Is your body working against you?
We all know that person who goes on vacation, eats all the yummy food and they maintain their body composition. We also know that person that as soon as they go on a diet the weight just falls off their body.
Meanwhile we also know people who just look at a cookie and put on 5 lbs and can fast for 5 days and weigh basically the same.
Obviously in these scenarios there are a ton of variables that we can’t control for, but we hear anecdotal stories like the ones above quite frequently and if you are someone who struggles with weight gain and loss, it can be quite frustrating.
What I often wonder when I hear these stories is are there people who, for whatever reason, tend to gain weight more easily and are also more resistant to losing weight?
Many of you may have heard of the term “metabolic adaptation”. It’s become quite popular to throw this term around as of late, and while I think it’s overused, over exaggerated, and thought to be a bad thing, it is a real thing and contrary to how the internet portrays it, it’s not necessarily a bad thing either.
Metabolic adaptation is essentially the slowing of the metabolism in response to the external environment in which you are living. The primary influence to metabolic adaptation is a mismatch in the amount of energy being expended versus the amount of energy being taken in. This mismatch can occur in two ways, you are purposely under-eating, for example in an effort to lose fat, or you are expending a lot of energy and not eating enough to match the energy expenditure. In either scenario the cause of metabolic adaptation is the same, you are taking in less energy than you need.
Your body senses the mismatch between your energy intake and expenditure and it will try to compensate for the mismatch by slowing down your metabolism in an effort to bring your intake and expenditure back in line. This is why over the course of a prolonged period of dieting you might have to adjust your caloric intake to a lower level one or more times because your body is doing what it can to keep your intake and expenditure in line.
This is also why when dieting is taken to the extreme we see health consequences like hormone imbalances, digestive issues, a down regulated immune system, increased soft tissue injury, and even decreased bone health…there is just not enough energy around to sustain all the processes the body needs in order to maintain health so its starts sacrificing non-critical functions in an effort to conserve energy.
The good news is that once you increase energy intake your metabolism will adapt in the opposite direction and increase its energy expenditure. This is a good thing as it means you don’t necessarily need to live at a low energy intake in order to lose weight.
However, unlike what the internet says, there is no magic in adding in more calories after dieting, once you reach your maintenance calories. If you eat more you will begin to gain weight. That doesn’t necessarily mean you will be gaining fat. If you combine this caloric surplus with more activity and strength training you will likely gain muscle.
This brings me back to my original point though, some people seem to have a metabolism that is very stubborn and seems to be working against them. Could it be that some people are being sabotaged by their own metabolism?
First I want you to realize that if your metabolism does drop when eating less food and does not speed up when eating more food, for most of human history you were actually better off than your counterparts!
Yup, that’s right you hit the evolutionary jackpot!
When food was not a guarantee as it is today, your metabolism was thrifty, it conserved energy when it needed to. Then when food was abundant, you stored more energy by not burning as much, setting you up to be in a better place the next time food was scarce.
When food and calories are no longer a concern, as it is for most of us today, this metabolic thriftiness seems to be a disadvantage.
Outside of the anecdotal experiences people have with thrifty metabolisms, is there any scientific evidence to back this up? If so, how detrimental can a metabolism that is highly responsive to under-eating and less responsive to overeating be to someone’s body composition goals?
A 2021 study sought to find out the answer to this question by measuring participants 24 hour energy expenditure under various conditions. The first condition was baseline energy expenditure without any dietary intervention at all. The second condition was their 24 hour energy expenditure after fasting for 24 hours. The third condition was their 24 hour energy expenditure when being overfed. Researchers overfed participants by 200% of their baseline energy expenditure and did it using various macronutrient diets:
- Balanced overfeeding with 50% carbohydrate and 30% fat.
- High-fat, normal-protein overfeeding with 60% fat and 20% carbohydrate.
- High-carbohydrate, normal-protein overfeeding with 75% carbohydrate and 5% fat.
- Low-protein, high-fat overfeeding with 3% protein, 46% fat, and 51% carbohydrate.
- High-protein, high-fat overfeeding with 30% protein, 44% fat, and 26% carbohydrate.
The researchers separated participants into two groups, those with thrifty metabolisms and those with spendthrifty metabolisms. They determined whether someone was thrifty or spendthrift by looking at the change in 24 hour energy expenditure from baseline in response to the 24 hour fasting condition.
The median decrease in 24 hour energy expenditure across all participants was 177 calories per day. If a participant’s decrease in energy expenditure after 24 hours of fasting was greater than 177 calories they were put in the thrifty metabolism group. If a participant’s decrease in energy expenditure after 24 hours was less than 177 calories they were put in the spendthrift group. (And yes I agree this is somewhat of an arbitrary cutoff, if your metabolism decreased by 177 calories you were thrifty but if your metabolism decreased by 176 calories you were spendthrift, but in reality there is no meaningful difference. I also understand that the researchers had to come up with some cut off so they picked the median of the participants. Understand this does not represent the population as a whole, see the end of this post for more discussion about this.)
Let’s spend some time talking about the metabolic adaptation that occurred during the fasting condition because this is the first thing I find fascinating about this study. First it’s important to realize that on average nearly everyone experienced a slowing of their metabolism after not eating for 24 hours. This is why I say metabolic adaptation is normal and is to be expected when we talk about long term diets. Even after an acute period of undereating we see the metabolism respond. However the really cool part is some of the differences in the individual responses to the 24 hour fasting period.
Here you can see the individual responses to 24 hours of fasting for each participant. If you start picking random dots you will notice more people have a lower energy expenditure after 24 hours of fasting, as expected.
However, not everyone did!
If you look at the 5 most spendthrift individuals, they actually saw an average of a 21 calorie per day increase in their metabolism in response to the 24 hours of fasting!
If you then look at the 5 most thriftiest individuals, they saw an average decrease of 313 calories per day in their metabolism in response to the 24 hours of fasting! That’s nearly double the amount when you compare it to the median decrease!
Clearly there is a huge individual variance in how someone may respond to being in a caloric deficit. This brings me back to something I reiterate time and time again, studies are based on averages and medians, they do not represent the individual. We can use them to inform decisions for the individual but ultimately we need to do the “mesearch” to find out what works for us!
That is not all though, there is another very important fact that revealed itself when looking at the thrifty vs spendthrifty groups. Those with thrifty metabolisms in response to the acute fasting period had a HIGHER 24 hour energy expenditure than the spendthrift individuals when at an energy balance state!
How much higher?
On average the thrifty individuals burnt 110 more calories a day than the spendthrift individuals in an energy balance state!
The other crazy fact is that when you look at the 24 hour energy expenditure between the thrifty individuals and the spendthrift individuals after 24 hours of fasting the energy expenditure was virtually the same! The thrifty individuals burnt 1834 calories while the spendthrift individuals burnt 1869, a difference of 34 calories…which is nothing in terms of calories, that’s like 5 almonds!
Here is how this shakes out visually
We have established at this point that most (not all) people will experience a downregulation of their metabolism in response to acute under-eating and the degree of that downregulation is variable. However, in general those who experience a greater downregulation tend to have a higher baseline energy expenditure to start with. This means that on average regardless of how much your metabolism slows in response to acute underfeeding it probably puts you in relatively the same situation from a caloric burn perspective as someone else who is similar to you!
Now on to the overfeeding condition, how is metabolism affected by the various overfed states?
This study was quite comprehensive in testing the body’s response to overfeeding various macronutrient combinations. Ultimately researchers found that those in the thrifty group saw smaller increases in energy expenditure in response to overfeeding than those in the spendthrift group. The strongest correlations in the various overfeeding conditions occurred in the low and high protein overfeeding conditions.
Those individuals in the thrifty group saw smaller increases in energy expenditure in response to overfeeding on low and high protein diets than spendthrift individuals. The below image shows the difference observed by researchers.
In the low protein overfeeding condition, spendthrift individuals saw an average increase of 100 calories due to overfeeding while thrifty individuals saw an increase of only 42 calories.
In the high protein overfeeding condition, spendthrift individuals saw an average increase of 302 calories while thrifty individuals saw an increase of only 237 calories. (You might be wondering why there is such a big difference in caloric burn across both groups in the high protein condition. My guess is that since protein takes much more energy than carbs or fat to break down, the body burns more energy in order to digest that food.)
In the low protein condition spendthrift individuals burnt 58 more calories than thrifty individuals and in the high protein condition spendthrift individuals burnt 65 more calories than the thrifty individuals.
Notice again that thrifty individuals have a greater 24 hour energy expenditure than spendthrifty individuals when in energy balance. So while the increase in caloric expenditure for thrifty individuals is less, their total energy expenditure is still higher than the spendthrifty individuals in response to low and high protein overfeeding. However the difference in total energy expenditure is still relatively small, maybe 50-75 calories (it’s kind of hard to tell based on the graphs).
When looking through this paper we can see that there does appear to be some evidence that people’s metabolisms will respond differently to under-eating and overeating. Some people will increase their metabolic rate less than others when they overeat and some people may decrease their caloric rate less than others when they undereat.
I think it is important to think about studies like this in real terms though and I think this study is a good example because we have some hard caloric numbers from the data.
If you are someone who has a “thrifty” metabolism, meaning your metabolism slows more than average when under-eating and speeds up less when overeating, you might assume that it is harder for you to lose weight and easier for you to gain weight. However the hard data from this study is not really all that bad for thrifty individuals.
First, thrifty individuals tended to have a higher metabolism when at energy balance. Basically this translates to you eating more and still maintaining your weight, which is certainly a good thing! Second, the difference between the slow down in your metabolism due to undereating when compared to non-thrifty individuals (spendthrifty) was 34 calories. In the overfeeding condition, the difference in metabolic rate was 58 calories less.
THESE ARE INCREDIBLY SMALL NUMBERS!!!!
If you are eating 2000 calories a day (and I think all adults should be eating that or more!), that’s between 1 and 3% of your total calories.
What does this mean in terms of weight loss and weight gain?
Maybe it means you need to eat a tiny bit less, or walk a tiny bit more to make up the difference and it would be so small it would be meaningless!
As we know body composition changes are less about acute changes and more about the long term trends. While this paper was quite comprehensive in testing a number of variables related to diet, they were limited on time and also did not consider the role other environmental impacts have on metabolism (like sleep, exercise, stress).
For one we cannot say that for example if you don’t eat for 1 day and your metabolism slows down by 300+ calories (like it did for some of the “thriftiest” people in this study) how that translates to a reasonable caloric deficit extended out over the course of weeks and months. We know there will likely be some metabolic adaptation but to what degree or extent that happens is unknown based on this study.
We also know that overeating over the long term will lead to weight gain. However, we also know that when people eat more they tend to be more active and in many cases people don’t gain weight, and sometimes lose weight . Also when you overfeed and combine that with a well structured resistance training program you may gain weight, but that could very well be due to gaining muscle!
At the end of the day, I think that worrying about fluctuations in your metabolism is not useful for most people. Will your metabolism shift based on whether you are under-eating, overeating, or at maintenance? Yes for sure!. Is that shift in your metabolism likely to be detrimental to your goal of gain, losing, or maintaining your weight? I am going to guess not.
What I would put your energy into is making sure you are consistent with your food intake, track your body composition changes with the scale plus something like circumference measurements, the way your clothes fit, pictures or any other measure you like, focus on moving throughout the day getting roughly 7-10k steps,incorporate resistance training and cardio into your exercise regime, and sleep like its your job!
If you do this with consistency, day in, day out, week after week, month after month, fluctuations in your metabolism will be meaningless towards your body composition goals. You will be able to eat an adequate amount of calories, feel full, physically and mentally perform at your best, and be healthy…what more would you want?!?!?!
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- (2021, July 5). Reduced adaptive thermogenesis during acute protein-imbalanced …. Retrieved September 21, 2022, from https://www.ncbi.nlm.nih.gov/pmc/articles/PMC8488870/ ↑
- (n.d.). Effects of weight gain induced by controlled overfeeding on physical …. Retrieved September 23, 2022, from https://www.ncbi.nlm.nih.gov/pmc/articles/PMC4254990/ ↑