It’s one of the most frequently asked questions in health and fitness:
How do I gain muscle and lose fat at the same time?
Some people phrase it differently, asking, “How do I replace muscle with fat?”
We’re here today to answer these questions.
The short answer:
Now we’ll explain why this is true.
Let’s start with some science.
First, remember that human bodies aren’t one giant clump of the same kind of tissue. Muscle and fat are two different functional ‘compartments’.
In other words, it is physiologically impossible to “replace” muscle with fat, or vice versa.
Second, let’s introduce the first law of thermodynamics, about the conservation of energy. It’s a law of energy balance.
This law states that energy can neither be created nor destroyed. Energy has to go somewhere and doesn’t just disappear.
An energy surplus will lead to storage. An energy deficit will result in energy leaving the body.
In nutrition terms, you may have heard this concept explained like this:
In order to lose weight, we must be in a caloric deficit (consuming fewer calories than we burn). In order to gain weight, we must be in a caloric surplus (consuming more calories than we burn).
To build muscle or fat cells, your body needs energy.
Your body will release energy when it breaks these cells down.
But the question is, once consumed, where do calories go — to muscle or fat tissue?
And when we burn calories (and energy leaves the body), will our body pull from our muscle or fat tissue?
This is the issue of ‘calorie partitioning’ and it’s an important precursor to understanding whether (and how) we could build muscle and lose fat at the same time.
The truth is: we can’t control where our calories go (fat or muscle) when we eat more or fewer of them.
In an ideal world, we’d want all our calories to go into muscle tissue and none into body fat, right?
And it would also be convenient if, when dieting, all calories used came from fat stores.
But, as physiologist and author Lyle McDonald explains, that’s simply not reality.
When talking about how calorie partitioning really works, researchers refer to something called the P-ratio .
This represents the amount of protein that is either gained (or lost) during over (or under) feeding.
Most of the P-ratio is outside our direct control (mostly genetic). McDonald says we can control around 15-20% of it with how we eat or train .
Hormones are very important in determining your P-ratio:
The tricky thing is that the P-ratio describes the overall effect on your whole body. It cannot predict the partitioning (in other words, when you’re in a surplus, it cannot predict the percentage of your energy that will go toward creating muscle cells vs. fat cells). 
In theory, if your body had enough stimulus (i.e. strength training), plus protein, plus water, it could pull energy from your fat cells to build muscle tissue. That would be about the closest we could get to losing fat and gaining muscle at the same time.
That’s theoretically possible, but it’s unlikely for most individuals.
(It’s also important to acknowledge that just because something is possible doesn’t mean it’s the ideal method or strategy. More on that below.)
The information above may have left you feeling a bit discouraged about your ability to gain muscle and lose fat simultaneously.
But hang on, there are three groups of people who probably can achieve this feat!
Several factors will impact whether you can gain muscle and lose fat at the same time, including:
Now let’s dive deeper into each of the three groups:
1. Beginners in the gym with a higher body fat percentage
Individuals who are new to strength training have a high potential for relatively rapid muscle gain. And with a higher body fat percentage, their bodies have more energy to spare and put toward muscle gain.
There is also a chance that these individuals are insulin resistant. Put simply, this means their fat cells “don’t want” more energy. 
When these individuals strength train, it sends a powerful growth stimulus to their muscles. This will have the effect of improving their insulin sensitivity and nutrient uptake within muscle cells. 
At the same time, their fat cells will remain insulin resistant. So, calories are more likely to go to muscle cells. 
These functions are what lead to simultaneous muscle gain and body fat loss in these individuals. The transformations of these individuals can be quite impressive — it appears as if the body takes calories out of fat cells and uses them to build muscle.
It’s important to keep in mind, however, that this only works if both factors are present: the individual is a strength training beginner and they carry a higher percentage of body fat.
Someone who is new to strength training but quite lean doesn’t have enough fat to “shunt” to the muscle.
And the longer you have been strength training, the slower your rate of muscle gain will be.
That is why the combination of being new to strength training and carrying a higher level of body fat is the “sweet spot”.
2. Athletes returning from time off
For athletes who took time off (either by choice or due to injury, etc.) and lost muscle mass and gained body fat, they will probably have a relatively easy time rebuilding that mass (thanks to ‘satellite cells’ and the ‘muscle memory’ effect). 
3. People using steroids
We aren’t going to spend a lot of time discussing this category, but you get the idea. Steroids essentially “override” normal physiology.
The answer: maybe. But, as mentioned above, the risk is that you’ll end up “spinning your wheels” and making minimal progress in either direction.
The leaner and more muscular you are, the less likely you are to successfully lose fat and gain muscle at the same time.
Put another way, the more “advanced” you are with your training and body composition, the slower “body recomposition” will likely be for you.
What tends to happen for many individuals is that neither their nutrition nor training program is optimized for fat loss or muscle gain.
For example, they may keep their calories too low to support muscle growth — but too high for fat loss.
This could lead to no changes happening at all (and how frustrating would that be?!).
For most individuals, we recommend breaking down your journey into specific phases (fat loss or muscle gain). This is likely to lead to quicker results.
If you’d like to learn more about how to effectively lose fat, we have some great resources including an article about what it takes to get leaner as well as one about how to ensure you lose fat without losing muscle mass.
And if you’re focused on gaining muscle, we’ve got an article about that too.
The most effective route may be hiring a coach who can guide you through the whole journey from start to finish, educating you along the way.
Our coaches are trained in the latest science around muscle gain and fat loss. They’ll work closely with you to create a personalized plan to achieve your goals (and they’ll set you up to ensure you sustain your results for the rest of your life). Add your name to the waiting list today!
McDonald, L. (2003). The Ultimate Diet 2.0. Lyle McDonald Publishing.
McDonald, L. (2020, April 22). Insulin Resistance and Fat Loss. Retrieved July 19, 2020, from https://bodyrecomposition.com/fat-loss/insulin-resistance-fat-loss
Helms, E., PhD, Valdez, A., MS, & Morgan, A., BS. (n.d.). The Muscle & Strength Pyramid: Nutrition (2nd ed.). Retrieved July 10, 2020.
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