Reducing Body Fat Percentage by Gaining Muscle – Q&A

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 25 pounds 180 pounds 13.8%
15 pounds 25 pounds 185 pounds 13.5%
20 pounds 25 pounds 190 pounds 13.1%
40 pounds 25 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 Fat Mass Total Weight Body Fat Percentage
5 pounds 20 pounds 165 pounds 12%
10 pounds 15 pounds 160 pounds 9.3%
15 pounds 10 pounds 155 pounds 6.4%
20 pounds 5 pounds 150 pounds 3.3%


See the difference?  Whereas a 5 pound gain in muscle only lowered body fat percentage by less than 1%, the same 5 pound fat loss lowers it a full 3%.  By the time the lifter has lost 10 pounds of fat, he’s dropped from 15% to 9.3% body fat; the same 10 pound muscle gain only lowered body fat percentage by 1.2%.  And where a 20 pound gain in muscle only lowered body fat percentage from 15% to 11%, the same 20 pound fat loss took him from 15% to full contest leanness.

Additionally, since fat can be lost much faster than muscle can be gained, it should be clear that losing fat is a far more effective method of lowering body fat percentage.  On a hard diet such as the Rapid Fat Loss Handbook, our lifter could achieve a true 5 pound fat loss in 2 weeks, compare that to the roughly 10 weeks it might take him to gain the same amount of muscle.

Even on a more traditional diet, a 5 pound fat loss should be easily achievable within 5 weeks; twice as quickly as the same gain in muscle mass and with a much greater impact on body fat percentage.  And where the 20 pound fat loss might take 20-24 weeks (taking the lifter from 15% to shredded), compare that to the 1-2 years it would take to gain the same amount of muscle mass.

Which is why I said yes and no originally.

Yes, gaining muscle mass can have an impact on body fat percentage but the effects are generally very small and I think it’s an inefficient way of approaching the goal.  Losing the same amount of body fat can not only be done much more quickly compared to gaining muscle, the equivalent fat loss has a much larger effect on body fat percentage compared to gaining muscle.

Of course, as you might imagine, gaining muscle while losing fat has the largest impact on body fat percentage (noting that the major effect occurs from the fat loss).  But achieving that is generally even more difficult although it can be achieved with complex cyclical diets such as my Ultimate Diet 2.0.

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