‘You have to eat enough calories every day, otherwise your body will go into starvation mode and you’ll never lose weight!’
I have to admit that I’ve said this more than a few times in the 21 years I’ve been an RD. But is it really true? If we skip breakfast, or eat too few calories, will our body flip a switch into ‘starvation mode,’ slowing our metabolism and making it harder to lose weight?
Is starvation mode real?
Let’s examine the science and evidence behind ‘starvation mode.’
In order to really get to the bottom of things here, we need to start with a brief discussion about metabolism. I’ve written a more in-depth post about the topic here, but it’s important to understand the basics as it relates to starvation mode, otherwise known as ‘adaptive thermogenesis.
What we know about ‘starvation mode.’
Adaptive thermogenesis (also used in this post interchangeably with ‘metabolic adaptation’ and ‘starvation mode’) is defined in the research as ‘a decrease in resting energy expenditure (REE, also known as BMR) beyond what is predicted from changes in body mass and composition.’
Here’s what that means:
When you lose weight, you’ll have less mass overall and need fewer calories to support that mass, which is normal. A smaller person needs less energy.
Adaptive thermogenesis though, is a disproportionate reduction in the number of calories needed.
In other words, not just the relatively small decrease you’ll normally experience when you lose weight, but a larger, more consequential decrease that prevents further weight loss and leads to weight regain.
In my experience, adaptive thermogenesis is when someone comes to me saying that their metabolism is ‘broken.’ Or, what we call, ‘starvation mode.’
Because our basal metabolic rate is responsible for around 70% of our energy usage – spent just keeping us alive – the body has a vested interest in making sure it rations what energy it does get, especially when energy (ie calories) are restricted.
When fat and muscle loss occurs, our hormones adjust our BMR to try to make up for it. The body has an innate need to ‘defend’ against weight loss, so that we have enough fat stores to survive a famine.
Conserving energy means that none of it is wasted on energy over and above what you need to actually survive (the basal metabolic rate, or BMR), which can cause you to be moody, sluggish, cold, and unable to concentrate.
BMR stays constant most of the time. But when we start to lose fat and muscle through dieting, the body senses this, and goes into a state of conservation.
It’s believed thy some that this is just one reason why it’s so hard to keep off lost weight with restrictive diets – because when your body thinks it’s starving, it ramps up your hormones to slow metabolism.
You can’t blame your body for acting this way. It’s like, when you suddenly discover your bank account has $5 left in it, are you going to squander that money on a new pair of shoes?
Same idea. But it’s not all that simple.
That’s because the effect of adaptive thermogenesis where it relates to weight loss, is controversial.
Nobody is arguing that adaptive thermogenesis doesn’t exist, because we know it does.
But what a lot of people get wrong – including me, full disclosure – is that it may not be as clinically significant as we thought.
What’s the research on starvation mode?
Metabolic adaptation to weight loss has been studied a lot, and the most up-to-date research around it seems to say the same thing: it happens, not to the same extent for everyone, and it’s not a huge reason why we regain lost weight.
Mind blown. I learned a ton from going through this research, and I’ve picked out highlights from the latest studies for you here:
A 2020 study on metabolic adaptation in the American Journal of Clinical Nutrition cites several studies that metabolic adaptation was present after weight loss, but BMR overall was not ‘significantly lower’ after weight loss, and 1-2 years later, was not associated with weight regain.
Another 2020 study in the American Journal of Clinical Nutrition called metabolic adaptation an ‘illusion,’ found that subjects experienced metabolic adaptation through their weight loss, but that it rebounded after a period of ‘weight stabilization.’
This study also found that metabolic adaptation was not responsible for weight regain at the one-year mark.
It’s important to note that weight stabilization – a period after weight loss where weight is stable – seems to lessen the impact of adaptive thermogenesis. It’s as though it gives the body time to ‘catch up,’ and the BMR to adapt somewhat.
A 2018 study in the American Journal of Clinical Nutrition also found that although adaptive thermogenesis exists, there’s not consistent evidence to show that it persists in people who have lost weight and have maintained that weight loss.
This same study (and this 2020 paper, as well) does agree that there are subsets who are more affected – as we already know, everyone is different – but overall, it’s not a given that losing weight results in a large, unexpected reduction in BMR.
Of course, we can’t talk about ‘starvation mode’ and not talk about The Biggest Loser.
During the show, participants lost huge amounts of weight, and their BMRs also decreased. It was determined that adaptive thermogenesis did account for a significant reduction in BMR at the end of the show.
Most participants gained some weight back after the show. Honestly, this is expected. Nobody can sustain the punishing craziness of The Biggest Loser (a whole other blog post).
But what happened to their BMR 6 years after the show ended?
Even with weight regain, the participants were still experiencing adaptive thermogenesis that accounted for around 500 calories per day. In other words, their metabolisms slowed more than expected, and didn’t bounce back. This was even more pronounced in contestants who lost the most weight.
But weirdly, the contestants who lost the most weight, kept the most weight off over 6 years. The above study found no correlation between adaptive thermogenesis and weight regain.
Why were The Biggest Loser contestants different than subjects in other research studies? Why did their adaptive thermogenesis persist for longer, and more significantly?
Was it the rate of loss? The amount lost? The methods used to lose weight during the show?
We don’t really know.
Kevin Hall, a researcher whose work I follow like a hawk, said in his 2018 review and response to the Biggest Loser research above that the extreme conditions during the show may be to blame for the adaptive thermogenesis that resulted.
Nonetheless, he believes that ‘Behavior is in the driver’s seat and REE is along for the ride’ – a statement that doesn’t sit well with me for the blaming of fat people for regaining weight, but also one that describes how behavior may be a driver for weight regain (or not), not adaptive thermogenesis.
As in, you’re going to need fewer calories after you lose weight, and if your behavior doesn’t change with your weight, you’ll likely gain the weight back.
So, is starvation mode real?
Overwhelmingly, the research points to ‘starvation mode’ as existing, of course, but being a lot less significant than we originally thought, and in most people, likely not a barrier to sustaining weight loss under normal (aka not Biggest Loser) circumstances.
Adaptive thermogenesis is often overblown in the media and by certain nutrition ‘experts.’ Skipping breakfast or intermittent fasting aren’t likely to put you into ‘starvation mode.’
Having a period of weight stabilization may help BMR recover to lessen the effects of adaptive thermogenesis.
While you might experience a drop in basal metabolic rate due to weight loss (over and above the drop in energy needed because you’re smaller), it’s not going to happen overnight, it probably won’t last forever, and it’s not likely going to be that consequential.