Back to Basics: All About Calories and Their Relation to Body Weight

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When it comes to weight gain or weight loss, people usually think about calories in simple terms: calories in and calories out. In other words – if you eat more calories than you spend, you gain weight; if you spend more than you eat, you lose weight. And if you eat just as many calories as you spend, your weight remains the same. Simple as that.

And they are completely right. This is basic thermodynamics that no one can argue with. But this picture, while true, is very simplified version of complex relationship between calorie intake and body weight changes over time. However, this simplified version is the basis for almost all weight loss diets. Almost all of them are based on a simple principle: eat less calories than you spend – that is, create a caloric deficit, until you reach your desired weight, and then you’re done! Clap, clap, success!

Yet 95% of all people who successfully lose weight, eventually re-gain it. The reason: the aforementioned simplistic philosophy works only in short term, because it ignores many important aspects of the relationship between calorie intake and body weight changes.

So let’s start finally exploring these aspects. This will completely change how you think of calories. So here are 5 basic principles/insights on how calories are related to body weight.

1. Bigger, heavier bodies need more energy

This is simple physics. It takes more fuel to move a truck, than it takes to move a car. An elephant eats more than a mouse. When your body is heavier, fatter, you need more energy (that is, calories) for everything. You spend more energy when you walk, more energy when you work, more when you do nothing… you spend more energy even for basic metabolism… even when you’re sleeping and your body just maintains its basic functioning, you spend more energy when your body weight is increased. If you look at two people of the same age, sex and height, having the same level of daily activity, and one being 30 pounds heavier than the other – the heavier person will spend significantly more calories while doing exactly the same things as the leaner person.

When it comes to weight changes over time, the important consequence of this is:

a) when you lose weight your calorie needs will also decrease… You won’t be able to maintain your new weight if you go back to your old eating habits, because your old calorie intake level is no longer appropriate for your new, leaner body… it’s now excessive.

b) when you gain weight, you need more energy to maintain that weight… This is why people, in spite of eating loads of food are not constantly getting fatter. Their body eventually becomes fat enough, so that they indeed need all these calories, just to maintain that heavy weight.

2. Calorie expenditure tends to match calorie intake

People say “calories in and calories out”, but they often forget a very important thing: “calories out” tend to match “calories in” in the long term. In other words, when you change your calorie intake, your calorie spending will also change in that same direction, until it becomes equal to your calorie intake. How does your body accomplish it? Simple – your body weight changes.

When you eat more than you need, you’re in a caloric surplus. Your body weight increases. Since heavier bodies need more energy, your spending of calories also increases, until it becomes equal to your caloric intake. When you reach a weight point at which your energy spending matches your energy intake, you reach a caloric equilibrium, you’re not in surplus anymore and you stop gaining weight.

The same thing happens when you eat less than you need. Being in deficit, you lose weight, which lowers your energy spending as well, until it matches your energy intake – at which point you reach an equilibrium and stop losing any more weight, in spite of your caloric intake remaining at that lower level.

Since calorie spending tends to match calorie intake, most people are either at or very near their caloric equilibrium. This is a bit counter-intuitive: if you ask people, they will usually tell you that obese people are obese because they eat more calories than they spend – however this is false. As long as they haven’t made any big changes to their caloric intake recently, most people spend almost exactly the same amount of calories that they consume. This includes obese people, thin people, normal weight people… everyone… So why are then, obese people obese? Well, because they eat at a high calorie intake equilibrium level. It is true that they spend pretty much all the calories that they consume, so yes, they are in equilibrium, but they consume a lot of calories, and they also spend a lot of calories, because at such high body weight, their caloric needs are elevated.

So a simple answer to “why obese people are obese” is even simpler than what most people think: it’s not because they eat more than they spend… it’s simply because they eat a lot. And they also spend all these calories, because their heavy bodies indeed need all of them to keep functioning at that weight. They are in equilibrium with both high intake and high spending.

This explains our everyday observations: we notice that most people have stable weight. Yes, there are some slow changes over very long periods, but most people are around their standard weight, regardless of what weight category they belong to. Obese tend to stay as obese as they always were, lean people tend to stay lean, etc… Almost everyone is at or very near their equilibrium. And even if people over 2 decades gain a lot of weight, during all these years they were still very near their equilibrium… 2 decades is a long time, and even a very slow increase in average caloric intake over that period can shift equilibrium to progressively higher weight levels.

3. The adjustment of your body to a new caloric intake is an asymptotic process

If you abruptly change your average caloric intake level, your body weight will quickly react to that change, but as time passes you will be gaining or losing weight ever slower. This is because your body weight will move in a direction where your caloric needs are approaching your new caloric intake level. The closer your body weight is to that new equilibrium, the smaller is your daily surplus or deficit, and the slower body weight change. This explains “the last few pounds” phenomenon where people go on a diet and successfully lose a lot of weight, but they seem to be unable to lose just “these last few pounds”. This is because their body weight is already at such low level, where they caloric needs are almost as low as their intake… so the deficit they were creating with their diet is now so small to be almost negligible.

4. Your long term average weight is mainly determined by your long term average calorie intake and not by any short term “surplus” or “deficit” that you “create” using diets.

Any short term surplus or deficit that you create becomes insignificant over the long term. What really matters is at what body weight level does your usual caloric equilibrium reside.

If you go on a classical diet that aims to produce weight loss by means of creating a caloric deficit, by default you will re-gain all your weight eventually. This is because classical diets work like this:

  1. you start at high caloric intake and high body weight level
  2. the diet puts you on a very low caloric intake level, so that you are in deficit. You quickly lose weight and successfully reach your target weight
  3. Since classical diets usually don’t take into consideration the fact that caloric needs depend on your body weight and that your weight over long term is determined by your average calorie intake over long term, and not by any short term deficits and surpluses, they now falsely make you believe that the problem is solved: you reached your target weight and now you can “eat normally, just pay attention not to overeat”
  4. Indeed you return to “eating normally”, that is, to your old eating habits. You’ll pay a bit more attention here and there, but basically, you returned to eating as you used to. But since your old eating habits are based on a much higher caloric intake level, that were appropriate for you when your body was bigger and when you really needed all these calories, now by simply “eating normally” you are in a surplus, because now your body is smaller and you don’t need all these calories, so now you’re re-gaining all the weight that you lost, until you reach your old, high weight level equilibrium again and you’re back where you started.

What a fantastic exercise in futility! This is in a nutshell how yo-yo effect works. Now, classical diets are not only futile, they are also harmful for a couple of reasons:

  1. drastic weight changes are a shock for your body – unless you’re sure you’ll be able to maintain your weight, any dieting will just give you unneeded stress, and will ultimately fail as you regain it all
  2. Very low calorie diets put you in a starvation mode, teaching your body to save energy, and to swiftly store any surplus calories… Your metabolism slows down, and when you return back to your “normal” way of eating, you’ll probably eventually reach even higher equilibrium weight than what you started with.
  3. They mess with you psychologically, and when they ultimately fail you lose confidence in yourself and you start believing that you’re supposed to be fat, and that you’ll never ever be able to reach your healthy weight.

5. To achieve permanent weight loss (given the same activity level) you need to permanently lower your average caloric intake. This is the ONLY way you can permanently lose weight and keep it at that level.

As we have seen what sort of havoc classical diets wreck on us, both physically and psychologically, it’s time to look for a different approach to weight loss.

Luckily such an approach exists and it is much simpler than any of the classical diets. All you need to do is to make a slight, but permanent change in your calorie intake level. Instead of complicating it with multiple steps and phases, all should be done in just one simple step. Such an approach is called Asymptotic nutrition, and all it asks you to do is to permanently lower your calorie intake level, but in a slight and sustainable way. (Alternative way to achieve exactly the same thing is to permanently increase your activity level, with the same food intake. However, it’s easier said then done, and while I encourage exercise, relying only on it for weight loss could be difficult in practice.)

This change in your caloric intake will cause you to asymptotically approach (hence the name) your new healthy weight equilibrium:

Here’s an example:

All the other details are up to you and are not required. However, of course – if you’re making a permanent shift to a healthier ways of eating and you decided to permanently lower your caloric intake, it would be highly recommended that you also learn basics of healthy nutrition, to make sure that you’re not only shifting to a healthier caloric intake level, but also to a healthier ways of eating in general.

Some of these changes will also make it much easier for you to stay at lower caloric intake, because when you’re eating healthier, more nutritious food, you will be more satisfied and you will have less cravings, even at lower caloric intake levels.

Take home message:

If you’re not willing to permanently lower your calorie intake level, don’t even start thinking about any kind of weight loss nutritional regimen. Any sort of temporary solution (aka “classical diets”) will only result in you eventually regaining all of your weight once you return to your old eating habits, and meanwhile you’ll be messing with your health and psychology as well.

You can also lose weight with the same amount of calories you always consumed, but in that case you need to permanently increase your activity levels, without compensating it with additional food. While this is possible, I could argue that lowering your food intake, slightly, but permanently is a safer bet then betting just on exercise. Ideally both things could be combined.


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