Metabolic Adaptation

March 26, 2021 7 min read

Metabolic Adaptation

Ever wonder why it gets harder and harder to lose weight the longer you diet?  

Or why do you see people eating 1200 calories and NOT losing weight?  

Metabolic adaptation. 

So what is metabolic adaptation?  

For us to be able to see weight loss, the body must be in a ‘negative energy balance’ - meaning we must burn more energy than we consume (i.e. calories).  There are a lot of camps that like to argue this, but research continues to show that a calorie deficit is necessary to see the scale drop.  

So why can’t we just eat 1000 calories each day and lose weight until the end of time?  Well, these bodies of ours were designed for two things: survival and procreation.  What this means is that when we try to ‘starve’ our bodies to get extremely lean, the body itself sees this as a threat to our well-being and this where metabolic adaptation comes into play.  

The body has certain defense and safety mechanisms in place to make sure we don’t diet ourselves to death.  What this means for us is that when we try to create a larger and larger calorie deficit with diet and exercise, the body works to conserve energy as much as possible in other areas to ensure it can hold onto enough body-fat to feel safe and protect itself. So what areas does the body adjust to stall out our weight loss?  

Hormonal Response 

There are a few main hormones that play roles in the regulation of our metabolism, body-composition, and our body’s ability to burn fat.  These each get impacted by a caloric deficit and weight loss in certain ways that can start to slow down progress. These are by no means the only hormones that get impacted, but these are some of the most notable ones: 

  • Thyroid Gland Hormones - Your thyroid is host to a number of different hormones, one being triiodothyronine (T3), which is known to play an important role in regulating metabolic rate - it is our ‘active’ form of thyroid that can speed up our metabolism.  When your thyroid is properly supported with enough calories and carbohydrates, it will show increases in circulating thyroid hormones, which often means an increase in metabolic rate. Whereas lower levels of intake or restriction have shown lowered thyroid levels which result in decreased overall metabolic rate [1].
  • Leptin - This is your ‘satiety’ hormone, which regulates fat storage and how many calories you eat/burn.  Caloric restriction and lower body-fat levels are shown to decrease the amount of leptin produced, meaning we don’t get as full as easily [2]! 
  • Testosterone - This hormone is primarily known for its role in increasing and maintaining muscle mass, which helps to increase resting metabolic rate.  Testosterone has also shown links in fat-tissue regulation. Research suggests that high testosterone may subdue adipogenesis, which is your body’s ability to form fat-cells.  In a calorie deficit, testosterone decreases, which then in turn may correlate with higher fat storage [3]. Low testosterone can also impact your libido and we often see a drop in sex drive with sustained caloric restriction.  
  • Cortisol - Cortisol is our stress hormone, and a calorie deficit is an additional stress on the body because we are not supplying the body with adequate energy to function optimally.  When energy is restricted (i.e. calorie deficit), research shows that cortisol will rise [4].  This rise in cortisol has shown to impact macronutrient metabolism and actually start to break down muscle mass instead of fat [5].  Cortisol also impacts other hormones like our thyroid and the female-dominant hormones estrogen and progesterone which all can have a downstream effect on body-fat.  

Metabolic Response 

Although everything we are talking about today is in essence a ‘metabolic response’ - these areas we are going to discuss next are the main contributors to caloric burn in a day.  They each get impacted negatively as well with a calorie deficit.   

  • Basal Metabolic Rate - This represents how many calories your body burns in a day at rest.  This is essentially to keep our organ function going, and to keep our immune system, nervous system, digestive system, etc. in check. When you lose weight, you are losing either fat tissue, muscle tissue, or both.  These are both forms of metabolically active tissue, meaning they burn calories at rest. So as we lose weight, we are losing total calories burned in a day as well [6].  The other important factor to realize is that this is one of the main areas that gets down regulated by the body when the calorie deficit gets too large to conserve energy and close the gap of the energy imbalance, which we’ll talk about later.  
  • Non-exercise Activity Thermogenesis - This refers to how much we move in a day aside from exercise.  Our fidgeting, how many steps we walk in a day, chewing food, standing, etc. It has been shown that when you are in a calorie deficit, NEAT naturally decreases subconsciously as a way for your body to conserve energy [7].  You’ll naturally move less, you’ll blink less, and your body works in miraculous ways to save every little bit of energy it can!  
  • Thermic Effect of Food - The thermic effect of food refers to the amount of energy it takes the body to break down, digest, and absorb nutrients from our food. It is estimated that about 10% of our total daily energy expenditure, or how many calories we burn in a day, comes from the thermic effects of food [8].  So as we eat less calories in an attempt to lose weight, we also see a decrease in the thermic effect total burn of calories as there is less food to breakdown and the overall calorie burn in a day.  

Why Exercising More Doesn’t Work 

Many people will turn to exercise to work to increase their energy balance and essentially try to create their calorie deficit with more cardio, more ‘active calories’ on their Apple Watch, or ‘burn the fat off.’  The body’s response to increased levels of exercise is definitely something that research seems to be a bit unclear about still, but from what we can tell, females tend to see more adaptation than men do.  

We will focus a bit more on the ladies in this section, because men - you have it a bit easier.  The male body and hormones do not adapt as easily or as quickly as the female body does, and the main reason is because the female body is the one which has to build and carry a human during pregnancy, and in turn, the female body’s defense mechanisms are a bit more sensitive and strong.  

One of the biggest misconceptions that females have about fat loss is that the more movement we do, the more calories we burn - and in turn, the more fat we burn. This is simply not true because the female metabolism in particular is reactive, not additive.  There was one study that demonstrated this perfectly, which compared total daily energy expenditure and total minutes of exercise per week.  What you may expect is that as energy burn increases, total daily energy expenditure would increase, but the metabolism doesn’t work like that.  Metabolic adaptation happens.  

As exercise calorie burn increased, the total daily energy expenditure actually pulled back and went down to compensate for the extra movement to drop overall daily calorie burn down (source).  This goes back to our basal metabolic rate that manages energy towards our organ function and systems within the body.  This is where energy gets conserved!  Instead of burning more calories, our body takes energy from our systems to bring energy balance back and close the gap.  It will pull energy from our immune system meaning we get sick more often or show lower white blood cell counts.  It pulls energy from digestion meaning we may end up with bloating, constipation, or other digestive problems.  It pulls energy from our reproductive system and we start to see irregular periods or no sex drive.  It lowers brain function and you’ll start to notice fatigue or brain fog more commonly.  

So instead of burning MORE calories, our body really just takes energy and conserves it in other places, making you feel run down, crummy, and stalled out with weight loss because like we said, the body really is designed to survive - it is not concerned about aesthetics.


Metabolic adaptation sounds like kind of a downer, right??  We get it, we know you have goals and want to lose that weight, but depending on how long you’ve been trying, your body may have other plans.  If you have been in a long-term calorie deficit, or you’ve stalled out with weight loss, some great ‘markers’ we look for to let our body tell us it needs a break, and to perhaps start eating a bit more or exercising a bit less are the following: 

  • Bowel movements have slowed down in frequency and/or constipation is happening often.
  • Sleep quality has decreased. 
  • Hunger levels are increasing. 
  • Performance in the gym has severely declined and/or injuries are starting to occur more frequently.  These are both signs your body is under-recovering. 
  • Mood swings are becoming more frequent. 
  • The scale is no longer moving and you may actually be seeing weight gain, even with adherence. 
  • You’re noticing more extreme fatigue/tiredness and lacking energy throughout your day.  
  • Sex drive has decreased.
  • Females: Periods have become irregular or stopped altogether. 
  • You’re getting sick more often. 

As we always tell our clients, listen to your body talk so you don’t have to hear it scream!   Our bodies are great guides for us to know when they need a bit more support, and when we can push them a bit harder as long as we’re willing to listen.  


 [1] - Kim B: Thyroid hormone as a determinant of energy expenditure and the basal metabolic rate. Thyroid. 2008, 18: 141-144. 10.1089/thy.2007.0266.

[2] Strohacker K, McCaffery JM, Maclean PS, Wing RR: Adaptations of leptin, ghrelin or insulin during weight loss as predictors of weight regain: a review of current literature. Int J Obes. 2013, 1-9.

[3] De Maddalena C, Vodo S, Petroni A, Aloisi AM: Impact of testosterone on body fat composition. J Cell Physiol. 2012, 227: 3744-3748. 10.1002/jcp.24096.

[4]Rossow LM, Fukuda DH, Fahs CA, Loenneke JP, Stout JR: Natural bodybuilding competition preparation and recovery: a 12-month case study. Int J Sports Physiol Perform. 2013, 8: 582-592.

[5]Rooyackers OE, Nair KS: Hormonal regulation of human muscle protein metabolism. Annu Rev Nutr. 1997, 17: 457-485. 10.1146/annurev.nutr.17.1.457.

[6]Leibel RL, Rosenbaum M, Hirsch J: Changes in energy expenditure resulting from altered body weight. N Engl J Med. 1995, 332: 621-628. 10.1056/NEJM199503093321001.

[7] Weigle DS, Brunzell JD: Assessment of energy expenditure in ambulatory reduced-obese subjects by the techniques of weight stabilization and exogenous weight replacement. Int J Obes. 1990, 14 (Suppl 1): 69-77. discussion 77–81.

[8] Tappy L: Thermic effect of food and sympathetic nervous system activity in humans. Reprod Nutr Dev. 1996, 36: 391-397. 10.1051/rnd:19960405.

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