I spend way too much time lurking on fitness forums.
And every so often (read: all the time), someone shows up panicking about the scale refusing to move, even though they swear they’re doing everything right. Soon, the helpful comments roll in, and if you wait long enough, you’ll see that comment:
“You’re probably just gaining muscle.”
It’s at this point that I, like an anime villain, let out a chortle, push up my glasses (I don’t wear glasses), and prepare to write a very long response–only to remember I have better things to do than argue with jacked_dude99, so I go back to mindlessly scrolling the interwebz.
But this one’s worth covering.
Can someone eating in a calorie deficit really gain that much muscle over a few weeks—or even months—to stall or reverse weight loss?
Let’s find out.
To start, you can absolutely gain muscle while losing fat, and this effect is more pronounced in beginners to weight training with higher body fat.
But could you gain so much muscle that it would completely eliminate weight loss or even lead to weight gain?
Unlikely.
To understand why, let’s get some numbers in place.
Assuming one pound of fat contains 3500 calories, you need to create a daily deficit of 500 calories to lose a pound per week.
For muscle growth, I’ll use two examples least favourable to my argument: untrained lifters not in a calorie deficit. In other words, individuals with the highest potential for growth.
Benito et al. found that untrained men gained an average of 1.5 kg (~3 lbs) of fat-free mass over 10 weeks of resistance training, while Hagstrom et al. found that untrained women gained an average of 1.45 kg of lean mass after 15 weeks. 1 2
So, let’s consider a common scenario. An untrained individual with extra body fat wants to get leaner. They start lifting weights a few times per week and clean up their diet, aiming for a daily deficit of 500 calories.
The image below shows their weight change after 10 weeks.
Despite gaining 3 lbs of muscle, the fat loss would still substantially outweigh the muscle gain, leading to a net weight loss of 7 lbs.
Let’s dig deeper and rerun the same scenario using the energy density of fat and muscle tissue.
To keep things simple, I’m going to use the 3500 kcal per pound rule for fat and 2500 kcal per pound for muscle.
I need to include a semantic note, or my brain won’t let me continue: One pound of muscle tissue contains around ~800 calories. 3 But there’s an energetic cost associated with building muscle. So that 2500 value is an estimate that accounts for both the energy stored in the muscle and the energetic cost of synthesising new muscle tissue. While it’s an estimate, most people are familiar with the 2500 value, so I’m sticking with it. Similarly, while pure fat contains about ~4200 calories per pound, the commonly cited 3500-calorie rule comes from the fact that human body fat isn’t 100% fat — it also includes water, connective tissue (like collagen), and other cellular material that add weight but don’t contribute usable energy. Similar to the muscle gain value, I’m sticking with 3500 calories for fat loss because it’s a familiar number.
Our hypothetical individual is in a 500-calorie daily deficit for 10 weeks, but at the same time, they’re gaining muscle.
Over 10 weeks, a 500-calorie daily deficit totals 35,000 calories. Usually, that would translate to about 10 pounds of fat loss. However, if they also gain 3 pounds of muscle, some of those calories are used to build that new muscle tissue.
In this case, about 7,500 calories go toward muscle gain. This leaves 27,500 calories to be pulled from fat stores, which equals roughly 7.9 pounds of fat loss.
Since the scale reflects both fat lost and muscle gained, the net weight change is -4.9 pounds.
Here’s a visual taking you through the process:
Putting hypothetical scenarios aside, we see a similar trend across the research.
The image below highlights several studies tracking body composition changes in previously untrained individuals following a calorie deficit of 500–1000 kcal per day, aligning with the recommended 0.5–1% of total body weight loss per week.
While most groups experienced muscle gain, fat loss consistently exceeded it, resulting in a net weight loss.
Here’s a real-world example from one of my coaching clients who achieved a pretty damn successful body recomposition:
Over 20 weeks, he averaged a daily deficit of 350 calories. During that time, he gained 2 cm on his chest, added 0.6 cm to his arms, maintained leg size, and lost 7 cm from his waist. Yet, despite the clear signs of muscle growth, he still dropped around 11 pounds.
To reiterate a point I made at the start––you can gain muscle while losing fat, but if you’re eating in a deficit with the intention of losing weight, it’s very unlikely you’re gaining so much muscle that it’s cancelling out losses on the scale.
Sure, you might lose less weight than anticipated, but this would be accompanied by changes in your measurements and visual changes in the mirror or progress photos.
If nothing is changing, well, you’re likely not in a calorie deficit. So that’s where you should probably start.
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