Quick note: This post was a response to a reader’s question in The Vitamin, who asked whether losing weight faster leads to greater metabolic adaptations than losing weight slowly.
Q: I’m 5’9″, 235 lbs, and my TDEE is approx 2400. I’m eating 1500 calories every day with enough protein and enough fat and carbs. My question is, will my metabolism be greatly reduced due to the size of my caloric deficit? I read somewhere that having a huge calorie deficit causes our metabolism to lower significantly. Is it true?
The short answer–
Based on your stats, you’re probably ok since your deficit isn’t that aggressive (it works out to around 0.8% of total bw/week vs the recommended 0.5-1%). Just be mindful of your hunger, energy levels, and training performance. As long as these are ok, I wouldn’t worry about the extent of metabolic adaptation. Your metabolism will drop since that’s the nature of weight loss. However, if you continue resistance training and eat enough protein, the degree of metabolic adaptation probably won’t be that much and will largely be reversed as you return to maintenance.
The deep-dive
Before we begin, I want to briefly explain what metabolism is since most people think of it as a single entity, but it consists of a few separate parts:
- Resting metabolic rate (RMR): The number of calories your body burns at rest to maintain basic functions like breathing, circulation, and cell repair. It’s also what most people think of when they hear the word ‘metabolism’, and research on metabolic adaptation primarily focuses on changes in RMR because it accounts for the largest portion of your total daily energy expenditure (around 60–70%).
- Thermic effect of feeding (TEF): The calories your body burns digesting and storing the food you eat.
- Physical activity energy expenditure (PAEE): The calories burned through intentional exercise.
- Non-exercise activity thermogenesis (NEAT): This is the number of calories burned via unintentional exercise, like walking around, fidgeting, etc.
All four of these components are collectively called total daily energy expenditure (TDEE) or metabolism––i.e. the total number of calories you burn each day.
With that out of the way, you could argue that losing weight faster could lead to a bigger drop in the total number of calories you burn each day because:
- A larger deficit is likely to cause more fatigue, so you’ll move around less, reducing the number of calories you burn via NEAT.
- A larger deficit also means you’re eating fewer calories, so your body isn’t using as many calories for storage and digestion.
- Finally, larger deficits could increase the risk of muscle loss, 1 which could reduce RMR since fat-free mass has the biggest influence on your resting metabolic rate.
With that said, there are two things to note. First, the biggest adaptation seems to occur in the NEAT component of the metabolism (up to 90%). 2 Second, the magnitude of metabolic adaptation seems linked with the total amount of weight lost––i.e. the more weight you lose, the more your metabolism adapts. You can see this in the image below, taken from a study by Ostendorf and colleagues. 3
So there’s a case to be made that when people lose weight faster, they lose more weight overall (in the same timeframe as someone losing gradually), and the greater reduction in their metabolism is just a byproduct of the total weight lost rather than how quickly they lost it.
This leads to the next question: Is there a difference in metabolic adaptation––i.e. how much metabolism is reduced during weight loss––when total weight loss is matched between people losing faster and slower?
When you review the research, you generally see that people who lose weight faster tend to experience a greater decline in their resting metabolic rate.
For example, a group of researchers published a meta-analysis in 2020, pooling all the studies that compared gradual vs. rapid weight loss and what effect the rate of loss had on body composition and RMR (resting metabolic rate). 4
Both groups lost the same total amount of weight, around 7kg/15lbs, but participants in the gradual weight loss studies lost an average of 0.5kg/week (~1 lb/week), while the participants in the rapid weight loss studies averaged 1.3kg/week (2.9lbs/week).
The researchers found that losing weight gradually preserved metabolic rate better (-87.5 kcal) than losing weight rapidly (–136.9 kcal). It can be tempting to take these findings at face value and conclude that losing weight faster is more detrimental to your metabolism. However, there are a few crucial things worth highlighting because they change how we view the study’s findings.
Firstly, the weight of a single study dominated the meta-analysis for resting metabolic rate (RMR).
One study contributed 88% of the weight, meaning that while the meta-analysis technically included multiple studies, the results heavily reflect this single dominant one. It’s like trying to determine the average height of a population but only measuring three people—if one of them is abnormally tall or short, the result will skew in their favour. Similarly, when one study carries that much influence, the overall conclusion becomes less about what all the studies say and more about what that one study says.
Secondly, only four studies were included in the RMR analysis.
This is a very small sample, and with so few studies, the meta-analysis becomes much more susceptible to being influenced by individual studies. It also limits how representative the findings are since the conclusions are based on such a narrow slice of the available evidence. The smaller the pool of studies, the less robust the overall conclusion.
Thirdly, there were inconsistencies across the included studies that make the results less reliable.
Some studies didn’t report the dietary contents of their interventions, which matters because things like protein intake can have a big impact on muscle preservation.
Fourthly, and possibly most importantly, most participants in the included studies weren’t resistance training or eating enough protein
(I say ‘most’ because only one study included resistance training.)
This is the biggest limitation because protein and resistance training are well-established strategies for preserving muscle mass during weight loss. Moreover, muscle mass is a major contributor to resting metabolic rate (RMR), and failing to preserve muscle can lead to more significant metabolic slowing.
So, a more accurate interpretation of this meta-analysis is that people who lose weight faster without adequate protein intake and resistance training might experience larger reductions in their (resting) metabolism than individuals who lose weight gradually. But we can’t extrapolate these findings to people who are lifting weights and eating enough protein.
It’s also worth highlighting the metabolism is flexible. As long as you implement resistance training and eat enough protein, any reductions in RMR will largely be reversed as you return to maintenance.
As one example relevant to this topic––Coutinho and colleagues conducted a randomised controlled trial comparing rapid to gradual weight loss. 5
In their study, RMR dropped significantly during the weight loss phase in the rapid weight loss group, by about 130 kcal, compared to a much smaller, non-significant drop of 24 kcal in the gradual weight loss group. During the weight maintenance phase, both groups experienced a rebound in RMR, with the rapid group showing an increase of approximately 90 kcal and the gradual group an increase of about 60 kcal.
So, what does this mean for you?
Based on your stats, your deficit isn’t particularly aggressive—about 0.8% of your total body weight per week. For context, I typically recommend losing 0.3–0.7% of total body weight per week, and 0.8% still falls within the generally recommended range of 0.5–1%.
As such, you’re probably ok as long as diet adherence isn’t an issue and your training performance isn’t negatively impacted. You will experience some metabolic adaptation since that’s par for the course when losing weight, and you might experience more or less metabolic adaptation than someone else since there’s a lot of inter-individual variability, but most of this will dissipate as you transition out of the deficit.
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