Why do we Gain Weight – the Myth of “Calories in, Calories out”

People often assume that the answer to the question why do we gain weight is obvious and it is because people eat more calories than they burn; the old “calorie in, calorie out” paradigm. That is, calories not used in some form of exercise are converted to fat and stored. If we assume that the cause of weight gain is that simple, then the solution must be equally simple; eat less and exercise more, right? But is it?

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People often assume that the answer to the question why do we gain weight is obvious and it is because people eat more calories than they burn; the old “calorie in, calorie out” paradigm.  That is, calories not used in some form of exercise are converted to fat and stored.  If we assume that the cause of weight gain is that simple, then the solution must be equally simple; eat less and exercise more, right? But is it?

Many Physicians and Dietitians continue to hold to this “calorie in, calorie out” model and to counsel their patients that in order to lose the stored fat, they need to eat less and exercise more.  If it is really that simple, then why are 4.7 million adults in Canada classified as obese and more than 40% of men and 27% of women classified as overweight? If it is that simple, why do obesity statistics continuing to rise? Because it isn’t that simple.

There is an underlying assumption that “calories in” and “calories out” are two independent events.  That is, if you reduce “calories in”, “calories out” is unaffected. The difficulty lies in that the body decides where it is going to “spend” the calories taken in. Let’s say we take in 2000 calories; some will be used for the energy we need during a 24-hour period by our body during resting conditions (Basal Energy Expenditure) and some will be used to synthesize muscle, bone or other proteins.  “Calories out” is not just exercise.

Another underlying assumption is that all calories are created equal. But are they? First what is a calorie?

A calorie is the amount of heat that is released when certain foods are burned in a laboratory.  It doesn’t matter if the food is protein, fat or carbohydrate, if they have 100 calories then 100 calories of heat is released when they are burned. So in the “calorie in, calorie out” model it really doesn’t matter what we eat whether it is broccoli or butter, in the end it only matters how many calories they add up to.  Period.

If we start with the assumption that “calories in” are independent of “calories out” then the only thing that really matters is how many calories we take in, not from what sources. In this model, since all food is burned and a certain amount of calories are released, then we arrive at the conclusion that weight gain is simply a matter of comparing what goes in (energy) to what goes out (energy expenditure).  In this simplistic view, weight gain is caused by having too many calories (energy) compared to too little exercise (energy expenditure).  But it is not that simple.

Some of the energy expenditure will be for building tissue, staying warm, cognitive function and our bodies determine whether “calories in” go to those involuntary functions over voluntary exercise Looking at weight loss in terms of the “calorie in, calorie out” model fails to take into account that the body will slow its metabolism in response to reduced calories, because it needs to use those calories for vital functions so restricting calories doesn’t necessarily translate to weight loss.

Furthermore, assuming that all foods can be boiled down to how many calories they contain fails to take into consideration that the composition of different types of foods actually increases or decreases hunger and thus eating when and what and how much we eat. The assumption by many health care professionals has been that obese people are overweight because their metabolism has slowed and that keeps them from burning off the calories they take in.

To explain this in terms of the “calories in calories out” model, say a slim person takes in 2000 calories and has a Total Energy Expenditure (TEE) [the amount of calories they burn per day] of 2000 calories, which means they don’t gain or lose weight. An obese person will take in the same 2000 calories, but assuming their TEE is lower, say 1500 calories, 500 calories are store as fat and they gain a pound. But is that in fact so?

A recent study however has disproven this.

When measured in the laboratory, obese people had a Total Energy Expenditure (TEE) of 3244 calories compared to lean people who had a TEE of 2404 calories. That is, when excess calories are eaten in someone who is already obese, the body will actually increase its Basal Energy Expenditure (BEE) to try and get rid of those calories.

So why are obese people obese?

Shouldn’t this increase in Total Energy Expenditure over time caused them to burn off those calories and become lean again? Actually, body weight, like other functions in our body is a closely regulated system and we have so something called a “Set Point” which acts to regulate it. When too many calories are taken in, the body tries to get rid of them and when too few calories are taken in, the body tries to conserve them. The body does this to maintain its ‘set point’. So if we are overweight, the body will adjust its processes to maintain that set point. More on that below. It is not about how many calories we take in but what changes the Set Point.

People also make the assumption that how much we eat (“calories in”) is voluntary; that is we can choose to eat or not eat, but there are a number of hormones such as leptin, ghrelin, cholecystokinin, and peptin YY that tell our body when we are hungry and we are not. Hunger and satiety (feeling full) are under hormonal control and as such, when we eat (“calories in”) is not voluntary.

People also assume that ‘Calories Out’ is voluntary; that we control how much exercise we do and assuming that our basal energy expenditure is stable and unchanging over time, we ignore it.  But it is not. This mistaken belief that the only variable that changes is the energy expended in voluntary exercise and that this consumes a major proportion of our calories leads to the conclusion that “diet” and “exercise” are equal partners in weight management and they aren’t.

The fact is, most of our Total Energy Expenditure is used for generation of body heat and other metabolic processes (called Basal Energy Expenditure). Furthermore, Basal Energy Expenditure is not stable and can increase or decrease by as much as 50%. This up-regulation and down-regulation of our body processes contributes way more to weight loss or gain than exercise does. But that is not what we’ve been told.  We have believed that if we just exercise more and eat less we will lose weight. Let’s look at this a little more closely.

What happens to our body if we suddenly restrict caloric intake? According to “calories in calories out” model, a reduction in calories will result in Total Energy Expenditure (TEE) using fat for energy and the person would lose weight.  Sounds great except that is not what happens.

In fact, Total Energy Expenditure drops substantially – by as much as 30-50%.  People complain of being unable to stay warm even with plenty of clothing and that is because calories are spared in heating the body. Heart rate and blood pressure drop to conserve energy (calories).  People even find it difficult to concentrate because the brain is very metabolically active and restricting calories suddenly turns that down. Calories are needed to move, so in sudden calorie restriction people feel weak during physical activity. In other words, metabolism slows.

Why does the body do this? It’s survival.

Consider a person normally eating 3000 calories a day suddenly starts eating 2000 calories a day.  If they were to continue to burn 3000 calories daily, they would soon deplete all their fat stores, then their protein stores and then they would die. The body tightly regulates body weight and compensates for this sudden decrease in calories by saving calories from its Total Energy Expenditure.  Instead of burning fat in storage, the body reduces its caloric expenditure on body functions to 2000 calories a day and restores balance.

The “calorie in calorie out” model does not factor in that basal energy expenditure is not stable.  It ignores that restricting calories results in down-regulation in Total Energy Expenditure. That is, “calories in” and “calories out” are not independent.

The “calorie in calorie out” model of weight gain also ignores that hunger, eating and fat storage are regulated by numerous hormones. Leptin (a hormone correlated to the amount of body fat) is one such mechanism, adiponectin (a hormone supressed in obese people) may be another mechanism and there are others being researched.  It is also believed that cortisol, the stress hormone may play a role.  But there is one well-known hormone that plays a very significant role in hunger, eating behavior and fat management and that is insulin. Insulin’s effect will be covered in detail in future blogs. A little ‘teaser’; we as health care practitioners have been focussing on blood glucose while overlooking insulin, which regulates it.

So in summary,

  1. “calories in” and “calories out” are not independent, but one affects the other.
  2. “Calories in” is not only under voluntary control (what and how much we choose to eat) but several hormone play a significant role in terms of hunger and fat storage.
  3. “Calories out” is not only controlled voluntarily through exercise but also involuntarily by up-regulating and down-regulating basal metabolic expenditure (tissue synthesis, heat generation, etc).
  4. Fat storage is not simply a result of having more “calories in” than “calories out” burned as exercise.

So what causes us to gain weight? This will be the topic of future blogs.


DeLany J P, Kelley D E, Hames K C et al, High energy expenditure masks low physical activity in obesity, International Journal of Obesity 37, 1006-1011 (July 2013)

Fung, Jason, Intensive Dietary Management, The Aeteology of Obesity, August 2013

Health Canada, Overweight and Obese Adults (2102), http://www.statcan.gc.ca/pub/82-625-x/2013001/article/11840-eng.htm


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