Probably the single most frustrating and unnecessary debate that crops up again and again in fitness communities is the one about muscle vs fat.
Is muscle heavier than fat?
It usually begins with the pictures. A lady in a bikini shares her before and after pictures.
She looks great in both. But in the first, she has a softer, curvy figure. And in the second she's standing on stage ripped to shreds with well-defined muscles and barely an ounce of fat on her.
Because she's lost the fat she looks lighter in the second picture, but she comments that actually, she's exactly the same weight. Whaaaat?
"Yes", she says, "because muscle is heavier than fat I look leaner but my weight hasn't changed."
Then all hell breaks loose!
"Muscle weighs the same as fat but it's denser!"
"Muscle and fat are the same weight but one takes up less space than the other!"
And my favourite "A pound of muscle weighs the same as a pound of fat!"
And the poor lady is left feeling confused and awkward.
So let's help her out and have a look at these "arguments".
"A pound of muscle weighs the same as a pound of fat."
No shit, Sherlock! And in that genius statement, we've established that every substance on the planet weighs exactly the same. That's very helpful!
A pound of muscle weighs the same as a pound of fat, just like a pound of feathers weighs the same as a pound of lead.
True as it is it's totally irrelevant.
In fact, that statement is totally misinterpreting the original comment, and shows a complete misunderstanding of basic equations. If you choose to make 1 pound the common denominator then her answer would not be muscle is heavier than fat but "fat is bigger than muscle".
"They weigh the same but muscle is denser."
Yes, muscle is denser. Good call.
But what's the definition of denser? The particles that make up the substance are more tightly packed.
Density = mass per unit of volume
When we compare the density of two substances we're comparing the mass in a given volume of each. We're not comparing the mass in a random volume of one versus a random volume of the other.
To compare variables we need to take at least one common denominator then we can make a meaningful comparison between two substances.
Not making sense?
Ok, let's take density as an example.
Density = mass per unit of volume and the standard measure used is...
density = kg/(cubic metre)
To compare the density of two substances we've taken a common denominator (a volume of 1 cubic metre) and compared the mass of each to define a figure for density.
Similarly for fat and muscle. Take one cubic meter of each and the one with the higher density (muscle) has the higher mass.
"Muscle and fat weigh the same but the muscle takes up less space."
Another true observation, but simply another way of stating the same thing.
If you take a piece of muscle and a piece of fat the same weight (1 pound now being the common denominator) because muscle is denser it takes up less space.
So we can agree that muscle is denser and that a given mass will occupy less space than the less dense fat, so the problem must be the word "heavier".
(We should clarify that "mass" and "weight" are interchangeable because weight is simply a measure of the force of gravity on a mass as measured by the scales. If we're both standing on the same planet then gravity is constant so weight can be assumed to mean mass.)
"Heavy" as a word is subjective (although taking a small amount of mercury doesn't seem to stop it being called a "heavy metal"). If I find a dumbbell heavy and you say it's light then that just shows you're stronger than me.
But assuming we're talking about the weight of a substance and not the seriousness of this conversation, the word "heavier" applied between two substances has a pretty definitive meaning. One has more weight than the other.
So does muscle have more weight than fat?
I think we've decided that due to it's greater density a cubic metre of muscle does indeed have more weight than a cubic metre of fat.
Does that mean muscle is heavier than fat? Only if we know how much of each we're weighing (in this case one cubic metre).
Right, so now it comes down to the volume. What size muscle are we weighing against what size of fat? We can't say "a muscle" is heavier than "a lump of fat" because the size is ambiguous. But what about "muscle tissue" versus "fat tissue".
Now we've taken away the ambiguity of size but we still need two substances to measure, so the sensible option would be to take an equal amount (volume) of each and weigh them against each other. We've got our common denominator (volume) and we can now compare our variable (weight).
Is it unfair to make that assumption? Well if we go back to our scientific definition of density there is an explicit assumption that there is a common denominator (cubic metre). The "concept" of density remains, the particles are always more tightly packed in the denser substance, but we can't put a number on that until we give it volume.
If it's good enough for science, and it's good enough for common sense, then in the absence of any amounts (specified or not) it should be totally reasonable to assume we're comparing like for like.
Still not convinced? Let me ask you another question.
What's the fastest animal in the world?
Did you say cheetah?
So is a cheetah faster than a warthog? NOT IF THEY'RE BOTH RUNNING AT 20MPH!
The unspoken assumption is that we are applying a common denominator (a fixed attribute) which in this case is top speed.
We can say a cheetah has a higher top speed than a warthog but we can't say it's faster... unless you assume a common factor—and we always do!
From Wikipedia - "The fastest land animal is the cheetah..."
If a cheetah is faster than a warthog then muscle is heavier than fat.
This is not saying any amount of muscle is heavier than any amount of fat. And it certainly isn't saying one pound of muscle is heavier than one pound of fat.
What it's saying is "all other things being equal, muscle tissue is heavier than fat tissue".
Getting back to our lady on the stage we can now see that she's lost 5 pounds of fat and, by luck or judgement, gained 5 pounds of muscle.
She looks smaller and weighs the same because, as she says, "muscle is heavier than fat".
What's most frustrating about the whole conversation is that everyone is correct, including the woman who says muscle is heavier than fat!
It is possible to lose fat, gain muscle, look leaner and still weigh the same. When it comes down to it, what matters for weight/fat loss is that people understand the principle of the argument. How you view that is just a matter of which side of the equation you approach it from.
More importantly, if you've been mocking people for saying muscle is heavier than fat, you should stop being a dick about it!
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