How Metabolic Adaptation Can Hinder Your Fat Loss

Losing weight is a big problem as far as your brain is concerned. As captain of the ship, its job is to make sure you survive long enough to pop out as many kids as you can and that means stocking up on as much fuel as you can hold on to. Can’t make babies if you can’t even get out of the harbour.

Your brain has three main priorities in life: 1. Survive. 2. Sex. 3. Repeat.

Anything that threatens these is met with great hostility, and we’ve had millions of years of evolution to hone this survival instinct.

It takes energy to do stay alive and breed, so we have two strategies to make sure there’s enough around. We can either conserve energy or go out and find more in the form of food. Sitting here in front of our screens in 2019 is thus far the ultimate energy-seeking calorie-sparing machine in human form that natural selection has to offer.

So naturally your brain has a little freak-out when you decide you want abs for Summer. Well, maybe more than just a little freak-out. It has mechanisms in place to sense when the fuel gauge is getting low – your level of bodyfat literally informs the brain via a hormone called leptin how much energy is available to do its two favourite things.

Not only this, but it has a multi-pronged strategy in place for when this happens. Faster depletion of energy reserves means a more drastic response – the harder you push on the accelerator, the more you deplete fuel and the harder your brain steps on the brakes.

To get past the analogies: the more weight you lose, the more your brain fights back by reducing energy output and increasing behaviours associated with energy intake.

How Energy Output Is Reduced

As your metabolism adapts to lower energy availability, we start to see energy output decrease. Weight loss leads to what is called adaptive thermogenesis, which means the number of calories you burn daily starts to go down.

This is partly to be expected. Your metabolic rate is closely tied to body weight, so as you lose weight on a fat-loss diet you’d expect that your metabolic rate would reduce accordingly. We have robust research-validated formulae that can work this out.

The problem is that it happens to a greater degree than we would expect. Let’s say you had two 70kg individuals. One’s been dieting and has lost 5kg so far whilst the other has been weight stable for several months.

Our formula would predict that both have roughly the same resting metabolic rate. However, research has shown that the dieter’s metabolic rate will have adapted to compensate for weight loss and is now lower than we predicted.

How can this be? It’s partly due to greater mitochondrial efficiency, meaning that your mitochondria can get better at deriving energy from the food you eat. We also tend to see a drop in hormones that control metabolism (specifically the thyroid hormone T3). This leads to a decrease in overall energy used at rest and while exercising. Coupled with the reduction in bodyweight and we’re looking at a significant reduction in total daily energy expenditure – perhaps as much as 10-15% below what our predictions say based on bodyweight.

The largest variability in daily energy output between individuals is non-exercise activity (NEAT). NEAT includes mostly subconscious movement like fidgeting, posture and generally how animated you are. It’s impacted to a varying degree between dieting individuals. Some people experience a very large reduction in NEAT and others very little. In fact, there can be up to a 2000 Calorie difference in daily energy expenditure between individuals.

To summarise, as your fuel stores drop your brain begins to step on the brakes in an attempt to preserve what’s left of them.

How Energy Intake Is Increased

Recap: When you lose weight, the fuel gauge goes down. The hormone leptin is the first mate who must go inform Captain Brain. Leptin mediates the reductions in NEAT and energy output, but it also increases hunger hormones. The major player here is called ghrelin alongside its pals insulin and cortisol.

Brain is smart. Brain like food. Brain make you eat.

That’s about the gist of it.

What next?

This all seems relatively straightforward but it’s a complex web of interactions between various hormonal messengers and tissues throughout the brain and body. It’s important to note at this point that if you’re panicking - don’t. These are all normal responses to weight loss that have evolved to protect us from starving to death. It doesn’t mean you can’t recover your metabolic rate and it doesn’t mean you can’t continue to safely lose weight.

However, given the inherent resistance to weight loss it would be great if there were some strategies to counteract metabolic adaptation and give us a smoother ride to shredsville.

That’s coming in the next blog post, so I guess you could stress out a bit until you get a chance to read that.

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