Your basal metabolic rate (BMR) tells you how much energy your body burns to just “keep the lights on” – it’s the energy used to power the basic functions of your vital organs, to accomplish sufficient protein and cell turnover to keep your tissues functioning properly, and more. If you didn’t leave your bed all day and didn’t move a muscle, your basal metabolic rate is the amount of energy you’d still burn in a day.
We’re approaching the end of our BMR series. We’ve discussed the (current) best formulas to estimate BMR, we’ve covered the determinants of BMR, and we’ve addressed how sex, age, and athletic status impact BMR. Now, we’ve reached our final pair of factors that influence BMR: weight gain and weight loss. This article will address weight loss, and the next will address weight gain.
Type and amount of tissue lost
For starters, losses of body tissue will decrease BMR, proportional to the metabolic rates of the tissues that are lost. In other words, muscle has a BMR of about 13 Calories per kilogram, adipose tissue has a BMR of about 4.5 Calories per kilogram, and the liver has a BMR of about 200 Calories per kilogram. So, if you lost 5kg, including 3kg of muscle, 1.9kg of fat, and 100g of liver tissue, you’d expect your BMR to decrease by about 67-68 Calories per day due to the composition of the tissue you lost. Losing the same 5kg, while only losing 5kg of fat, would only be expected to decrease BMR by 22-23 Calories.
| Illustration of how tissue composition affects BMR changes with weight loss | ||||
| Scenario | Tissue | Loss | Tissue-specific BMR | BMR decrease |
| A) Greater loss of fat-free mass | Muscle | 3kg | 13 Calories/kg | 39 Calories |
| Adipose Tissue | 1.9kg | 4.5 Calories/kg | 8.55 Calories | |
| Liver | 0.1kg | 200 Calories/kg | 20 Calories | |
| Total | 5kg | Varies based on tissue composition | 67.55 Calories | |
| B) Exclusively losing fat | Adipose tissue | 5kg | 4.5 Calories/kg | 22.5 Calories |
So, the impact of weight loss on BMR will depend, in part, on the type and amount of tissue you lose. If you don’t lose much weight, you shouldn’t expect your BMR to change by very much, whereas greater weight loss generally brings larger decreases in BMR. Furthermore, if you lose a considerable amount of weight, but you primarily lose fat tissue, you should expect to see a smaller decrease in BMR as a result, since fat tissue has such a low tissue-specific BMR. However, if you experience a larger decrease in fat-free mass (especially in the form of high-metabolic rate tissues), you should expect to see a considerably larger decrease in BMR.
Thus, to mitigate reductions in BMR as you lose weight, you should aim to lose weight gradually, since faster rates of loss and larger energy deficits lead to larger losses of fat-free mass (per unit of weight loss) than slower rates of weight loss and smaller energy deficits. Furthermore, exercise – and resistance training in particular – will also help you lose more fat and less fat-free mass per unit of weight loss, further mitigating reductions in BMR. Finally, maintaining a relatively high protein intake will help you preserve more fat-free mass as you lose weight, especially when paired with exercise and a relatively slow rate of weight loss. If you do all three, you might even be able to gain fat-free mass as you lose weight. All three of these things will help cushion the reductions in BMR that accompany weight loss.
Metabolic Adaptations to Weight Loss
Unfortunately, the amount and composition of the tissues you gain or lose only tell you half of the story. As you lose weight, your BMR tends to decrease more than would be predicted due to tissue losses. This phenomenon is referred to as “metabolic adaptation.”
The research on metabolic adaptation can be a bit muddled at first glance, because the term is sometimes used to refer to a few different concepts that are extremely similar, but meaningfully distinct.
- Sometimes “metabolic adaptation” refers to decreases in BMR that are larger than would be predicted based on changes in total fat-free mass (or changes in total fat mass and total fat-free mass).
- Sometimes “metabolic adaptation” refers to decreases in BMR that are larger than would be predicted based on granular body composition changes, including gains or losses in specific organ masses.
- Sometimes “metabolic adaptation,” or the more general term “adaptive thermogenesis,” is used to refer to all reductions in energy expenditure (including decreases in energy expenditure due to changes in activity patterns) that occur during weight loss. That’s not what we’ll be focusing on in this article.
This is an important distinction, because research using the first definition (reductions in BMR adjusted for changes in total fat-free mass) tends to report greater metabolic adaptation than research using the second definition (reductions in BMR adjusted for changes in granular body composition). Ultimately, the first definition is more relevant for most people in most contexts, whereas the second definition is more useful for researchers who are interested in gaining a deeper understanding of the physiology underpinning the phenomenon (and the third definition is relevant for an entirely different conversation).
To illustrate, let’s assume that you lose 10kg, including 3kg of fat-free mass. Based on the 1991 Cunningham equation, you’d expect a 3kg decrease in fat-free mass to result in a BMR decrease of 21.6 × 3 = 64.8 Calories. But, let’s assume that your BMR instead decreases by 100 Calories.
Using the first definition, you’d say that your BMR decreased by about 65 Calories due to the loss in fat-free mass, and the additional 35 Calorie decrease is due to metabolic adaptation.
However, an MRI might reveal that your liver and kidneys decreased in size as you lost 10kg. This loss of high-metabolic-rate tissue would lead one to anticipate a larger decrease in BMR than the “raw” decrease in total fat-free mass would predict. So, based on the granular body composition changes that occurred with weight loss, you might expect your BMR to decrease by 85 Calories.
So, using the second definition, you’d say that your BMR decreased by 85 Calories due to the specific composition of the tissues you lost, and the additional 15 Calorie decrease is due to metabolic adaptation.
Ultimately, I don’t think either definition is inherently better or worse. It’s just useful to be aware of the distinction.
The second definition is extremely useful for researchers who are interested in understanding why BMR decreases with weight loss. With the first definition, you can’t disambiguate between decreases in BMR resulting from the loss of high-metabolic-rate tissues, and decreases in BMR resulting from a generalized metabolic slowing in your remaining tissues. But, if you fully itemize specific tissue losses, the excess decrease in BMR after accounting for losses in each specific tissue compartment can more confidently be attributed to generalized metabolic slowing (i.e. “true” metabolic adaptation).
However, as mentioned before, I tend to think the first definition is a more useful definition for most people, most of the time. You might be able to roughly estimate your body fat percentage (and, by extension, your total fat-free mass) as you lose weight, but it’s extremely unlikely that you’ll get full-body MRIs and estimate the mass of each of your organs before and after a weight loss attempt. If your BMR decreases by 200 Calories when you’d expect it to decrease by 100, you’ll probably have no way of knowing if you lost a bit more high-metabolic-rate tissue than expected, or if your remaining tissues experienced a decreased rate of energy expenditure. All you’ll know is that your BMR decreased by 100 Calories more than you thought it would.
A 2022 study by Martin and colleagues helps illustrate these distinctions. Over 12 months, 109 subjects lost an average of 8kg of total body mass, including 7.2kg of fat mass. During that time, BMR decreased by an average of about 100 Calories per day.
Based on the minimal decrease in lean mass observed (-0.8kg), you’d expect BMR to only decrease by about 17 Calories according to the Cunningham equation (21.6 Calories per kilogram of fat-free mass x 0.8 kg of fat-free mass = 17.28 Calories). So, you’d conclude that BMR decreased by about 17 Calories due to decreases in fat-free mass, and the additional 83 Calorie decrease was due to metabolic adaptation.
If you wanted to directly account for decreases in fat mass as well, you might predict a further decrease of 4.5 Calories per kilogram of fat lost (32.4 Calories, since fat mass decreased by 7.2 kilograms), for a total decrease of about 50 Calories. So, you’d conclude that BMR decreased by about 50 Calories due to losses in fat mass and fat-free mass, and the additional 50 Calorie decrease was due to metabolic adaptation.
Finally, you could do what the researchers in the study did: take full-body MRIs to estimate changes in specific organ masses. The subjects lost about 100g of high-metabolic-rate organ tissue, leading to a slightly larger predicted decrease in BMR: about 60 Calories per day. So, the researchers concluded that BMR decreased by about 60 Calories due to specific tissue losses, and the additional 40 Calorie decrease was due to metabolic adaptation.
So, how much metabolic adaptation actually occurred? Was it 83 Calories, 50 Calories, or 40 Calories?
For metabolism researchers, probably 40 Calories. For you, the reader … probably either 83 Calories or 50 Calories, but I also truly don’t think the answer to the question matters. Either way, your BMR decreased more than would be expected, given how much weight (and how little fat-free mass) you lost.
How much metabolic adaptation occurs?
Using the first definition of metabolic adaptation provided above, we tend to see metabolic adaptation on the scale of around 5-10%; in other words, during weight loss, BMR decreases to a level that’s 5-10% lower than would be predicted, based on changes in total fat mass and fat-free mass. So, if your BMR before weight loss was 2200 Calories per day, and you lose an amount of tissue that would be expected to decrease your BMR to 2000 Calories per day, your BMR will likely be about 5-10% lower than that: closer to 1800-1900 Calories per day.
You can certainly find reports of significantly greater metabolic adaptation, especially when people are exposed to very large energy deficits, or when they achieve extreme levels of leanness. For instance, metabolic adaptation was perhaps most famously observed in the infamous Minnesota semi-starvation study, where men were fed approximately 1200 Calories per day for six months while doing hard labor. In that study, BMR decreased to a level that was more than 20% lower than would be expected, based on changes in total fat mass and fat-free mass. Similarly, a case study in a competitive bodybuilder found that his BMR decreased from approximately 2500 to 1400 Calories per day during the leadup to a bodybuilding contest, despite only losing about 3kg of fat-free mass. His BMR dropped from about 28.5 Calories per kg of fat-free mass to 16.5 Calories per kilogram of fat-free mass, which suggests that he experienced metabolic adaptation of approximately 40%. But, during this time, he got down to 4.5% body fat, and his resting heart rate dropped all the way to 27 beats per minute; he was clearly in a much more physiologically extreme state than most people would experience when dieting (and, for what it’s worth, most physique athletes don’t experience quite that much metabolic adaptation when dieting).
On the flip side, considerably less metabolic adaptation is frequently observed when dieting. In the Martin study cited above, a sizeable minority of the subjects actually experienced a (typically small) increase in BMR despite tissue losses. However, most of these subjects didn’t lose a ton of weight to begin with, and their rate of weight loss was very slow (less than 1kg per month).
But, on the whole, metabolic adaptation of about 5% is fairly typical when you’re not losing much weight (<10% of initial body weight), and when weight loss is fairly slow. Conversely, metabolic adaptation of about 10% is more typical with greater total weight loss and faster rates of weight loss (on top of the expected BMR decreases due to larger losses of lean mass).
As previously mentioned, there’s considerable inter-individual variability – some people will experience more metabolic adaptation, and some people experience less. Some people even experience (typically small) increases in BMR during (typically minor) weight loss. Furthermore, if you plan to go on a complete crash diet, or if you plan to diet to extreme levels of leanness, you might experience metabolic adaptation exceeding 15% or even 20%. But, for most people, most of the time, 5% is fairly typical when losing weight gradually, and when losing less than 10% of your initial body weight, and 10% is fairly typical for more aggressive diets (in terms of either rate of weight loss, or total weight loss achieved).
Why does metabolic adaptation occur?
There are two different ways to answer the question posed by this section header: a teleological answer, and a physiological answer.
The teleological answer1 relates to survival of the species. During periods of famine, the human body evolved to conserve energy, in order to increase the probability that an organism will survive and reproduce. Your body doesn’t “know” that you want to fit into a smaller pants size or achieve a shredded six pack. It just knows that you’re consistently consuming less energy than you’re burning, and if that trend continues, you won’t be able to pass on your genes. So, it conserves energy as much as it can in an attempt to keep you alive.
The physiological answer is still being investigated. I don’t think we have a full accounting of precisely why metabolic adaptation occurs, but we do at least have a partial answer.
The “classic” explanation is that fat loss decreases levels of a hormone called leptin. Leptin is sensitive to both short-term energy status (are you in an energy deficit or surplus?) and long-term energy status (are your total fat stores higher or lower than they used to be?). Decreased leptin concentrations have a wide range of effects that make weight loss more difficult, including a down-regulation of energy expenditure.
Other research has found that metabolic adaptation is related to changes in thyroid hormone levels. Thyroid hormones – specifically T3 – regulate energy expenditure at the tissue level.
Furthermore, mitochondrial adaptations may influence metabolic adaptation. On average, human mitochondria are about 40% efficient. In other words, they convert about 40% of the chemical energy in food into ATP that can be used to power various cellular processes – the rest of the energy is lost as heat. But, the “40% efficiency” heuristic isn’t an immutable rule – mitochondria can adapt to be more efficient or less efficient in response to a variety of stimuli. For our purposes here, there’s some research (primarily in rodents, though there is a bit of human evidence) indicating that weight loss increases mitochondrial efficiency. That’s “good” in a general sense – all else being equal, greater mitochondrial efficiency enhances physical performance – but it also directly decreases the amount of energy you expend per unit of fat or carbohydrate your body metabolizes.
Finally, if we include losses in organ mass as a component of metabolic adaptation (using the first definition above) … losses in organ mass contribute to disproportionate decreases in BMR per unit of fat-free mass. Losses in organ mass may be driven by some of the hormonal changes mentioned above, or they may simply be the result of eating (and typically burning) less. If you’re consuming and producing fewer biomolecules that your liver needs to metabolize, and you’re producing fewer waste products your kidneys need to filter, it shouldn’t be too surprising if they shrink a bit in response to a reduced workload.
Can we reduce metabolic adaptation?
The biggest thing you can do to reduce metabolic adaptation is to lose weight at a slower rate.
In a 2020 meta-analysis by Ashtary-Larky and colleagues, the researchers pooled the results of seven studies examining the effects of gradual weight loss (averaging about 0.5kg per week) and rapid weight loss (about 1.25kg per week) leading to similar amounts of total weight loss (about 7.5kg). On average, rapid weight loss led to a loss of 1.6kg of fat-free mass, and a reduction in BMR of 137 Calories per day – about 100 Calories more than you’d expect, given the typical loss of fat-free mass. On the other hand, gradual weight loss led to a loss of 0.6kg of fat-free mass, and a reduction in BMR of 87.5 Calories per day – about 75 Calories more than you’d expect, given the typical loss of fat-free mass.
So, a slower rate of weight loss meant that it took subjects about 2-3 times longer to achieve their weight loss target, but they lost a bit less fat-free mass in the process, and experienced about 25% less metabolic adaptation.
| Effects of Gradual vs. Rapid Weight Loss (Basic Summary of the Meta-Analysis by Ashtary-Larky) | ||
| Rapid | Gradual | |
| Rate of weight loss | -1.25kg per week | -0.5kg per week |
| Total weight loss | -7.7kg | -7.5kg |
| Loss of fat-free mass | -1.6kg | -0.6kg |
| Total decrease in BMR | -136.9 Calories | -87.5 Calories |
| Metabolic adaptation* | -102.3 Calories | -74.5 Calories |
| *Decrease in BMR beyond what reductions in FFM would predict, based on the 1991 Cunningham Equation | ||
A 2024 meta-analysis by Poon and colleagues had similar results. This meta-analysis examined the effects of continuous versus intermittent dieting strategies. But, the net effect of most of the intermittent dieting strategies was simply to reduce the average rate of weight loss. For example, a continuous dieting strategy may involve 18 straight weeks of maintaining a daily deficit of 500 Calories, while an intermittent strategy may involve 36 weeks alternating between a 500 Calorie daily deficit for two weeks, followed by two weeks at maintenance (twice the duration, but the average daily deficit is just 250 Calories per day).
Total weight loss was similar with both continuous and intermittent strategies (about 5-5.5kg), and losses of fat-free mass were similar as well (0.99kg with intermittent, and 0.65kg with continuous). But, despite losing slightly more fat-free mass, subjects in the intermittent dieting groups experienced smaller reductions in BMR (about 39 Calories, versus about 92 Calories with continuous strategies).
And … that’s about it. The two other strategies we tend to look toward to get us out of most metabolic jams are exercise and increased protein intake. Unfortunately, they’re not of much help in this instance.
To be clear, higher protein intakes do help you maintain more fat-free mass when dieting. But, metabolic adaptation refers to decreases in BMR in excess of losses in fat-free mass. So, higher protein intakes will help you maintain a higher BMR because they’ll help you maintain more fat-free mass when dieting, but I’m not aware of any research showing that higher protein intakes reduce metabolic adaptation.
As for exercise, there’s an entire relevant body of literature that’s basically investigating the impact of energy deficits in athletes: RED-S research. RED-S stands for Relative Energy Deficiency in Sport. It’s not worth getting into the nuances of RED-S in this article, but a defining characteristic of RED-S is low energy availability. Energy availability in this context is calculated by subtracting exercise energy expenditure from total energy intake, and dividing the resulting value by fat-free mass. Values under 30 Calories per kilogram of fat-free mass are defined as “Low Energy Availability.” Since BMRs are typically around 30 Calories per kilogram of fat-free mass, being in a low energy availability state essentially means being in an energy deficit … unless you have a reduced BMR due to chronically low energy availability.
RED-S and low energy availability in athletes are reliably associated with lower BMRs in both male and female athletes. In fact, having a BMR that’s at least 10% lower than would be expected (based on the athlete’s fat-free mass) is frequently used as a screening tool to assess an athlete’s risk of RED-S.
So, much like higher protein intakes, exercise will help you maintain more fat-free mass when dieting, and as a result, it will likely lead to smaller total BMR reductions while you’re losing weight. But, the RED-S research suggests that people who exercise a lot experience about the same magnitude of metabolic adaptation as everyone else.
Does metabolic adaptation last forever? Are you “damaging” or “crashing” your metabolism?
Thankfully, no. Most of the metabolic adaptation that occurs during weight loss is reversed once you stop losing weight and return to energetic maintenance.
All the way back in 1999, Astrup and colleagues meta-analyzed the studies comparing BMRs in formerly obese subjects (who’d previously lost a substantial amount of weight) to matched control subjects who’d never been obese. They found that BMRs were only about 3-5% lower in formerly obese subjects than subjects who’d never been obese.
In the same year, a study examined the BMRs of individuals in the National Weight Control Registry (NWCR). To join the NWCR, you need to lose at least 30 pounds, and maintain that weight loss for at least one year. When compared to weight-matched control, the subjects from the NWCR had virtually identical BMRs.
More recently, Nunes and colleagues performed a systematic review in 2021, investigating the impact of weight loss on metabolic adaptation. They found that most (though not all) studies report a significant reduction in metabolic adaptation – or even a complete abolishment of metabolic adaptation – after a period of neutral energy balance and weight stability.
So, although metabolic adaptation of about ~5-10% is typically observed during periods of significant weight loss, most of that effect goes away once you spend some time at energetic maintenance. Until an updated meta-analysis is conducted, I think the results of the Astrup meta-analysis give us a decent estimate of the typical metabolic adaptation that persists following significant weight loss.
To be clear, your total BMR will likely decrease quite a bit if you lose a lot of weight, but MOST of that reduction will simply be the result of now living in a smaller body. To illustrate, your BMR before significant weight loss may have been 2000 Calories per day. By the end of your diet, it decreased to 1500 Calories per day. After a period of weight maintenance and neutral energy balance, it stabilizes at 1600 Calories per day. So, in total, it decreased by 400 Calories. But, the average BMR for people with a similar body size and body composition (who’d never needed to lose a significant amount of weight) may by 1650 Calories per day. Thus, the total persistent reduction in BMR may be fairly large, especially if you lose quite a lot of weight, but the persistent metabolic adaptation (reductions in BMR below what would be expected) may be quite small, or even nonexistent. You’re not “crashing” or “damaging” your metabolism – metabolic adaptation is a perfectly normal part of weight loss, and it largely reverses once you stop losing weight.
What about “The Biggest Loser” study?
Before wrapping this article up, I’d be remiss if I didn’t at least mention the infamous “Biggest Loser” study. Many people believe that BMRs are permanently reduced to a dramatic degree following weight loss, and this is the study most strongly influencing those beliefs.
Just for context, “The Biggest Loser” was an American TV show that turned fat shaming into a twisted form of entertainment and crash dieting into a competitive sport. It’s thankfully off the air now, but during its run, contestants would eat as little as possible and exercise as much as possible to lose up to 250 pounds over 30 weeks.
In 2016, a study was published detailing metabolic and body composition changes during one season of the Biggest Loser competition, with a follow-up six years later.
On average, subjects weighed about 150kg before the competition, 90kg at the end of the competition, and 130kg six years after the competition. They started with 75.5kg of fat-free mass, ended the competition with 64.4kg of fat-free mass, and had 70.2kg of fat-free mass six years later. Finally, their average BMR was reported to be about 2600 Calories per day before the competition, 2000 Calories per day after the competition, and 1900 Calories per day six years later.
| Summary of the findings of “The Biggest Loser” Study | |||
| Baseline | End of competition at 30 weeks | Follow-up at 6 years | |
| Age (y) | 34.9±10.3 | 35.4±10.3 | 41.3±10.3 |
| Weight (kg) | 148.9±40.5 | 90.6±24.5 | 131.6±45.3 |
| % Body fat | 49.3±5.2 | 28.1±8.9 | 44.7±10 |
| FFM (kg) | 75.5±21.1 | 64.4±15.5 | 70.2±18.3 |
| Measured BMR (kcal/d) | 2607±649 | 1996±358 | 1903±466 |
| Predicted BMR (kcal/d) | 2577±574 | 2272±435 | 2403±507 |
| Metabolic adaptation (kcal/d) | 29±206 | −275±207 | −499±207 |
| Anthropometric and energy expenditure variables in 14 of the original 16 study subjects who participated in the 30 week Biggest Loser weight loss competition. The predicted RMR was obtained using a linear regression equation developed using baseline data on body composition, age, and sex in the full 16 subject cohort. From Fothergill et al. (2016) | |||
Based on a regression model developed from the subjects’ pre-study data, it was reported that subjects experienced 275 Calories of metabolic adaptation during the competition. Furthermore, instead of metabolic adaptation attenuating over time (as subjects re-gained weight), it was reported that metabolic adaptation actually increased to nearly 500 Calories over the next six years.
The only two things that truly need to be said about this study are:
- It’s a pretty notable outlier in the scientific literature, both in terms of the total magnitude of metabolic adaptation that occurred, and since it reported that metabolic adaptation increased over time, instead of attenuating.
- The way subjects lost weight was particularly extreme. So, if you don’t intend to crash diet, do intense exercise for over three hours per day, and lose an average of 2kg per week (or up to nearly 4kg per week for the contestant that lost the most weight), you should probably pay more attention to the larger body of research using much more reasonable approaches to weight loss.
But, there are two more things worth pointing out about this study.
First, the subjects had extremely high BMRs at baseline. To bring back the graph showing BMRs as a function of fat-free mass in athletes from a prior article in this series, you can see where the Biggest Loser contestants would fall at baseline, relative to the trendline.
As you can see, their baseline BMRs would be quite a bit above average for competitive athletes, and extremely high for people who had relatively low levels of activity before they went on the show. According to the 1991 Cunningham equation, their predicted BMRs at baseline would be almost exactly 2000 Calories per day (meaning their measured BMRs were about 600 Calories higher than would be predicted, based on their fat-free mass).
So, to the extent that metabolic adaptation occurred and persisted in the Biggest Loser participants, their BMRs were still somewhere between “very high” and “extremely average” relative to their fat-free mass at all time points. Thus, one potential explanation for the outlier findings in this study is just that the baseline BMR values were unrealistically high for some reason, and they settled back to more normal values in subsequent measurements. For instance, short-term overfeeding can elevate BMR by about 10% (as we’ll discuss in the next article), and I find it plausible that the contestants may have tried to “bulk up” a bit before going on the show to make it a bit easier to lose a lot of weight during the early weeks and avoid elimination. If something like that occurred (or if baseline BMR was unrepresentative for any other reason), that would mean subsequent estimations of “metabolic adaptation” would all be overstated.
Second, and more importantly, different metabolic carts were used to assess BMR throughout the study. The MAX-II metabolic cart was used for the baseline and post-competition measurements, and a ParvoMedics cart was used for the six-year follow-up. That may not mean much to most readers, but if you’ve spent much time reading metabolism research, you’re definitely familiar with ParvoMedics; they’re one of the top manufacturers of indirect calorimeters, and Parvo carts are trusted by metabolism researchers world-wide due to their excellent track record of accuracy and reliability. But, much like myself, I suspect you weren’t familiar with the MAX-II – this was the first study I’d seen using this particular metabolic cart.
Two years after The Biggest Loser study was published, a validation study by Kaviani and colleagues showed why the MAX-II isn’t frequently used in research. It found that the MAX-II overestimates VO2 by about 7%, and VCO2 by about 4.5% (whereas both Parvo carts tested in the same study had errors of at most 1.2%). So, the MAX-II would be expected to overestimate BMR by around 6.5%.2
Given the bias of the MAX-II cart used to take the pre- and post-competition measurements, the magnitude of the persistent metabolic adaptation was likely quite a bit smaller than the study reported. Instead of a baseline BMR of about 2600 Calories per day, the actual average baseline BMR was likely closer to 2435 Calories per day. Instead of a post-competition BMR of about 2000 calories per day, the actual average post-competition BMR was likely closer to 1865 Calories per day. So, rather than decreasing by nearly 100 Calories per day while subjects re-gained weight over the next six years, that would mean their BMRs actually slightly increased from about 1865 to about 1900 Calories over the next six years. That still wouldn’t paint the rosiest picture in the world, but it would slash the persistent metabolic adaptation by about a third – from about 500 Calories to about 330 Calories.
| Estimated Metabolic Adaptation in “The Biggest Loser” Study After Adjusting for Max-II Bias | |||
| Baseline | End of Competition | 6-year Follow-up | |
| Measured BMR (Calories/day) | 2577 | 1996 | 1903 |
| BMR Adjusted for Max II bias | 2409.5 | 1866.3 | 1903 |
| Reported metabolic adaptation (Calories/day) | -275 | -499 | |
| Bias-adjusted metabolic adaptaton (Calories/day) | -257.1 | -331.5 | |
Put it all together, and a slightly different picture emerges: even with the errors produced by the MAX-II cart, the contestants STILL had quite high BMRs at baseline. Following the competition, they experienced some metabolic adaptation. Over the next six years, their BMRs increased a bit as they regained weight, though perhaps still not quite as much as one would predict. However, both post-competition, and six years later, they were experiencing around 300 Calories of metabolic adaptation – a little less than 300 immediately post-competition, and a little more than 300 six years later, but the difference isn’t worth writing home about. Furthermore, their BMRs at both post-competition time points were perfectly normal compared to the general population, and maybe slightly on the high side, relative to their fat-free mass.
In short, this study doesn’t show that being on the Biggest Loser totally and permanently cratered the participants’ metabolisms. It does still suggest that a significant amount of metabolic adaptation occurred, and that the metabolic adaptations persisted – and perhaps even increased – for the next six years. But, the study likely overstated the magnitude of the persistent metabolic adaptation by nearly 50% (it was likely closer to 330 Calories than 500 Calories), due to issues with the metabolic cart used to take the baseline BMR measurements. Furthermore, at all time points, the contestants had normal-to-high BMRs relative to the general population, and relative to their fat-free mass. And, just to reiterate, the results of this study do still run counter to most of the research on the topic, and the approach to weight loss used on the Biggest Loser is still unrepresentative of how most people lose weight. So, it would be unwise to assume that the Biggest Loser study is representative of what happens following weight loss in most populations, most of the time.
Wrapping it up
This article was quite long, so I think we’re due for a brief recap:
- Your BMR will likely decrease as you lose weight. However, most of that decrease is simply due to losses in body tissue. It takes less energy to power a smaller body.
- Preserving fat-free mass by losing weight gradually, exercising (and resistance training in particular), and eating plenty of protein will help you lose more fat and less fat-free mass, which will mitigate reductions in BMR.
- Metabolic adaptation – reductions in BMR in excess of what you’d predict based on the magnitude and composition of the tissue you lose – is a normal part of weight loss.
- Metabolic adaptation of about 5-10% is pretty typical. More metabolic adaptation is generally seen with greater total weight loss, and faster rates of weight loss.
- Reducing your rate of weight loss can help mitigate metabolic adaptation. Smaller consistent energy deficits and intermittent dieting strategies seem to be similarly effective at reducing metabolic adaptation.
- Metabolic adaptation mostly goes away when you return to maintenance and your weight stabilizes. Though, slight metabolic adaptation may persist – around 3-5% seems typical (which usually amounts to less than 100 Calories per day), though many studies report that it goes away entirely.
- Don’t let your perception of metabolic adaptation be dictated by the Biggest Loser study. It used an extreme intervention, it’s an outlier in the literature, and its findings were likely exaggerated due to an equipment issue.
The next article in this series will discuss how weight gain affects BMR. Then, we’ll wrap up the main part of this series by using everything we’ve learned to improve upon the (current) best BMR equations. You can also try our BMR calculator, which incorporates all of the information covered in this series, in order to estimate your BMR as accurately as possible.
- Teleology is the branch of philosophy interested in the purpose a phenomenon serves, rather than the specific cause of the phenomenon ↩︎
- To give the researchers some credit, they did do some extremely preliminary validation work to justify their use of the MAX-II cart, but not enough to give me much confidence. Of the eight measurements they took with the MAX-II, half of them produced errors of around 8-12%. See the supplemental materials in the study. ↩︎
