How Metabolic Adaptation Can Hinder Your Fat Loss

Part Two – What To Do About It

In the previous article, I played Herald of Doom and delivered the bad news about how adaptive thermogenesis coming over uninvited to empty your liquor cabinet and kick your dog. We established how our pesky genes like us to survive and mate and that they’re sensitive to changes in bodyweight.

As we lose weight, the hormone leptin tells the brain that the fuel tank is being depleted. Brain steps on the brakes (the faster the weight loss, the harder the brakes are applied) which amounts to a coordinated hormonal response to prevent further reductions in energy availability – increased hunger, reduced movement, increased movement efficiency and so on.

I left you with a nice silver lining: “Don’t worry! This is all normal 😊 good luck with your fat loss!”

…Great. Helpful.

Actually, there are some strategies we can employ to mitigate the adaptive response to weight loss. Since the problem is partly psychological (you feel hungry and tired) and partly biochemical (leptin, ghrelin, thyroid and co) these are based on behaviour and environment modification in combination with some strategic diet manipulation.

The most obvious place to start is the amount of weight loss. There are a few key factors that contribute:

1. The size of the energy deficit
2. The length of the diet period
3. How often you diet
4. Your starting bodyweight

To be very clear: metabolic adaptation occurs specifically in response to a reduction in body weight. The above factors will influence the degree of metabolic adaptation because they affect how much weight you lose and how fast.

The size of the energy deficit

Eating below maintenance calories is a pre-requisite for weight loss and the size of the deficit will determine how quickly bodyweight is lost. It’s not unusual to see very small deficits do very little to change a physique because the degree of metabolic adaptation can undermine reductions in energy intake.

Put more plainly, when you eat less to lose weight, your brain recognises this and kicks its protective mechanisms on – it hits the brakes. Your calculated energy deficit will be reduced as a result of you moving less and becoming a bit more efficient. If your calorie deficit is small, it could become negligible.

In my experience a deficit of around 10% or less from your original maintenance intake is not likely to produce significant results.

On the other hand, an aggressive deficit will lead to faster weight loss. Research has shown this to be very motivating and can encourage people to stick to their diet. The downside is that adaptation is likely to kick in much harder as your brain drops anchors to try and stabilise your weight.

I’d consider an aggressive deficit to be somewhere around 40% or more from maintenance. But it’s not the size of your deficit is that matters, it’s how you use it.

The length of the diet period and How often you diet

Without framing the deficit in a timeline, you’re navigating without a compass. An aggressive deficit of 40% from maintenance isn’t particularly challenging or likely to incite a great deal of metabolic adaptation if you do it for a week or two, but once you start talking months – good luck!

Obviously, the longer you spend below maintenance energy levels the more weight you’ll lose.

Similarly, the more frequently you decide to reduce bodyweight, the more metabolic adaptation you can expect.

Your starting bodyweight

Let’s zoom out another layer. Where you start matters quite a lot – are you already seeing hints of a six-pack poking through or are you trying to get rid of a keg? If you’re carrying a lot of body fat, then a faster rate of weight loss may be warranted and is certainly more sustainable. Therefore, it affects the size of your initial deficit and how long you can diet for.

 Taken together, the way metabolism is affected by dieting represents a multi-factorial model. We need to contextualize weight loss within this framework.

Asserting that a larger deficit causes more metabolic adaptation (via greater weight loss) is technically true if all else is equal, but it really depends on how long you’re dieting for, how much you have to lose and how the diet is set up.

It’s also important to keep in mind that there is an individual metabolic response to dieting. Remember in the previous article where I said that NEAT can vary by as much as 2000 kcal between individuals?

This means that a huge deficit might not incite the same response from Person A’s brain as person B’s.

With that in mind, let’s look at some strategies we can use to mitigate the effects of weight loss on your metabolism.

Slow losers

So here’s the rub: Brain thinks we’re going to starve and miss out on sex if we keep losing weight. Fundamentally, we need to convince Brain we’re not actually starving to death and there’s a few ways we can do this.

Firstly, we can lose weight more slowly. Imagine you’re driving your car and you have a long trip ahead of you, with many kilometres/miles to shed before you reach your destination. You could press down on the accelerator, start knocking down distance much faster and get there sooner – but that means your fuel gauge will also drop much faster.

Brain doesn’t like seeing rapid fuel depletion. If the fuel gauge is quickly ticking downward, Brain’s likely to panic and hit the brakes. If we can instead take a smooth and steady approach, we might be able to convince Brain that everything is hunky dory as we coast into our destination with minimal intervention.

In practicality this means making sure weight loss is on average not too drastic from week to week. I’d recommend aiming to lose around 0.5-1% of your total bodyweight each week on average (bearing in mind that weight loss is usually not linear).

Diet breaks

We could also let Brain know we’re doing just fine by eating at maintenance every so often. Remember that leptin is the fuel gauge that tells your brain how much fat is left in storage? It starts kicking and screaming at the brain to do something when we lose body fat and mediates the hormonal changes that contribute to adaptive thermogenesis.

It turns out leptin calms down a bit when we have enough energy coming in. It’s hard to fool though – having a one-off cheat meal or a day at energy maintenance isn’t particularly convincing to Brain that we’re not starving.

Imagine that scenario in our evolutionary history: the brain kicks in multiple mechanisms to help us survive when there’s no food available, we stumble across a half-eaten carcass and gorge ourselves silly for a day – and then what? We’re hardly out of the woods yet; it takes more than one lucky day of eating to prevent death by starvation.

To soothe leptin’s signals to the brain, we probably need a few days of eating at maintenance. I typically recommend at least 1 week at maintenance, and sometimes up to 2 weeks. This seems to be enough time to convince leptin that we’re not going to die hungry.

It seems that carbohydrate intake has a more powerful effect than other macronutrients, but I’ll save that for the next article.

Importantly, this strategy means that the rate of weight loss over time is slower.

10 weeks of continuous dieting at a 40% deficit will produce more weight loss than 4 weeks of a 40% deficit-> 2 weeks at maintenance -> 4 weeks at 40% deficit. In the latter scenario we’re actually only getting 8 weeks of being in a deficit.

Put it another way: if we have a diet break of 1-2 weeks, it’ll take an extra 1-2 weeks to achieve the same amount of time in a 40% deficit – hence a slower rate of average weight loss.

See how it all ties back into the magnitude of weight loss?

Mind games

I mentioned earlier that there is a behavioural element to this. So far, we’ve been mostly talking about hormonal changes, and while hormones certainly affect our behaviour, we can’t ignore the psychological element of dieting.

Both previous sections have an important part to play in this. The size of the deficit will determine rate of weight loss. Some people may find a slow and steady approach suits them better psychologically, whereas others may find that while a quick and dirty deficit is challenging, it yields faster results which props up motivation.

Diet breaks have a role to play here too. Not only can we mitigate hormonal adaptation to weight loss by taking a break every so often, we can also refresh ourselves mentally and reduce social friction. I’ve often pre-empted a diet break around social events like weddings, holidays or birthdays to relieve the pressure of a tough or prolonged diet.

But wait, there’s more!

This article is starting to get long in the tooth, but I’ve yet to touch on a couple of details around energy availability and structuring weight loss diets. I’ll save this for another article in this series.

For now, here’s a quick summary: a diet must be contextualised as there are a few dimensions to a simple calorie deficit that all interact with each other. These include how long you diet for, how big your deficit is and where you’re starting from. Fundamentally, we’re trying to drain our fat-battery while convincing our brain that there’s still plenty of juice left to keep on living and breeding.

References/Further reading

Rosenbaum M, Leibel RL. Adaptive thermogenesis in humans. Int J Obes (Lond). 2010;34 Suppl 1(0 1):S47–S55. doi:10.1038/ijo.2010.184

 von Loeffelholz C, Birkenfeld A. The Role of Non-exercise Activity Thermogenesis in Human Obesity. Endotext [Internet]. 2018