What is Metabolic Adaptation and How to Manage It

When most people start dieting they think its a fairly linear process. For example, they think that there is a magic calorie amount that will get them to lose weight and lean out.

Or there are magic foods that get them to burn fat.

But the body is more complex than that.

Your body isn’t a fan of large bodyweight changes at one time. Whether this is an increase or decrease in weight. It has mechanisms in place to fight this.

This is why you will see a lot of people lose a bunch of weight and then it comes back on fairly quickly.

Or maybe you are trying to gain weight and you gain weight to build muscle but then once you go back to normal eating your body goes back to where its comfortable.

As I have talked about in the past most people don’t have a problem losing weight, but its keeping that weight off. If you are interested in that topic read here.

Just like when you place the body under stress it will adapt to that, that’s why your first workout was tough, but now if you did that workout it wouldn’t do anything.

Our bodies are very adaptive.

And when it comes to weight change or if you eat in a different way than what you are used to, your body will adapt to this.

What is metabolic adaptation?


“The process by which the body alters how efficient it is at turning the food you eat into energy.”

This makes sense if you think about it from an evolutionary standpoint.

Back in the day food was scarce and you didn’t know when you would get your next meal.

Therefore your body had to be good at adapting to the amount of energy it was given for survival purposes.

It was useful back then, but not so useful in today’s world. Our bodies are the same though.

Before we go on it’s important we go over energy balance and its components.


The energy in part is all of the food you eat on a regular basis, even those small handfuls of chips you eat periodically throughout the day still count as calories, unfortunately.

Some foods have more calories than others.

Energy out (burning calories) is a bit more complicated and is usually broken up into four categories.

They are:

Resting metabolic rate (RMR): is the energy burned for basic bodily functions. One way to think of it is the amount of energy used when you are at rest. This makes up a large portion of the energy out side of the equation, about 60-75% (going to vary from person to person).

Thermic Effect of Food (TEF): is the amount of energy burned via absorption and digestion. Yes, the calories you consume require calories to digest and absorb. This makes up about 8-10% of the energy expenditure side.

Exercise Activity (EA): this is the activity that is deliberate exercise: running, weight lifting, sports, etc. This factor can have a high variability from one person to the next. For example, a sedentary person could burn 100 calories per day, whereas an athlete could burn 3,000 calories per day from activity.

Non-exercise activity thermogenesis (NEAT): this is all of your movements that are not deliberate. Talking, blinking, fidgeting, walking to do everyday chores like going to the bathroom, cleaning etc. In a previous article, I went over how NEAT goes down as you lose weight and how to increase NEAT levels.

These four factors make up the energy expenditure side of the equation. There can be a huge variability between two people (which is why there is no magic calorie amount to lose or gain weight).

Summary: Energy balance is energy in (calories consumed) – energy out (RMR, NEAT, EA, TEF).

If you want to lose weight we need energy out to be greater than energy in.
If you want to gain weight we need energy out to be less than energy in.
If you want to maintain weight then you need energy out to match energy in.

A few more things:

  • Energy balance varies from person to person
  • Energy balance in itself is fairly easy to understand, where it gets complicated is how the energy out portion can vary from person to person. Here are some ways it changes from person to person.

Things that affect the energy out portion:

  • Body size- those who are bigger, tend to burn more calories. On the other hand, smaller people burn fewer calories
  • Age- The older you get the fewer calories you burn. Compared to younger people, who burn more calories.
  • Genetics- Some people just burn more calories than others because of their genetics.
  • Gender- Men tend to burn more calories than women. Most likely due to men being bigger on average.
  • More lean mass- Those who have more lean mass (think muscle), will burn more calories than someone who has less lean mass.

For example, an active tall male that has a good amount of muscle and genetically has an active metabolism uses much more energy throughout the day than a sedentary female, who is short, does not have much muscle, and genetically has a metabolism that is much more efficient with its energy. This is why some people can eat 3000 calories per day and still lose weight, while someone could be struggling to lose weight on 1,200 calories.

What is the best way to find out if you are in a calorie deficit, surplus, or maintenance?

Tracking your scale weight. If it’s going down over time, then you are in a deficit. If it’s not moving much, you are in maintenance, if it’s going up then you are in a calorie surplus.

If you aren’t sure how to track your fat loss progress, read this article.

Metabolic Adaptation 101

As if energy balance isn’t confusing enough, there are some other things at play when you start to eat less OR more.

When you start to reduce your calorie intake here are some things that will happen:

  • Less food being eaten = less calories burned from TEF. This may not be a huge decrease but since you are eating less food you simply arent burning as many calories digesting/absorbing food.
  • If you are in a calorie deficit you will lose weight. The smaller you are, the less energy your body burns overall. This is your RMR that decreases. Smaller body=less calories being burned.
  • The less calories (energy) you bring in, the more your body tries to conserve as much energy as possible (NEAT decreases). Therefore, less calories are being burned. Side note: Read this blog on how NEAT decreases when dieting.

**2022 Update Hormonal Adaptations to Fat Loss

I wanted to update this blog and go over some hormonal adaptions that also happen when you are in a calorie deficit.

These will also play a role in why its tougher to diet after a long period of time.

-Thyroid: This will decrease as you restrict your calories. Your thyroid is a regulator of energy. What this means is that you can expect a downregulation in your metabolic rate and it will affect other hormones when your calories are lower.

A slower thyroid is also going to show up as low energy, poor digestion, lower hormone production. And it will also impact your sex hormones.

The good news is that this can go back up once you increase your calories.

-Cortisol: This increases the longer you diet and the more you restrict. This is considered your stress hormone.

Chronic high cortisol can lead to inflammation, loss of lean body mass, increases insulin resistance, and can negatively impact sleep.

As you can see this will indirectly affect many other things, take sleep for example.

-Leptin: This will decrease with fat loss and a decrease in calories. This is a satiety hormone. It tells your body its full and doesn’t need to eat more.

When leptin is low you can expect a lower energy expenditure, an increase in hunger, and it can impact other hormones.

-Ghrelin: This will increase with fat loss and a decrease in calories. This is considered the hunger hormone. Its job is to get you to eat more to keep you alive.

This will show up as an increase in hunger.

-Testosterone: This can decrease with a calorie deficit and the leaner you get. However, if you arent super lean (sub 10% males, sub 15-17% females) then you wont need to worry about leanness, just the magnitude of the deficit.

Low testosterone can increase the risk of muscle loss, it can also show up as low energy, poor mood, and low libido.

-Estradiol and Progesterone: Your ratios of these hormones could go out of whack. This is going to impact reproductive health, mood, thyroid, and bone health.

Its important to note that not all of these may happen. But its important to be aware of them.

Overview of Metabolic Adaptation

All of these (hormones and metabolic adaptation) combined means that what was once an energy deficit for you, may not be an energy deficit weeks into your diet.

For example, when you first start dieting, 1,800 calories maybe your energy deficit, but once you lose weight you’re smaller, you’re eating less food, and your body is conserving more energy, 1,600 calories could be your new deficit.

This could be why if you were losing weight with a certain calorie amount and then all of a sudden weight loss slowed down. At this point, you will either need to decrease calories more or increase physical activity.

This is a good reason why you cant just be in a fat/weight loss phase forever. Eventually, you will have to do so much activity and eat so little food that it just isn’t worth it.

So does that mean your metabolism is broken for good? Nope.

Now lets look at the opposite.

Say you are trying to gain weight, here are some things that will happen:

  • More food being eaten = more calories burned from TEF. This may not be a large increase but since you are eating more food your body uses more energy to absorb and digest.
  • If you are in a calorie surplus, you will gain weight. The larger you are, the more energy your body burns overall. This is the RMR portion that increases. Larger body = more calories burned for every movement.
  • The more calories you bring in, the more energy you will have. Therefore we will usually see an increase in the amount of calories burned from NEAT.

All of these combined means that what was once an energy surplus, may just be maintenance now. This is why when someone is in a building phase and they were gaining weight at 2,500 calories but then eventually their weight gain plateaued. And they now maybe need to increase their calories to 2,700 or decrease activity levels to gain weight moving forward.

Genetic Component

What’s even worse, is that some people have more of adaptive metabolism than others.

For example, one person’s body may fight weight loss hard, meaning they have a very adaptive metabolism.

Another person’s body may not fight it so much, meaning they have a less adaptive metabolism.

Let’s say your body fights weight change, then you will likely have to make more adjustments during your gaining or cutting phases.

Some people have to go lower calorie than others to lose weight, some people have to go higher calories than others to gain weight.

This is why tracking is so important for not only weight loss, but weight gain as well.. (Read here on how to track).

**2022 Update How to Mitigate Metabolic Adaptation

Ok so great, you know that your metabolism is going to adapt to what you do to it. How do you actively work with this rather than against it?

With online clients, this is where we will use strategic nutrition periodization.

  1. Weekly or Monthly Periodization

With this option, you can do things like diet breaks, refeed days, calorie cycling etc.

If you need more information on these topics read HERE.

2. Long term Periodization

This is where we will break things up into larger phases. For example, a fat loss phase followed by a reverse diet into a maintenance phase.

Some examples:

-Fat loss phases: Most fat loss phases last 8-12 weeks. But having planned times of a deficit can help with metabolic adaptation.

-Reverse diet. Here you slowly bring your calories back up to maintenance.

-Maintenance phases. Here you focus on maintaining weight loss or gain. Get your body used to staying the same rather than changing.

-Building phases. After a fat loss phase, you may want to do a building phase to increase your metabolic rate and increase lean body mass which can make fat loss easier in the future.


People talk about a slow metabolism and try to avoid it at all costs, but if your metabolism isn’t slowing down then you probably aren’t losing weight.

But as you can see, just because it slows down during weight loss doesn’t mean it won’t come back up eventually.

However, since it will be slower after weight loss, this is where having a professional help you get back to normal is valuable.

Your metabolism is based on your:

-Current food intake
-Current body size
-Current movement

Your metabolism isn’t broken. It’s just adapted.

This is a fairly complex topic so if you have any questions please reach out.

If you have struggled with this and need a coach to guide you, that’s exactly what we do with our 1:1 online coaching.

You can read more about it HERE.

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