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No, you can't get fat by eating too little, but here's why it's easy to get that impression.

Can you gain weight by Not Eating Enough?

No, you can't get fat by eating too little, but here's why it's easy to get that impression.

You monitor very carefully the calories you eat and your physical exercises, and yet you don't see the changes you want. Has your metabolism slowed down? Is it really possible to gain weight because you eat too little? Here's what's really happening and how to get over it. 

How is it possible to eat so little and still gain weight?
Have you ever felt this? As a personal trainer you hear this remark quite often. And this state creates frustration, confusion and even anger. Despite the fact that you do everything you can, including eating less - sometimes much less - you still don't lose weight. Sometimes it is even possible to gain weight. 

A short search on the internet and you will find a lot of explanations for this phenomenon. Some say that the laws of thermodynamics cannot be broken, and probably the cause is faulty calorie counting (you eat more than you think). Others say that the body goes into "starvation mode" and all kinds of strange hormonal and metabolic problems appear. 

Can you really gain weight because you eat not enough? Let's find out!

The truth: the laws of thermodynamics cannot be broken!
Maybe you've heard of the laws of thermodynamics (you learned them in physics), and maybe you know that they also apply in fitness. In terms familiar to fitness, the laws of thermodynamics refer to fattening and losing weight, or calories eaten versus calories burned. Let's see what it actually means. 

Thermodynamics is a way of expressing the way in which energy is used and exchanged. Simply put, we bring energy into the body in the form of food and burn energy through activities such as:

Basic metabolic functions (breathing, blood circulation, brain activity, etc.)
Movement (daily commutes, physical exercises, etc.)
Heat production (thermogenesis)
Digestion and excretion. 
Energy balance determines body weight! This is a pure truth! If you absorb more energy than you consume, you gain weight. If you absorb less energy than you consume, you lose weight. It is a certain fact, tested and retested. Practically, it is a scientific certainty! 

Yes, there are many factors that influence both sides of this seemingly simple equation, which can easily put you in a fog : basal metabolic rate, physical exercises, non-sporting physical activity, the thermal effect of food (these are factors that burn energy); appetite, living environment, metabolizable energy, the influence of the food consumed on the reward system in the brain, stress, mentality, etc. (these are the factors that influence energy intake).

People do not defy the laws of thermodynamics
No matter how much we try to avoid them, we must obey the laws of thermodynamics. But what about unexplained changes in weight? When you ate a bigger dinner than usual and woke up the next day lighter? Or when you feel like you're doing everything by the book and you still don't lose weight? 

Even if you have the impression that you somehow evaded the laws of thermodynamics, in fact, you didn't!

Even that guru who tells you that you don't lose weight because of carbohydrates and insulin (or other hormones) that turn your energy equation upside down, is not right. Hormones can influence the proportions between lean and fat mass that you gain or burn, but they still do not cancel the energy balance equation. 

But as I said in the title, it's easy to think you're gaining weight even though you don't eat much. 

Measuring metabolism is difficult
Metabolic needs and responses are not so easy to measure. It is possible to APPROXIMATE the basal metabolic rate - the energy cost to be alive. But the measurements are only as accurate as the tools used to make them.

For measuring metabolism, the best tools are hermetically sealed metabolic chambers, but it's kind of hard to get access to something like that, isn't it?

We can estimate our metabolism through formulas or through fitness bracelets or through applications based on food labels and measurements; these estimates can have deviations of 20-30% in young and healthy people. The deviations are probably greater in the older population.

If we could measure with maximum accuracy how much we eat and how much we burn, we would really know if we are eating too little for the daily caloric requirement.

Even if we could know this outside the laboratory (which is not possible), it still wouldn't help us much. Energy burns are dynamic, that is, a modified variable leads to changes in all other variables. In order to have an accurate measurement, we should monitor the burning minute by minute throughout the day.

So we are left with estimates, and estimates are usually not very precise.

"Eating too little" is subjective
What does eating too little mean to you?

Less than you usually do?
Less than you were told to eat?
Less than you feel you should?
Less than you need to be healthy?
Less than the estimated basal metabolic rate?
Less than the exact basal metabolic rate?
And how often does this apply? You eat too little:

At backgammon?
One day?
Some days, but other days you eat too much?
Without clarifying these questions, you can see how difficult it is to realize if you are really eating too little, or if it just seems to you that you are eating little when in fact you are eating more than you would need to lose weight.

The problem is a perception
People are very bad at estimating the calories they eat. We tend to think that we eat less and burn more than we really do, sometimes with deviations of up to 50%. (interesting is that very thin person, who try to gain weight, have the opposite problem - they overestimate their caloric intake and underestimate how much they burn).

In the modern world, when the portions and plates are bigger than ever and the foods are very energy dense, it is all the more difficult to correctly estimate how much you eat.

When you start watching carefully what you eat, measuring everything exactly, you are shocked at how much you eat compared to what you thought.

Sometimes you can do everything right in most of the meals, but some calories will still enter the diet through small escapes. You can eat all week in an average caloric deficit of 500 calories per day, but on the weekend have two meals with a caloric surplus of 1500 calories and you've practically canceled the whole week.