Losing fat While building muscle

May 28, 2023
Goal of losing fat while

Question: I want to reduce my body fat percentage but I also want to gain muscle and would rather not diet. A guy at my gym told me that if I gain muscle, this will have the effect of reducing body fat percentage, is this true?

Answer: Yes and no. Strictly speaking, yes, if you can gain muscle without any accompanying fat gain, you will reduce your body fat percentage. However, the reality is that when you work the math, the impact of gaining muscle mass is minuscule approaching irrelevant, especially compared to the impact of actually losing fat through diet/activity.

To illustrate this, let’s consider an average lifter who is 170 pounds with 15% body fat. As I showed in Body Composition Calculations, we can determine the total amount of body fat (in pounds) that this person is carrying by multiplying their weight by 15% (or 0.15). So our lifter has

170 pounds * 0.15 = 25 pounds of body fat and 145 pounds of lean body mass. We don’t actually need the lean body mass number for any of the calculations I’m going to do.

Let’s look at how much of an impact gaining pure muscle mass has in terms of changing body fat percentage. For these calculations, I’ll assume that the lifter is gaining 100% muscle and no fat; please note that this is not usually a good assumption. But it makes the math easier.

The table below demonstrates how various increases in muscle mass affect body fat percentage; note that his fat mass will stay static at 25 pounds throughout the calculations. So all I’m doing is dividing total fat mass (25 pounds) by the new body weight after adding the muscle that was gained. For the unadulterated hell of it, in addition to more reasonable numbers, I’ve done the calculation assuming this lifter can gain a whopping 40 pounds of true muscle mass with zero fat gain.

Impact of Muscle Gain on Body Fat Percentage

Muscle Gain Fat Mass Total Weight Body Fat Percentage
5 pounds 25 pounds 175 pounds 14.2%
10 pounds 180 pounds 13.8%
15 pounds 185 pounds 13.5%
20 pounds 190 pounds 13.1%
40 pounds 210 pounds 11.9%

As you can see, adding muscle mass doesn’t really have the impact on body fat percentage that you might hope. Every 5 pounds of true muscle gained reduces body fat percentage slightly (by about 0.4%). Sure, if our lifter can gain a tremendous 40 pounds of muscle mass with no fat gain, he will reduce his body fat percentage by nearly 4% but we need to consider the time frame involved here.

As discussed in What’s My Genetic Muscular Potential, with realistic rates of muscle gain, it might take this lifter 3-4 years to gain that 40 pounds of muscle mass. That’s if he gains it at all (i.e. it may be beyond his personal genetic potential).

As well, it would be staggeringly unlikely for this lifter to gain that much muscle without gaining some fat; and by ‘staggeringly unlikely’, I mean basically impossible.

Now, for comparison, let’s look at the impact of fat loss on body fat percentage. The calculations here are a little more complex because both fat mass and total weight are changing. The table below demonstrates how losing fat impacts on body fat percentage, using values similar to the above. For what should be obvious reasons, I can’t do the calculation for a 40 pound fat loss since our lifter only has 25 pounds to start with. If he lost 40 pounds of fat, he’d be long dead.

Impact of Fat Loss on Body Fat Percentage

Fat Loss
165 pounds 12%
160 pounds 9.3%
155 pounds 6.4%
150 pounds 3.3%

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