more and exercise a bit less, or is there something in our physiology that is causing it... such as hormones?
If it's behavior that is causing the increased calorie intake (weight gain) then what is driving the behavior?
The fact is that all our thoughts, desires and actions are controlled by hormones and neural circuits.
Saying that it's "greed" or "laziness" that is causing the increased calorie intake totally disregards the complex physiological processes that control our behavior and how the foods we eat can directly affect these processes.
Bottom Line: Saying that weight gain is caused by excess calories is true, but meaningless. It tells you nothing about the actual cause.
In my opinion, one of the biggest fallacies in nutrition is to think that all calories are created equal.
Different macronutrients (proteins, fats and carbohydrates) go through different metabolic pathways.
Let me show you this with two examples... fructose and protein.
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.
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.
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.
Bottom Line: Different foods go through different metabolic pathways. Some foods can cause hormone changes that encourage weight gain, while other foods can increase satiety and boost the metabolic rate.
Changing your macronutrients can affect your appetite in a dramatic way.
The best example of this is seen in studies comparing low-carb and low-fat diets.
Whereas people on low-fat diets must be calorie restricted in order to lose weight, people eating low-carb (and high fat and protein) can usually eat until they feel satisfied and still lose weight.
In these studies, the researchers need to actively restrict calories in the low-fat groups to make the results comparable, but the low-carb dieters still lose more weight (10).
In this study, the low-fat group is calorie restricted while the low-carb group is eating until fullness (11):
The low-carb dieters automatically start eating less calories, because their appetite goes down.
These studies show that there is no need to consciously focus on calories in order to eat less of them. This can happen automatically, simply by changing the types of foods you eat.
Bottom Line: Being aware of your calorie intake is NOT necessary to lose weight, as long as you eat in a certain way. Cutting carbs while increasing fat and protein is proven to lead to automatic calorie restriction and weight loss.
Another thing to keep in mind that long-term dieting will reduce your metabolic rate.
If you were to cut calorie intake by 10%, it would only work for some time until your metabolic rate would adapt and you would stop losing. Then you would have to cut calories again, then again...
The body tries desperately to maintain its fat mass. This is called the body fat setpoint and is regulated by the hypothalamus. If you don't change your diet, only the amount of foods you eat, then your set point won't change.
If your weight goes below your set point, your brain responds by decreasing your calorie expenditure (calories out) and increasing your calorie intake (calories in).
Bottom Line: The body tries to resist changes in body fat levels by increasing hunger and reducing calorie expenditure.
Most people believe that the increased calorie intake is driving the weight gain.
But what if we've got things backwards and the fat gain drives the increased calorie intake?
When a teenage boy grows rapidly in height, he eats more calories than he expends. Instead of turning into fat, the calories are used to build muscle, bones, skin and organs.
It is not the increased calorie intake that is driving the growth, but hormones, growth factors and physiological processes that are causing the growth and the growth drives the increased calorie intake. That makes sense, right?
What if obesity is similar? What if calories are a consequences of the weight gain, not a cause?
In the same way that a teenage boy's muscles and bones grow because of hormones, an obese person's fat mass may be growing because of hormones.
One example of this is drugs like some antidepressants and birth control pills, which often have weight gain as a side effect.
There are no calories in these pills, but they alter the physiology of the body (brain and hormones) to cause weight gain. In this case, the increased calorie intake is secondary to the change in hormones.
Bottom Line: It is possible that we are confusing cause and effect. Perhaps it's not the increased calorie intake that drives the fat gain, but the fat gain that drives the increased calorie intake.
Humans aren't robots.
We don't walk around and make decisions about our behavior based on mathematical calculations. It is against our nature.
We make decisions based on our emotions, how we feel and what we want to do. The "logical" part of our brain often doesn't have much control over the part of our brain that is regulated by emotions.
Some might call this weakness, I call it human nature. Changing behavior based on logical, rational decisions can often be impossible.
Ever made a decision not to drink coffee after 2pm? Always do homework right after school? Only sleep in on Sundays?
Making these kinds of changes in your life is often very difficult and the same applies to eating behavior like making the decision to eat 500 calories below your maintenance every day.
Even though some highly motivated individuals are able to control their food intake completely (like athletes and bodybuilders), this really isn't representative of the general population.
This is very difficult for most people and especially for people who have a tendency to gain weight.
Let me use breathing as an example of how it is difficult to "control" a physiological function that is regulated by the brain.
Breathing is almost completely subconscious, although you can control your breathing for a short amount of time if you manage to focus on it.
If you made the decision to skip 1 in 10 breaths, then you could probably do it... but only for a few minutes. Then you'd get distracted and start doing something else.
This is only possible while you're consciously focusing on it... and even if you did, you might unwillingly compensate by breathing a little heavier in the other 9 breaths, or you'd start to feel uncomfortable and stop doing it altogether.
If you think this is a ridiculous example and not applicable to eating, then you're wrong. Eating is controlled with the same types of homeostatic mechanisms.
Some people may be able to consciously eat less calories and manage it with portion control and / or calorie counting. But they have to stick with it for life.
Bottom Line: Eating behavior is largely subconscious, controlled by hormones and neural circuits. It can be downright impossible to control these sorts of behaviors in the long term.
One of the key problems with the "calories in, calories out" way of thinking is that it doesn't account for other health effects of foods.
The fact is that different foods can have varying effects on our health.
Another example is fructose. When consumed in large amounts (from added sugars, not fruit), it can lead to insulin resistance, elevated cholesterol and triglycerides and increased abdominal obesity (14).
There are many examples of foods having harmful effects that have little to do with their caloric content.
Also, being at a healthy weight does NOT guarantee that you are healthy, in the same way that being obese does not necessarily mean that you are unhealthy.
Even though these metabolic problems are more common among obese individuals, many obese individuals are metabolically healthy and many lean people have the metabolic syndrome and can succumb to heart disease and type II diabetes (15).
Optimal nutrition and disease prevention go way beyond just calories.
Saying that weight (or health for that matter) is simply a function of "calories in, calories out" is completely wrong.
It is a drastic oversimplification that doesn't account for the complex metabolic pathways that different foods go through, or the effects that foods have on our brain and hormones.