Can Eating Too Little Damage Your Metabolism?

There’s a lot of discussion in the fitness and health industry about whether crash dieting can cause metabolic damage. In this article I’d like to help separate fact from fiction and finish up by teaching you exactly why crash diets might be linked to struggling to maintain weight in your future.

Is this you? 

Despite working out consistently and intensely, plus eating carefully, you’re not losing weight (or not losing it as fast as you’d like or expect).

Or you were losing weight consistently… until recently. Now you’re stuck — even though you’re working as hard as ever.

Or when you were younger, you were super fit; but not anymore. Maybe you were an athlete. Maybe you did some crash diets. But now, even when you put in the same effort, you just can’t seem to get as lean.

“Is my metabolism damaged?” you may ask yourself. Patients ask me this question all the time.

Can months or years of dieting do some kind of long-term harm to the way the human body processes food?

Not exactly.

But gaining and losing fat can change the way your brain regulates your body weight.

To understand this answer let’s explore how human metabolism actually works. Then we’ll talk about whether the metabolism can actually be damaged.

*Note* This post delves into the science of energy balance, thermodynamics, and metabolic regulation. If you love learning this stuff, feel free to really dig. 

If, on the other hand, you want to jump right to the “HOW TO” for weight loss, fat loss, and breaking plateaus, feel free to skip to this article: 10 Ways to Boost Metabolism.

Energy Balance: It’s A Law of Physics (and not that simple)

You absolutely need a certain amount of energy (in the form of calories) to stay alive, as well as to move around. You can get this energy from food, or you can retrieve it from stored energy (i.e. your fat tissue).

In theory:

  • If you eat less than you work, you should lose weight.
  • If you eat more than you work, you will gain weight.

Body Weight = Energy In – Energy Out

But it’s not that simple. This relationship between ‘energy in’ and ‘energy out’ is called the Energy Balance Equation and it’s the most commonly accepted model for calculating a person’s energy balance and how much weight they’ll lose or gain over time.

While the Energy Balance Equation determines body weight… it doesn’t really tell us about body composition, which is influenced by hormones, macronutrient intake (especially protein), exercise style / frequency / intensity, age, medication use, genetic predisposition and more.

AND… this is why people get frustrated. Often times the numbers simply don’t add up or their results don’t match the expectations. (Which is a good lesson about the importance of adjusting expectations to match observable reality!)

This is Important…

The mismatch between expectations vs reality is not because the Energy Balance Equation is wrong, or a myth. I promise, no ‘body’ can defy the laws of physics… but the equation is more complicated than it sounds.

Many factors affect the energy balance equation and what you do to “ENERGY IN” affects what happens to “ENERGY OUT” and vice versa.

I will say that “Eat less, Move More” is a good start for most; but not everyone. It really just depends.

Let’s take a look at some of these factors, starting with the ‘energy in’ part of the equation.

“Energy In” is trickier than you think

Reason 1 – The number of calories in a meal likely doesn’t match the number of calories on food labels or a menu. 

This may be hard to grasp, but the FDA permits inaccuracies of up 20-25%.

And even if the labels were correct…

Reason 2 – The amount of energy a food contains in the form of calories is not necessarily the amount of energy we absorb, store and/or use.

Remember that the food we eat has to be digested and processed. And how we individually digest and process food is completely different due to genetics, pre existing health conditions and daily activities. There are so many factors involved in digestion > processing > absorption > storage and use.

For instance:

  • We absorb less energy from minimally processed carbohydrates and fats, because they’re harder to digest.
  • We absorb more energy from highly processed carbohydrates and fats, because they’re easier to digest. [The more processed a food is, the easier they’ve made it for us to digest and absorb].

For example, research has shown that we absorb more fat from peanut butter than from whole peanuts. In fact, researchers determined that almost 38 percent of the fat in peanuts was excreted in the stool,  rather than absorbed by the body. Whereas seemingly all of the fat in the peanut butter was absorbed.

In addition:

  • We often absorb more energy from foods that are cooked (and/or chopped, soaked, blended) because those processes break down plant and animal cells, increasing their bioavailability.

When eating raw starchy foods (like sweet potatoes), we absorb very  few of the calories. After cooking, however, the starches are much more available to us, tripling the number of calories absorbed.

Interestingly, allowing starchy foods to then cool before eating them decreases the amount of calories we can extract from them again.  (This is mostly due to the formation of resistant starches).

So technically speaking, eating potatoes that have been cooked and then cooled will result in less calorie absorption than eating them hot!


  • We may absorb more or less energy depending on the types of bacteria in our gut.

Some people have larger populations of a Bacteroidetes (a species of  bacteria), which are better at extracting calories from tough plant cell  walls than other bacteria species.

Take Home Message: By eating a diet rich in whole, minimally processed foods, the number of calories you absorb can be significantly less than what you expect. Plus… it takes more calories to digest them.

Opposite to that… you will absorb more calories from processed foods and burn fewer calories trying to digest them.

“Energy Out” Varies From Person to Person

There are 4 key parts to this complex system:

1 – Resting Metabolic Rate (RMR)

RMR is the number of calories you burn each day at rest, just to breathe, think, and live. This represents roughly 60 percent of your ‘energy out’ and depends on weight, body composition, sex, age, genetic predisposition, and possibly (again) the bacterial population of your gut.

A bigger body, in general, has a higher RMR.

For instance:

  • A 150-pound person might have an RMR of 1583 calories a day.
  • A 200-pound person might have an RMR of 1905 calories.
  • A 250-pound person might have an RMR of 2164 calories.

RMR varies up to 15 percent from person to person. If you’re that 200-pound individual with an RMR of 1905 calories, another person just like you on the next treadmill might burn 286 more (or fewer) calories each day with no more (or less) effort.

2 – Thermic Effect of Eating (TEE)

This may surprise you, but you’ll burn calories just from digesting food. Digestion is an active metabolic process.

TEE is the number of calories you burn by eating, digesting, and processing your food. It represents approximately 5-10% of your “energy out”.

In general, you’ll burn more calories by digesting and absorbing protein (20-30 percent of its calories) and carbs (5-6 percent) than you do fats (3 percent). So you burn more calories depending on the macros (i.e. protein, carbs, fats) you choose to eat.

This is why increasing your protein intake is a solid weight loss recommendation!

And as noted before, you’ll burn more calories digesting minimally processed whole foods compared to highly processed foods.

3 – Physical activity (PA)

PA is the calories you burn from purposeful exercise, such as walking, running, going to the gym, gardening, riding a bike, etc.

Obviously, how much energy you expend through PA will change depending on how much you intentionally move around.

4 – Non-exercise activity thermogenesis (NEAT)

NEAT is the calories you burn through fidgeting, staying upright, and all other physical activities except purposeful exercise. If you sit all day long, your NEAT is going to be low. If you have a standing work station or work in construction… your NEAT is going to be high.

So what does all this mean?

Energy Out = Resting Metabolic Rate + Thermic Effect of Eating + Physical Activity + Non-Exercise Activity

In short… it’s pretty dang complex and highly variable. This means that the ‘energy out’ side of the equation may be just as hard to pin down as the ‘energy in’ side.

So, while the Energy Balance Equation variables make it hard to know or control exactly how  much energy you’re taking in, absorbing, burning, and storing.

Here’s the entire equation:

Changes in Body Stores = [Actual Calories Eaten – Calories Not Absorbed] – [RMR + TEE + PA +NEAT]

Basically… when you try to outsmart your body, it’ll outsmart you back; and sometimes harder. When energy input goes down, energy out goes down to match it. You burn fewer calories in response to eating less and burn more calories in response to eating more.

Even that last statement is not that true. This doesn’t happen perfectly and not in everybody the same way. But… it’s how the system generally works and how our bodies were able to avoid unwanted weight loss and starvation during times of famine. Don’t be mad.

It’s the reason why humans survived for 2 million years.

The good news is that when “energy in” goes up, “energy out” tends to go up too! So you can burn more calories when you eat more calories.

To illustrate this point, here’s how your body tries to keep your weight  steady when you take in less energy and start to lose weight*:

  • Thermic effect of eating goes down because you’re eating less.
  • Resting metabolic rate goes down because you weigh less.
  • Calories burned through Physical activity go down since you weigh less.
  • Non-exercise activity thermogenesis goes down as you eat less.
  • Calories not absorbed goes down and you absorb more of what you eat.

In addition to these tangible effects, reducing actual calories eaten also causes hunger signals to increase, causing us to crave (and maybe eat) more. The net effect leads to a much lower rate of weight loss than you might expect. In some cases, it could even lead to weight re-gain.

To add insult to injury, a rise in cortisol from the stress of dieting can  cause our bodies to hold onto more water, making us feel “softer”  and “less lean” than we actually are. Interestingly, this is just one  example of the amazing and robust response to trying to manipulate  one variable (in this case, actual calories eaten). There are similar responses when trying to manipulate each of the other variables in the equation.

For example, research suggests that increasing Physicalactivity above a certain threshold (by exercising more) can trigger:

  • Increased appetite and more actual calorieseaten
  • Decreased calories not absorbed as we absorb more of what we eat
  • Decreased RMR
  • Decreased NEAT

In the end, these are just two of the many examples we could  share. The point is that metabolism is much more complicated (and  interdependent) than most people think.

So Does Dieting Damage Metabolism?

Despite what you may have heard:

Losing weight won’t “damage” your metabolism.

But because of the adaptations your body undergoes in response to  fat loss (to prevent that fat loss, in fact), ‘energy out’ for those who  have lost significant weight will always be lower than for people  who were always lean.


Losing weight, and keeping it off, is accompanied by adaptive metabolic, neuroendocrine, autonomic, and other changes.

These changes mean that we expend less energy — around 5-10  percent less (or up to 15 percent less at extreme levels) than what  would be predicted based on just weighing less.

Unfortunately, because of this adaptive response, someone who has  dieted down will often require 5-15 percent fewer calories per day to  maintain the weight and physical activity level than someone who has  always been that weight.

(Or even less, potentially, because as we learned in the very  beginning, the RMR of people of the exact same age/weight/etc. can  still vary by up to another 15 percent.)

This means someone who was never overweight might need 2,500 calories to maintain their weight, while someone who had to diet down to that weight may need only 2,125-2,375 calories to hold steady.

We don’t know how long this lowered energy expenditure lasts.  Studies have shown that it can hang around for up to 7 years after weight loss (or more, 7 years is as far as it’s been studied). This likely  means it’s permanent, or at least persistent.

This is extra relevant for people who have repeatedly dieted, or for fitness competitors who may repeatedly fluctuate between being extremely lean and being overweight in the off-season.

We don’t have data to back this up (to our knowledge no one has studied it), but adaptive thermogenesis seems to react more strongly or more rapidly with each successive yo-yo of extreme body fat fluctuations.

All of this explains why some people can feel like they’ve “damaged” their metabolism through repeated dieting. (And why some experts suggest “metabolic damage” is a real thing.)

But nothing really has been “damaged”.

Instead, their bodies have just become predictably more sensitive to various hormones and neurotransmitters. Their metabolic rates are understandably lower than predicted by various laboratory equations.

So, where does this leave us?

Body change is going to be harder for some people, and easier for others.

That can mean all physiological changes: weight loss or gain, fat loss or gain, and muscle loss or gain.

But even if your body might resist weight loss, you can still lose weight, gain muscle, and dramatically change your body.