CAN YOU LOSE FAT IN A CALORIE SURPLUS... Or build muscle in a deficit?

I often hear people say that you can’t build muscle unless you’re in a caloric surplus. I’d like to explain why this, like most things, is context dependent. Perhaps even more interestingly, can you lose fat despite eating more calories than you burn?... Before we get into it, I'd like to run through a scenario with you.

Let’s imagine a scenario where you have a well-trained lifter who’s never done any direct bicep training & usually maintains his weight with around 2,500 calories. He’s currently running a 12 week cut where he’ll be dieting on 2,000 calories (a 20 percent deficit). Since he’ll be losing body mass over the next 12 weeks, the question is: If he starts doing direct bicep training with 16 sets per week, will he gain significant muscle in his biceps?

Keep in mind, he will be in a deficit the entire time. The short answer is YES, unless he’s some kind of non-responder to training or is just really under-eating protein.

To understand how it’s possible to build muscle in a caloric deficit or lose fat in a surplus, the first thing we need to consider is the ‘First Law of Thermodynamics’. The confusion seems to come when people assume that an anabolic process like building muscle couldn’t happen concurrently with a catabolic process like losing fat.  There are a few ways that we could explain why this is false.

Common Sense Approach

Because fat tissue & muscle tissue are separate systems, it’s possible to lose a bunch of fat due to the caloric deficit while at the same time build muscle from the resistance training & sufficient protein. It would be similar to having two different bank accounts where you could withdraw £1,000 from one account, while depositing £100 into the other account. Note how you’d still have withdrawn a net amount of £900, despite gaining money in one account.

Muscle Math

1kg of muscle contains 1,800 calories, but 1kg of fat has 9,400 calories. This huge difference between muscle & fat exists because muscle is mostly made up of water. If someone gained 2kg of muscle across 6 months while losing 1kg of fat, that would equal a gain of 3,600 calories of muscle vs a loss 9,400 calories of fat. Even though they have gained 1kg body weight, there must have been a total net energy deficit of 5,800 calories across those 6 months.

Because there are about 180 days in six months, this would only come out to about a 32 calorie deficit per day.

Scientific Research

One 2016 study showed an average loss of about 5kg of pure fat while gaining about 1kg of total bodyweight. This was despite being in a deficit as the subjects were all on a progressive resistance training program & eating a high protein diet. We can’t all expect the amazing body results as depicted here because the likelihood of anyone building muscle in a deficit depends on a few factors. It’s easier to build muscle in a deficit if you:

1.     Are a beginner

2.     Have a higher starting body fat percentage

3.     Don’t crash diet (the smaller the deficit the better)

4.     Eat a higher protein diet (1.5g+ per kg bodyweight ideally)

Next, let’s move along to an even more interesting question: Is it possible to eat in a surplus & still lose body fat?

Time for some more quick maths. If you gained 6kg of muscle, while losing 1kg of fat, you would have had to eat a surplus of 1,400 calories while losing body fat. If the transformation happened over a year, that would only be about a 4 calorie surplus each day, which is obviously not practical. Because of the differences in energy (calorie) density between fat & muscle, this proves is that it’s possible to lose fat while in a surplus if you build a lot of muscle very quickly. However, the practical side of this doesn’t really add up as when we ask this question, we are usually referring to a much larger surplus than 4 calories per day. So, in 99.9% of real life cases, it’s not possible to lose fat in a deficit, we just can’t build muscle quick enough to use up the extra calories.

That’s your lot for today, see you next time.

Elliot Hutchinson // 20.04.2025

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