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What Is Metabolic Adaptation and Why Does It Occur?

For a long time, people have been aware that total daily energy expenditure (TDEE) changes during phases of dieting. Personally, my first insight into this phenomenon was when I saw the phrase “metabolic damage” a bit over 5 years ago at some stage in my nutrition degree at university. My understanding of the concept was that if somebody was in a calorie deficit for an extended period, their “metabolism” (which I didn’t fully understand at the time) would slow down.

My take on the main difference between metabolic damage and metabolic adaptation, is that ”damage” indicates that there is an issue, whereas adaptation indicates that the body is making an adjustment. Damage could indicate that the metabolism is broken and needs to be fixed. It could also insinuate that people who have spent long periods of time following low calorie diets may continue with this damaged metabolism long-term even after no longer being in a calorie deficit, unless something is done to fix it. The biggest issue with the terminology of metabolic damage though is that there is minimal scientific literature identifying it in the way it was described. There are some individual studies that have results that potentially support the concept, but the weight of the literature appears to strongly indicate to me that metabolic adaptation would be a more appropriate term.

Metabolic Adaptation

Metabolic adaptation is a physiological response that has been documented consistently in the research. Specifically, the terminology used in the literature is adaptive thermogenesis. This is the concept of where the body’s energy expenditure drops to a greater extent than what you would predict with a formula while dieting, or alternatively energy expenditure increases more than a formula would predict based of weight and activity while in a calorie surplus.

It is part of the body’s way of manipulating its role in energy conservation. If you have more calories available, you burn a little bit more. If you have less available, you burn less, and your body prioritises its highest needs first.

This is a controversial topic in the fitness community, but I think it is mostly controversial due to people misunderstanding the magnitude of these effects on average. For example some people think it makes a massive difference whereas other think the difference it makes is so minimal it is irrelevant, whereas the answer is likely in the middle somewhere.

Variables of TDEE That Are Effected

TDEE is made up of exercise activity thermogenesis (EAT), non-exercise activity thermogenesis (NEAT), thermic effect of food (TEF) and your resting metabolic rate.

Your RMR is going to change related to changes in body weight and composition, since that is a big factor, but those changes are factored into the formulas used for predicting TDEE, which makes it irrelevant for the discussion of metabolic adaptation. But it is worth being aware of for identifying reasons for changes in calorie needs e.g. if you diet and lose 10kg, then your RMR will drop in part due to the loss of body mass.

TEF likely will drop a little bit while in a calorie deficit as you are eating less total food. Keeping protein and fibre high can help though.

EAT will likely decrease due to the reduced body mass while exercising. There also appears to be an increase in exercise efficiency so to speak where exercise burns less calories while in a deficit, beyond what you would expect related to changes in body mass.

NEAT is the most interesting one for me though. When people are in a calorie deficit for an extended period of time, their NEAT appears to decrease significantly. Often this will mean fidgeting less and doing less overall incidental activity that burns calories.

From a hormonal perspective, it looks as though leptin, triiodothyronine (T3) and thyroxine (T4) are also decreased during times of caloric restriction, which could also contribute to reduced energy expenditure.

During a Calorie Surplus Energy Expenditure Increases

One of the reasons I like that the phrase metabolic adaptation has caught on is that during a calorie surplus, your body adapts in the other direction too. A higher calorie intake leads to a higher calorie expenditure.

Using NEAT as an example: when people enter a calorie surplus, their NEAT often significantly increases, and in some people this makes far more difference than for others.

To put this in context for those wondering how big of a deal this is, this study tracks people gaining 10% body weight and the participants energy expenditure increased by an average of 400-500kcal higher than what would have been predicted. This seems to be on the higher end of what is commonly seen in research and in practice, but it seems consistent that under most circumstances there is an increase above what a formula would predict.

Practical Applications

Given the difficulties associated with self-reported calorie intake, while there are a lot of cases of self-reported metabolic adaptation that seem severe, they may not all actually be accurate. People are notoriously poor at reporting their intake.

One study that stands out to me involved 224 participants attempting to provide a 24hr recall of their food intake to the researchers, but on average their reported intake was 47% lower in calories than what they actually did. Somebody reported an intake roughly 80% lower than their actual intake and the closest anybody got was within 20%, meaning every single person underreported significantly. Based on this and the other research on the topic, it is worth taking self-reported information lightly.

This makes it difficult to assess what is truly going on when working with a client or listening to somebody explain their situation. But there is no doubt that metabolic adaptation effects some people significantly more than others.

Taking all that out of the equation though, I wanted to do a case study to explain how this could look without the underreporting aspect being involved.

Case Study

Imagine a male lifter named James who is 183cm tall, 22 years old, 90kg and 15% body fat. They have been eating at maintenance calories for two weeks, but they were previously in a muscle gain phase and their energy expenditure is still 10% above what you would expect due to metabolic adaptation related to the surplus.

James wants to lose 10kg over 20 weeks. To achieve this he will need to maintain a little bit more than a 500kcal deficit per day on average.

As the time spent in the deficit progress, his TDEE will drop accordingly.

Let’s say he starts off at 3000kcal at maintenance calories (matching his TDEE) and drops to 2500kcal initially to attempt to create that 500kcal deficit (although we know it will be a bit less than 500kcal due to some immediate impacts of reduced calories).

While his TDEE starts at 110% of what is predicted, in this example, after 20 weeks of dieting it ends up at 90% of what is predicted – which is roughly in line with what would be expected on average in this scenario. His predicted TDEE at the start is 2722kcal per day and at the end when he weighs 80kg at 9% body fat, it is 2668kcal per day. That’s not a massive drop.

But because his TDEE is now 90% of what is predicted, his TDEE is actually at 2401kcal.

While the prediction has gone from 2722kcal to 2668kcal per day, the reality has gone from 3000kcal to 2401kcal. This is a big difference and enough to be relevant for people who are dieting.

What’s more is that from a practical perspective, James would need to still be consuming a 500 calorie deficit throughout the process. So in the final week of his diet, he would be consuming 1901kcal per day. This is now a full 1100kcal below where he originally started.

Summary

To dismiss metabolic adaptation as irrelevant would be silly in my opinion. From a short-term standpoint, it is important because it means that adding/subtracting 500kcal from somebody’s diet doesn’t mean they will actually end up in a 500kcal surplus/deficit. The difference will be smaller due to adaptation. From a longer term impact though it helps us to understand ways we can potentially achieve better outcomes and it also highlights how strategies such as diet breaks could potentially help keep energy expenditure higher and make the dieting process easier.

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