The Mythical Calorie Equation

Calories In – Calorie Out = Change in Body Fat

This ubiquitous equation represents the idea that the difference between the number of calories we consume and the number of calories we expend determines the amount of fat we gain or lose. It’s a wonderfully simple conception, although not an accurate one, and actually a very dangerous one.

It’s truly amazing to me that this equation has stuck around this long. The entire concept is illogical to begin with, not to mention the mounds of evidence against it.

Yet, the dogmatic adherence to this equation has informed so much of the current nutrition recommendations. This is especially true when it comes to fat loss, where “eat less and exercise more” is the common mantra. But it has also been applied to the general health recommendations to eat foods that are less calorie-dense, drink more water, and consume more protein and fiber to keep us full for longer, among many others.

While the fact that we associate calories with fat gain and loss is bad, the fact that we associate them with health is even worse.

But before we dive in too far, let’s start by exploring the extensive evidence that refutes the validity of equation.


Isocaloric Studies

If the calorie equation was valid, then consuming diets containing different foods with the same number of calories would have the same effects on body fat. But, many studies that have tested this hypothesis have shown that this isn’t the case. Let’s take a look at a few of them.

This first study, performed in a metabolic ward, had subjects on isocaloric reduced-fat or reduced-carbohydrate diets (1) (isocaloric means the diets had the same number of calories). They found that subjects on the reduced-fat diets lost more body fat than subjects on the reduced-carb diets. And, those on the reduced-fat diets lost less weight than those on the reduced-carb diets, meaning that a much higher percentage of the weight they lost was fat.

[This study also provides more evidence against the “burn fat for fat loss” fallacy, as those on the reduced-carb diets burned more fat than the reduced-fat group even though they lost less body fat.]

This next study also compared subjects on isocaloric diets with varying amounts of carbs and fat, although this one was performed on rats instead of humans (2). They found that the rats on the high-fat diets had higher total body fat by the end of the study than those on the control diet, but those on the high-carb diets didn’t. Again, remember that the high-fat and high-carb rats were fed the exact same number of calories!

In this study, human subjects were given 1 of 3 isocaloric diets that contained varying amounts of carbohydrates, and they found huge differences in weight loss, fat loss, and percent weight loss based on the amount of carbohydrates in the diet (3). In fact, those on the diet that was highest in carbohydrates lost 14 pounds more of body fat than those on the lowest carbohydrate diet during the 9-week intervention! In reference to these results, the authors commented that “no adequate explanation [could] be given for [the] weight loss differences.”

There are many more isocaloric studies that provide similar evidence against the calorie equation, including one on rats where two groups were fed the exact same diet except for the type of protein and ended up with a 26% difference in fat mass (4) and other studies showing that changing the amount of fat or carbohydrate in the diet affected the amount of body fat gained or lost, even when consuming the same number of calories (5, 6).

It’s clear based on these studies that the calorie equation isn’t quite as simple as it was made out to be. The types of foods that are eaten have a substantial effect on changes in body fat, rather than simply the number of “calories-in.”

Now, the argument can be made that changing the types of foods simply changes the number of “calories-out.” And, while this is true, it’s often entirely ignored in favor of the recommendations to simply “eat less and exercise more.”

Even when not ignored, the fact that the type of food, rather than simply the number of calories, affects the number of “calories-out” creates problems of its own.

It’s known that different diet compositions can vary in their metabolizable energy (the amount of energy in the food that’s actually digested and absorbed) and thermic effects (the amount of energy required for digesting and using the food). It’s also acknowledged that different diet compositions can cause changes in the resting metabolic rate and physical activity levels. But, some of the effects of different diet compositions can’t be fully explained yet.

This review, aimed at figuring out whether we can account for all the energetic effects of different diet compositions, looked at many calorie-controlled weight loss studies (7). They found that “neither macronutrient-specific differences in the availability of dietary energy nor changes in energy expenditure could explain [the] differences in weight loss” and concluded by suggesting that “further research is needed to identify the mechanisms that result in greater weight loss with one diet than with another.”

In other words, even if we account for differences in metabolizable energy, thermic effects, resting metabolic rate, and physical activity levels, (which very few people are doing) there are still unknown variables that would need to be factored in to make the calorie equation work.

This presents a glaring problem with this equation, but it doesn’t end here. There are other, more serious problems with the calorie equation that arise because it’s predicated on several false assumptions.


Caloric Restriction, Metabolic Adaptation, and Behavioral Compensation

The first of these assumptions is that our bodies’ energy usage doesn’t change based on our environment.

There have been many times where researchers have tried to apply the calorie equation to fat loss. They set up experiments where they reduce people’s food intakes and increase their exercise, thereby reducing the number of “calories-in” and increasing the number of “calories-out.”

And these studies do result in weight loss. But the amount of weight loss is nowhere near the amount predicted by the calorie equation.

This discrepancy boils down to two main factors:

1. Metabolic Adaptation

Metabolic adaptation is how our bodies internally adjust, or adapt, to changes in caloric intake (“calories-in”) or caloric expenditure (“calories-out”). When we reduce our caloric intake or increase our caloric expenditure, our bodies reduce the amount of energy they use. And when we increase our caloric intake or decrease our caloric expenditure, our bodies increase the amount of energy they use (8, 9, 10, 11).

This is accomplished through various hormonal and enzymatic changes, which allow for adjustments in energy usage, metabolic fuel selection, and energy partitioning.

These adaptive responses throw another wrench in the calorie equation. If we factor metabolic adaptation into the equation, we would have to reduce our caloric intake even more to achieve our desired weight loss.

2. Behavioral Compensation

Behavioral compensation is how we respond externally to changes in caloric intake or expenditure. This could mean reducing the amount we exercise when we eat fewer calories or increasing the amount we eat when we exercise more.

One review that considered many weight loss studies found that, because of behavioral compensation, the amount of weight loss from caloric restriction averages to be 12-44% less than what’s predicted by the calorie equation (12). They also found the amount of weight loss from increased exercise to be 55-64% less than was expected due to behavioral compensation.

Those are HUGE differences between the mythical calorie equation and reality!

And, behavioral compensation is an even bigger factor when we increase the amount of “calories-in.” The amount of weight gain in these situations is up to 96% less than is expected from the calorie equation! (12) You read that right, 96%! That basically means that we’re really good at compensating when we eat more food than normal, to the point that it’s pretty hard to gain weight that way.


But of course, metabolic adaptation and behavioral compensation don’t mean that the mythical equation is wrong. They’re just two more variables we have to factor into the equation that at this point is unrecognizable compared to the original “calories in – calories out = change in body fat.”

Up until this point we’ve only been dealing with adjustments to the calorie equation, but after considering this next false assumption it’ll become quite clear that this equation has no place in the health or fat loss arena.


What Are Calories?

This brings us to one of the most egregious errors that underlies the calorie equation: Calories are not relevant to human physiology.

Calories are simply a measure of energy, specifically heat energy.

In the context of nutrition, we think about the number of calories, or potential energy, held in certain foods. But, we can consider the number of calories in anything, whether it’s grass, wood, or gasoline.

Now, we all know that our bodies can’t convert the energy (calories) in grass, wood, or gasoline to usable energy in our bodies. So why do we assume that we’ll equally convert the energy in all food to usable energy in our bodies?

We consider that a mango has 200 calories, a steak has 600 calories, and a doughnut has 300 calories, but we don’t consider how much of that potential energy can or will be converted to usable energy.

There are many factors that affect how much of the energy, or calories, in the food we eat will become usable energy in our bodies.

The food must first be digested and absorbed. However, the amount of the food that actually gets digested and absorbed varies greatly based on the food and our gut function, and varying amounts of energy are required for this digestion and absorption.

Once absorbed, the carbohydrates, fats, and protein from the food we eat can be used to produce energy. This occurs in the mitochondria of our cells through a process called respiration.

During this process, the food we eat is converted into a form of usable energy that’s held in a molecule called ATP. But, there are tons of factors that affect whether the carbohydrates, fats, and protein even get to the mitochondria, let alone whether they’ll be used to produce ATP.

The different macronutrients (carbs, fats, and protein) vary significantly in this regard. Carbohydrates are our primary energy source while fat is our secondary energy source. Protein, on the other hand, is not likely to be converted to usable energy through mitochondrial respiration and is instead used primarily for building and rebuilding tissues.

So, it makes little sense to equate the potential energy, or calories, between the different macronutrients, as the likelihood that they’ll be used to produce ATP varies greatly. Equating the calories between the different macronutrients actually violates the second law of thermodynamics, as is explained here:

“A review of simple thermodynamic principles shows that weight change on isocaloric diets is not expected to be independent of path (metabolism of macronutrients) and indeed such a general principle would be a violation of the second law [of thermodynamics].” “The second law of thermodynamics says that variation of efficiency for different metabolic pathways is to be expected. Thus, ironically the dictum that a “calorie is a calorie” violates the second law of thermodynamics, as a matter of principle.” (13)

And, the factors that affect mitochondrial respiration go far beyond the differences between macronutrients. They also include the hormonal state, the amount of nutrients available, the toxins present, the time of day, the ambient temperature, the amount of sunlight present, the psychological state, and virtually every other aspect of our environment.

In other words, our bodies don’t run on a calorie currency, as they must convert the energy in food that we call “calories” into energy that they can use. And the amount of calories in our food is barely relevant considering the many factors that affect its conversion to usable energy.

The problems with the misplaced focus on calories has been nicely summed up in this quote:

“It is increasingly clear that the idea that “a calorie is a calorie” is misleading… Different diets… lead to different biochemical pathways (due to the hormonal and enzymatic changes) that are not equivalent when correctly compared through the laws of thermodynamics. Unless one measures heat and the biomolecules synthesized using ATP, it is inappropriate to assume that the only thing that counts in terms of food consumption and energy balance is the intake of dietary calories and weight storage.” (6)

And, as if all of this weren’t enough, the amount of usable energy that we do produce or use doesn’t say anything about where or what that energy is used for, which is vitally important to consider for both fat loss and health!


The Dangers of Caloric Restriction

The final false assumption of the calorie equation is that any excess energy we have will become fat, while any energy deficit will result in fat loss.

This assumption that fat gain is the product of excess energy and fat loss is the product of a lack of energy is probably the single most harmful misconception that exists in the health and nutrition field.

The 1st law of thermodynamics is often cited in support of this idea, which explains that energy cannot be created or destroyed. But, this has nothing to do with where energy is used or the physiological processes of fat gain and loss.

Energy is used for virtually everything we do. It’s used to accomplish our basic daily functions, like breathing and digesting, in addition to handling stressors, like exercise and mental work. Energy is also used to maintain, repair, and grow our tissues, like muscles, bones, fascia, and organs.

All the factors we’ve mentioned so far, including our hormonal state and the many aspects of our environment, determine the partitioning of energy, or where energy is used.

For example, in the rat study that was mentioned earlier, the amount of energy from consumed protein that was allocated to fat and muscle tissue differed based on the type of protein that was consumed (4).

However, the calorie equation doesn’t acknowledge the concept of energy partitioning and instead relies on the extremely common fallacy that any excess energy we have will be used for body fat and any energy deficit will come from body fat.

While this ignores a key physiological concept, there’s another problem with this fallacy that we haven’t addressed: body fat is a storage of fuel, or potential energy. In other words, the potential energy in food is stored as body fat when it’s NOT converted to usable energy.

While this is somewhat acknowledged, it’s assumed that energy production occurs until we have enough energy, at which point any potential energy left over becomes fat. But, that couldn’t be farther from the truth.

As was mentioned earlier, energy production, or mitochondrial respiration, is affected by all sorts of variables, from the amount of nutrients available to the time of day. When energy production is inhibited based on these many factors, the potential energy from our food is stored as fat and we’re left without enough energy to properly function.

This is what underlies fat gain and the obesity epidemic. Not too much energy, but too little energy (14).

As such, the recommendations to further reduce our available energy only make the problem worse, as is explained in this quote:

“In conclusion, the presented metabolic mechanism of environmentally caused obesity indicates energy deficiency as an engine working toward development of obesity. It is obvious that all efforts to stop this engine by further decreasing the energy cannot be successful. Thus, the low calorie diets and exercise regimens seem to be a redundant burden of already exhausted people.” (14)

Unfortunately, it doesn’t end there. Our bodies adapt to energy deficiencies by further inhibiting energy production and storing more fuel as fat in order to conserve energy, resulting in a vicious cycle (10, 14, 15).

This is the biggest reason why the calorie equation is destroying our health. We already lack energy due to the inhibition of mitochondrial respiration by nutrient deficiencies and many other problems, and then we eat less and exercise more, which further reduces the amount of energy we have available to properly function, encourages fat gain, and leads to the chronic diseases we see in epidemic proportions today.

We need to put an end to this terribly misguided approach to fat loss and health and instead shift our focus to the process of energy production. By increasing the amount of energy we produce, we can reduce the storage of food as body fat which will allow us to lose fat while also improving our health.


As I’m sure you’ve already gathered, there are quite a few factors affecting energy production. If you want to learn more about those factors and learn to lose fat the healthy way, sign up for the free mini-course below!


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