Metabolic Adaptation: What Is It and Significance to Weight Loss or Gain Explained

The body's regulation of body weight is a dynamic process.

Governed by drivers of food intake and factors that affect energy expenditure and how these respond to changes in energy balance.Energy balance is the term used to describe the difference between energy intake from food and our expenditure.

Basically, a negative energy balance means weight loss, positive weight gain...

We can think of this equation as a summation of our ‘metabolism’

Interestingly, as we shift our energy balance in either direction, both acutely and chronically, our ‘calories out’ part of metabolism is also influenced.

This can affect rates of fat loss and weight gain.Metabolic adaptation, therefore are the reductions or increases in our energy expenditure that occur as a response to energy deficits or surpluses.In this article we focus on the main constituents of energy expenditure and how they adapt to changes in energy intake, these are…

• BMR (Basal metabolic rate): The energy we use to keep our body’s functioning, at rest, if we were to stay still for the whole day!
• NEAT (Non-exercise Activity Thermogenesis): The energy we use to do activities of daily living. Everything from making a coffee to doing our dishes.
• EAT (Exercise Activity Thermogenesis): This is any activity that we would consider an intentional & structured movement that contributes to energy expenditure.

TEF (Thermic Effect of food)

Now before we go any further, it is important to note that this isn’t always strictly the definition of Metabolic Adaption.

In scientific research, it is often defined as the changes in energy intake or expenditure that are greater than which we would predict from changes in body mass.What this means, is that as we gain or lose weight then by default we are going to change our energy expenditure both at rest and during activity.

In many studies, they use the date from the ‘before’ fat loss participants' data to create a predictive equation for their basal (BMR) or resting metabolic rate (RMR)...there are slight differences between the two but right now that’s not important!...This equation can be based on several variables to match the equations predictions as closely as possible as the participants in the studies data and will have some component of ‘mass’ in that equation.This means when the person loses (or gains) weight, we can plug in these new numbers to see if these match our prediction from what we might expect from changes in body weight.

• Related Article: Foods with High Thermic Effect

There is, however, a slight problem with this approach

It is pretty clear as to why weight loss studies are going to use overweight and obese people as the participants. Now imagine we take this already overweight group and do our equation making magic.Do you see the issue?If we accept that being overweight could be associated with higher than normal RMR, then this could be high in the opposite direction. Thus, any significant reductions in RMR noted could also be due to (at least in part) a ‘normalisation’ in RMR rather than an excessive level of adaptation. In any case does it matter practically if this is a relative or absolute energy expenditure adaptation? For the sake of what most of us are probably concerned about… NO!What most of us really care about is that as we lose weight our energy output, in some component of metabolism, might decrease and this could of course have an impact on rates of fat loss and even perhaps even bring it to a standstill. Another concern might be, if we do adapt and even if we lose weight, might this make maintaining weight harder?On the other side of the coin those looking to gain weight might adapt the other direction and then struggle to put on mass.The fortunate thing about these two different directions of adaptation is that it appears the apparent causes work in pretty much the opposite direction.

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As we are going to focus on the weight loss aspect for the remainder of this article, for those who may be worried about why they can’t gain weight think the exact opposite might be happening!

What regulates these metabolic adaptations?

Our body fat is not just some passive tissue that stores energy. It also produces hormones and other important signallers that tell our brains about the state of energy balance in the body.

One of the key players in the process is a hormone called leptin and is considered a bit of a fuel gauge hormone. The amount of leptin produced is related to fat mass. People with more body fat produce more leptin and this acts centrally to increase hormones that regulate metabolism.As we lose body fat leptin levels typically reduce, and this is one of the proposed mechanisms that as we lose fat we then get reductions in hormones that regulate metabolism and in turn have the potential to lower metabolic rate. Interestingly, although reductions in leptin in line fat loss do align somewhat with changes in resting metabolic rate. This tends to follow a pattern of a return to normal range leptin levels (suggesting a potential normalisation rather than excessive reduction). It also does not explain with enough accuracy the individual variation that we see in changes in metabolic rate as a response to fat loss. This means that when we do see changes in BMR as a response to fat loss, this is not likely to be fully accounted for by changes in leptin and hormones like thyroid that regulate metabolic rate.The truth is, at present, although leptin and thyroid are likely to influence metabolic rate. They do not fully account for changes that we do see in resting metabolic rate in many people. There are other candidates that may influence individually, or perhaps together our energy expenditure… however, that’s probably a whole other article within itself.

Is this the same as metabolic damage or starvation mode?

It is clear that during fat loss our calories ‘out’ can become reduced. What is more interesting is that the component that most greatly accommodates this change is actually our activity, not in fact our ‘metabolism’ in the sense of our resting metabolic rate!Terms like ‘starvation mode’ are thrown around without clear definition, and are in every sense an unscientific way to talk about metabolism that is often used to give the impression that at some point the body can enter a state that even on low calories and what should be an energy deficit that the body will cling on to fat and not lose weight. The fact is that energy balance matters. Yes, weight loss can slow and even stop if we have enough adaptation to our energy output through reductions in BMR and other energy expenditure. But once these have been accounted for through an increased focus on increasing/maintaining energy output and adjustments to calorie intake, then weight loss will occur… This means that when people feel they have stopped losing weight no matter how ‘low calorie’ they go, this means that either a) they aren’t being honest or accurate with their calorie intake or expenditure or b) that they are actually losing fat mass, but scale weight is lagging behind for some reason, i.e. that they are holding water weight and they need to sit tight and be consistent until the scale eventually moves.Metabolic damage is a term that is another unscientific term and although it is more balanced in terms of the view that metabolism adapts during weight loss, this is not a damaged metabolism but an adaptation, that is completely normal and actually serves as an important mechanism in our survival.Another term used in regard to metabolic adaptation is ‘adaptive thermogenesis’ (AT). If we think back to our different components of metabolism (BMR, EAT, NEAT, TEF), we can see that one of the key products of metabolism is heat energy. AT is therefore a legitimate term that means the same as metabolic adaptation just with an arguably more appropriate phraseology.

Do we need to worry about metabolic adaptation?

This is a ‘how long is a piece of string’ type of question. But we should really follow some basic rules before deciding if we have adapted (and in what ways!) or not. Are we certain fat loss has stalled or is it just weight ‘holding’ for some other reason. Are we noticing any other positive changes in weights or pictures and has it been enough time to definitely be a plateau?Are we being as consistent in our nutrition and have old eating habits crept back possibly consuming more calories than you had been? Have we been as consistent with exercise and daily activity or have we become ‘lazy’ with our energy expenditure?When it comes to our BMR, even using quite extreme fat loss approaches, this will only reduce in the region of 10-20%. For context, if you had an estimated BMR of 1300 Calories at the start (we discussed the issues with predicting BMR previously), then this would be reduction of 130-260 Calories(ish). Now depending on the deficit we are in, this may be enough to slow or cause a plateau in fat loss if we are using only very slight deficits each day. However, for most people this alone would not be the main adaptation that would cause us to stall completely. But we might want to adjust things slightly at this point if we want to keep rates of fat loss consistent.The biggest adaptation is in terms of day to day and exercise expenditure, and this can be huge, several hundred Calories difference pay day, in those who are highly adaptable. There is a large individual response in this adaptive component and this could 100% cause a fat loss plateau (or stop us from gaining weight if that was our goal!). The good news is, that although we can’t be super accurate in terms of activity and day to day energy expenditure, we can use activity trackers to at least see if we are being consistent in our approach. To achieve this it does take some pretty careful monitoring of intake and expenditure, at least periodically just to make sure that our numbers add up to fat loss.

How do we prevent metabolic adaptation?

There are certain components of adaptation that we cannot prevent. For example, as we lose weight we are in effect going to be using less energy doing day to day things or during exercise because we are carrying less of us around! Although we can’t necessarily change this directly (although we could wear a weighted vest to replace lost weight, this has been done in research to highlight this point), we can look to increase our activity doing a little bit more, or with more intensity, as we lose weight. This can often happen as a result of becoming fitter and stronger in overweight people, and intuitively they increase expenditure as they lose weight for this reason. In terms of the ‘deeper’ types of adaptation to our BMR the research is inconsistent/unclear when it comes to the type, duration and ‘aggressiveness’ of the dietary approach. However, in most of the extreme diet research, there is a subtle indicator that deficits of under 500kcal per day do not have a significant impact on BMR.These less extreme approaches may also help to preserve our energy expenditure because we do not feel so fatigued, ‘lazy’ and underperforming in our day to day life and during exercise.

Summary of Metabolic Adaptation

Metabolic adaptation occurs as a natural result of weight loss and weight gain. It can affect all our components of ‘metabolism’, but not as much to our BMR as many people are led to believe. Terms like starvation mode and metabolic damage are not only unscientific, these are used in the world of health and fitness by some people in a negative way that can lead people away from understanding in a more meaningful context what it is that is actually stopping them losing weight. This is because these terms are easy to ‘blame’, instead of understanding the behaviours that are underpinning success or failure in their physique goals. Metabolic adaptation in some form will likely take place. But as long as we are collecting appropriate information, understanding where we may be adapting in the things we can measure, then progress can be sustained.Poor sleep, long periods of more extreme calorie deficits (over 500kcal a day) may also have more adaptive response, but the evidence for this is still unclear and it is likely that more aggressive deficits are ok for shorter durations.