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Is Metabolic Adaptation Real?

Is Metabolic Adaptation Real?

Metabolic adaptation is a hot topic in the fitness industry right now, largely driven by fitness influencers suggesting it's the primary reason people struggle to lose weight and keep it off.

But what is it, and is it something you really need to worry about?

What is Metabolic Adaptation?

To understand metabolic adaptation, you first need to know how you burn energy on a daily basis.

Your "Total Daily Energy Expenditure" (or TDEE for short) is the term used to describe the total amount of energy you burn each day. This is broken down further into four categories, being:

  • Exercise activity thermogenesis (EAT), which is the energy you burn through formal exercise (i.e., weight training and cardio)
  • Non-exercise activity thermogenesis (NEAT), which is the energy you burn moving about throughout the day.
  • Thermic effect of food (TEF), which is the energy you burn breaking down and digesting the food you eat, and:
  • Resting metabolic rate (RMR), which describes the energy you burn performing the cellular functions your body needs to stay alive.

Now of these factors, the largest determinant of your TDEE is your RMR, which accounts for around 70% of your total energy expenditure.

With this in mind, metabolic adaptation describes a process whereby your energy expenditure decreases, meaning the amount of energy you need to consume to stay the same weight (or lose weight) decreases [1].

Why Does Metabolic Adaptation Occur?

Metabolic adaptation is thought to occur when you are exposed to a prolonged period of low energy availability (i.e., during a diet).

Historically speaking, this process makes sense. 

Back when humans were hunter-gatherers, food was scarce. And during times when food was hard to come by, metabolic adaptation would be desirable because it would help you sustain essential body fat levels and survive without food.

However, in modern day, we don't have a food scarcity problem. Instead, the most likely instance of metabolic adaptation is when someone tries to lose weight. 

The Drivers of Metabolic Adaptation

When it comes to metabolic adatpation, there are two main drivers: a physical loss of weight and a change in hormone secretion.

The first one is simple to understand.

The heavier you are, the more energy you burn moving around on a daily basis. You also spend more energy on the physiological processes that keep your body running because there is simply more of you to run.

But, when you lose weight and get lighter, the amount of energy you burn in this manner decreases.

This is one of the key factors that contribute to lasting (but expected) changes in energy expenditure (and, by extension, metabolism) after you finish dieting and lose weight.

The second is a little more complex and related to the hormone leptin [2].

Leptin is a hormone produced by fat cells, but its production is only high when those fat cells are full. When you lose weight, your fat cells store less energy and literally shrink in size. This causes a reduction in leptin production and secretion.

Now, leptin plays a number of key roles in the human body, some of which include the regulation of hunger and energy levels.

With this in mind, when you enter an energy deficit and start losing weight, your low leptin levels are likely to cause an increase in hunger, a reduction in energy levels, and, therefore a reduction in NEAT -- all of which further reduces your daily energy expenditure.

So, considering this, it should be apparent that metabolic adaptation is a very real phenomenon -- but is it something that has a large impact?

The Impact of Metabolic Adaptation

Given the level of interest in metabolic adaptation, it should come as no surprise that it has been researched quite extensively -- and the results have indicated that it can indeed have a measurable impact on energy expenditure.

A well-regarded review by Rosenbaum and Leibel explored the topic in detail and found that if someone loses ~10% or more of their body weight, their TDEE will drop by somewhere between 20 and 25% [3].

But where exactly does this come from?

As I have already outlined, some of this reduction is explained by simply being lighter, but this cannot explain all of it. 

In fact, the change in TDEE tends to be slightly larger than what you would expect based on changes in body weight alone -- which means there must be an additional adaptive component.

And this can almost entirely be explained by reductions in NEAT.

Research has shown that if someone loses ~25% of their body weight, they will see a reduction in TDEE that is ~25% greater than what could be caused by just a loss of physical mass [4]. 

However, their resting metabolic rate changes will only account for about ~2% of this. This means that reductions in NEAT explain the other ~23% of metabolic adaptation.

NEAT is Important

Most people balk when they hear this because they perceive NEAT to be somewhat unimportant -- but I want to reiterate that reductions in NEAT go beyond just doing less incidental activity.

It implies that you are subconsciously becoming more efficient.

This might mean making more efficient movement strategies to get around the house. It might mean unknowingly fidgeting less to preserve energy. It might even mean taking the elevator instead of the stairs because you feel too fatigued to take another step.

Changes in NEAT are very real, and they are often outside our control.

How Long Does Metabolic Adaptation Last?

So we know that Metabolic Adaptation does happen, but the good news is that it doesn't always happen and doesn't last forever.

Firstly, metabolic adaptation is not going to happen to a notable degree unless someone is spending a very long time in an energy deficit and looking to lose a large amount of weight, or if someone is looking to get down to extremely low levels of body fat (i.e., less than 8-10%).

In both of these instances, we would expect to see a larger degree of metabolic adaptation due to the more extreme nature of the energy deficit. But if you are looking to lose a little bit of weight, it will not be a huge concern.

As you move through a weight loss phase, you will be forced to reduce calories to continue losing weight. Although you will not be able to reverse the adaptation coming from being a lighter body weight (unless you regain that weight, which probably ruins the point), the reduction in NEAT tends to return to baseline soon after returning to maintenance calories.

The reason is that losing weight is not the primary cause of metabolic adaptation, whereas being in a constant state of energy restriction is.

To minimize the already small amount of metabolic adaptation, the goal should be to increase back to maintenance calories relatively quickly after reaching your goal body weight [5]

Final Thoughts

Metabolic adaptation is indeed a very real phenomenon.

But it is unlikely that you need to worry about it unless you are looking to lose a considerable amount of weight or get as lean as a bodybuilder on stage.

If you are someone who is simply looking to lose a bit of weight so you can feel more comfortable at the beach, it is good to know that it exists, but know that it is not going to impact you in a particularly negative manner.

References:

  1. Trexler, Eric T., Abbie E. Smith-Ryan, and Layne E. Norton. "Metabolic adaptation to weight loss: implications for the athlete." Journal of the International Society of Sports Nutrition 11.1 (2014): 7.
  2. Kelesidis, Theodore, et al. "Narrative review: the role of leptin in human physiology: emerging clinical applications." Annals of internal medicine 152.2 (2010): 93-100.
  3. Rosenbaum, Michael, and Rudolph L. Leibel. "Adaptive thermogenesis in humans." International journal of obesity 34.1 (2010): S47-S55.
  4. Weigle, David S., et al. "Weight loss leads to a marked decrease in nonresting energy expenditure in ambulatory human subjects." Metabolism 37.10 (1988): 930-936.
  5. Hall, Kevin D. "Metabolic adaptations to weight loss." Obesity 26.5 (2018): 790-791.
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