What is Body Composition?

Howdy-ho, all!

Today I’m going to give you a primer on Body Composition, specifically looking at the differences between Fat Tissue and Muscle Tissue. There are a lot of misconceptions out there; lets start busting them!

What is Fat Tissue?

What we know as body fat is called adipose tissue. It’s a type of loose connective tissue made up primarily of adipocytes, though it also contains a small cocktail of other cell types which deserve their own articles. Briefly, these other cells include things like preadipocytes (cells that can be stimulated to form adipocytes) and fibroblasts (cells that help form the basic structure of cells and tissues). We’re most concerned with the main component, so lets break that down a little more.

Brown Fat

These cells appear to be brown because they contain a lot of mitochondria, often called the “power house” of the cell. These nifty little buggers are the major force responsible for energy production in our cells. Mitochondria are kind of like bacteria, in that they have their own separate genome and DNA, but they’re incredibly helpful for, you know, keeping us alive. Anyway, brown fat has a lot of mitochondria interspersed throughout the cell along with small droplets of lipid. These cells are primarily used to provide heat to the body, so they’re actually pretty useful. Don’t worry about Brown Fat.
Fat Cells

White Fat

These cells appear white in color, made up of a big drop of lipid surrounded by cytoplasm. The lipid is basically chemical, liquid fat made up of triglycerides and cholesterol ester. This is where energy is stored after digestion, but before it’s needed. An average adult has about 30 billion white fat cells, accounting for about 30 lbs of total weight. Keep in mind that this includes both the subcutaneous fat you see just underneath the skin, and the visceral fat that surrounds and insulates your organs.

White fat cells are typically about 0.1mm in diameter, but when excess fat is stored as an adult, these cells can expand up to four times their normal size. As long as you don’t go too high, the number of cells stays the same (i.e. you don’t develop *new* fat cells). However, once the fat cells get too big, they divide and increase the total number of fat cells accordingly. The good news is there is evidence that this increase of fat cells only happens in childhood and adolesence, and when people become obese as adults their overall number of fat cells doesn’t change; they just get bigger.

So what’s the bad news? You can’t naturally lose fat cells! When you lose body fat, what you’re doing is “emptying” white fat cells to reduce the overall average size of the cells you have. This means that if you *did* become obese as a kid, you have more fat cells in your body than someone of similar age, gender, and build who was leaner as a kid. Here’s the really crappy part: if you were fat as a kid, it’s harder for you to stay lean as an adult because of the extra fat cells you developed.

White Fat Cells are pretty active, too. Adipocytes are capable of creating estrogen out of androgen, with the excess causing potential fertility issues in obese men and women, and feminization of obese men. In addition, white fat regulates a hormone called Leptin, which plays a big role in your feelings of hunger. If someone is overweight, they literally feel more hungry than someone who has less body fat. This, in turn, generally causes the overweight person to eat at a caloric excess, gain more fat, and continue the cycle.

Don’t get me wrong, we need a certain amount of white fat in our bodies for energy storage, and if you’re prone to situations where you don’t eat for a few dats at a time you’ll want that extra enery laying around. But otherwise…white fat kind of sucks.

What is Muscle Tissue?

Man, talking about fat is kind of a downer. Let’s change gears and talk about the most awesome tissue to ever tis’: muscle!

Fun Fact: The word “muscle” comes from the Latin word “musculus” which can have two meanings (that I found). The first, “little mouse”, seems to be the most common interpretation. The justification is that some muscles may appear mouse-like in shape OR that when you contract your muscles it may look like little mice are running under you skin (that one is creepy). However, I put forth that the secondary meanings of “musculus” are actually more accurate of an interpretation, those being “mantle” or “shielding”. This is a military usage, but to me it makes more sense to think of muscle as the “shielding” for the body as compared to rodents under your skin. Maybe it’s just me?

Moving on! There are three types of muscle in your body. The one we care most about for this article is skeletal, but lets touch on the others quickly as well.

Smooth Muscle

Found in the walls of organs and other structures (like your esophagus) that need to function outside your conscious control. They’re also called “involuntary muscles” because you can’t make them do anything. Smooth muscles contract based on signals from the body that tell them to contract when they need to. A good example is your lungs, which can breathe even when you aren’t paying attention. Smooth muscle can contract for long, sustained periods of time (sometimes almost permanently).

Cardiac Muscle

Also called myocardium, makes up the structure of your heart and is pretty neat. It works like smooth muscle in that your unconscious brain functions control what it does and when, but it looks like skeletal muscle in structure. Myocardium is also involuntary muscle, but it is also striated into groupings of muscle cells called sarcomeres. Cardiac muscles contract in short, intense bursts which you already know because you know what your heartbeat feels like. The sarcomeres in cardiac muscle are special in that they connect in irregular branches all around the heart, forming a web of muscle tissue that performs a “squeezing” motion.

Interesting Note: When your involuntary muscles no longer contract on their own, you’re basically brain dead, but you would have stopped noticing well before that point.

Skeletal Muscle

Also called voluntary muscles, make up about 42% of the average adult male and about 36% of the average adult female (based on body mass). Like myocardium, skeletal muscles are made up of sarcomeres: groupings of myofibrils (muscle fibers) that are banded together in long sections of tissue. Generally, tendons connect muscles to bone and fasciae connect muscle to muscle; both are basically collagen-based connective tissue. Skeletal muscle is what lets you maintain posture (i.e. standing up) and move your body parts wherever you want them to go. There are a few sub categories of skeletal muscle as well.

The muscle fiber types are listed as Type I, II, etc. This can get confusing to read through and keep everything straight since they look so similar. I’m going to use the following terms to make it more clear what we’re talking about in each case:

Type I is “Red Muscle”
Type IIx (humans) or Type IIb (other mammals) is “White Muscle”
Type IIa is “Pink Muscle”

If you want more information, you can search by the Types. Be warned, there’s a ton of good stuff out there.

Red Muscle

Also called “slow twitch fibers”, these are the muscles which provide function for aerobic endurance. They are thick with capillaries, so their blood supply is very high, and it is relatively easy to allocate oxygen into the muscle tissue for energy production. Red Muscle is also high in mitochondria, which is the workhorse of aerobic energy production, and myoglobin, which is responsible for carrying oxygen around the muscle tissue (just like hemoglobin does in the blood stream). The high concentration of myoglobin is what gives the muscle it’s color. Though Red Muscle is really good for long-term usage (like running a 5k) it is very poor at providing short term power (like when throwing a kick). These fibers have the least potential for developing mass and size, but they can be developed for longer term endurance.

White Muscle

Humans and other mammals each have White Muscle, though the version in non-human mammals differs slightly. This difference is mostly academic for our needs and has to do with how the fibers function when interacting with the cells “instant” power supply, ATP (adenosine triphosphate). Anyway, White Muscle is the “fast, powerful” fiber that is responsible for all the very strong contractions in your muscles. If you remember our example from earlier, where Red Muscle is the “marathon running” muscle, the White Muscle would be the “kick down the door” muscle.

These fibers provide a lot more power than Red Muscle, but they get tired very quickly. Their energy source is ATP/anaerobic glycolysis; neither uses oxygen to produce energy, but that means the contractions aren’t sustainable over time. The capillary network and myglobin in White Muscle are both low, which limits their aerobic endurance to basically nothing and causes them to appear white in color.

Pink Muscle

This is the coolest muscle, by far. Pink Muscle has many of the good qualities of both Red and White Muscle, with few drawbacks. Like Red Muscle, Pink Muscle has a well developed capillary network so it gets a great supply of blood and oxygen when in use. It also has medium-high myoglobin levels, so it can make use of the extra oxygen available to it for aerobic energy production. This gives it a distinct advantage over White Muscle in that it can maintain activity for a longer duration.

Speaking of activity, Pink Muscle is much stronger than Red Muscle, almost equaling White Muscle in the sheer amount of force it can produce with each contraction. This is because Pink Muscle has access to the same anaerobic energy systems as White Muscle, able to pull a lot of energy in a short amount of time. So, you have the longer term endurance of Red Muscle mixed with an almost equal ability to contract forecefully found in White Muscle. That’s pretty awesome, in my opinion.

Fiber Distribution

Muscle groups are made up of all fiber types, with different muscles showing different concentration of each fiber type depending on their function. For instance, the quadriceps (top of the thigh) are about 52% Red Muscle, while one of the principle muscles of the eye, the orbicularis oculi, is only about 15% Red Muscle. For the average, sedentary human the body is almost equal with an average 55% Red Muscle and 45% White/Pink Muscle. This average also applies to most young children,

This differs in people who participate at a high level in certain sports. For instance it’s been found that “marathon” athletes tend to have a greater proportion of Red Muscle, “sprint” athletes tend towards more White/Pink Muscle, and “middle distance” athletes tend to have about equal distribution. Now, the problem here isn’t the difference, it’s in why the difference exists. There is some argument to suggest that these athletes are attracted to, and excel at, their sports because of their muscle distribution. However there is also some evidence that suggests this difference can be caused by the act of training for the sport itself.

There is no good concensus how whether or not Nature (your natural distribution of muscle) or Nurture (your training) has the bigger effect on your muscle make-up at the fiber level (could be both or neither, as well). I will say, however, that as soon as I figure out how to turn everything into Pink Muscle, I’ll let you know!

Comparing Muscle and Fat

So, we’ve talked a lot about Fat and Muscle each in their individual “vaccuums” so to speak. Now lets start comparing them a bit.

Does Muscle weigh more than Fat?

No, but this is really just poor wording to describe a real concept. If you have a pound of anything, it will weigh the same as a pound of anything else. Weight is a single measurement related to some thing, and refers to how much the force of gravity pulls that thing towards the center of gravity. Weight on Earth is a measure of how strong gravity pulls the thing towards the center of the Earth.

What we’re really talking about here is Volume, or the amount of space occupied by something relative to it’s weight. This is the fundamental difference between Muscle and Fat: at the same weight, Muscle tissue takes up less space than Fat tissue. This means that if two people both weight 150lbs, the one with the higher body fat will take up more space than the other one.

As an example: for every 10lbs of Fat you have on your body, you are carrying around the equivelent of 2.5 two-liter bottles of soda. In comparison, 10lbs of muscle is equal to about 2.14 two-liter bottles. This is about a 15% difference in volume. I know, I know. It doesn’t seem like that much of a difference, mathematically. Keep in mind that Fat tissue is pretty, well, squishy while Muscle tissue is more tightly bound. Even at the same weight, that change in volume makes a huge difference in your appearance because of how tightly compacted Muscle tissue is compared to Fat tissue.

Muscle, Fat, and Calories

Another place where these tissues differ is in caloric needs and caloric provision. One pound of fat contains 3,500 calories of stored energy while one pound of muscle contains about 750 calories of stored energy. If you burn 3,500 calories in a week (either through diet restrictions or exercise) while maintaining a good muscle routine, you should lose about one pound of fat that week. The same cannot be said for eating an excess of calories, however. Just because you eat a 750 calorie excess or a 3,500 calorie excess doesn’t mean you’ll gain muscle or fat. It actually takes more calories to build the tissues, than what you get out of them when they break down.

There really isn’t a known formula for how many calories to eat to build Tissue XYZ, but it is known that in order to build muscle you generally need a caloric excess and to burn fat you generally need a caloric deficit. I’ve seen estimates that say you need 45,000 calories to build a pound of muscle, and others saying you need as low as 2,200 for the same thing. The same basic principle applies to building fat. Your metabolism needs a certain amount of energy, also pulled from the calories you eat, in order to form new tissues, so you have to add that to the energy inherent to the tissue itself.

Not only do the stored calories differ between Fat and Muscle mass, but the number of calories required to maintain those tissues also differs. We know that every day, each of your major tissues takes the following number of calories to maintain themselves:

Muscle: 13.0 cal/kg
Fat: 4.5 cal/kg
Bone: 2.3 cal/kg

Based on these figures we can see that Muscle burns way more calories than either Fat or Bone. This means that a lower body fat at the same weight will give you an advantage in maintaining that weight long term.

Can Muscle transform into Fat (and vice versa)?

Short Answer:

No. Not ever. If someone tells you differently they’re either lying, misinformed, or selling something.

Long Answer:

Fat and Muscle are two different tissues with very different characteristics, functions, and attached processes. They have completely separate metabolic processes for their formation and breakdown, though they are interrelated in the same way that all bodily processes are interrelated. Fat is built by eating an excess of caloric energy, generally consumed in the form of sugar, where the excess is stored in adipose tissue after passing through the digestive system. Fat is broken down based on the energy needs of the body, either to maintain bodily systems or to feed muscles as they move. Generally Fat stores are only used after running out of free carbohydrates in the blood stream.

In contrast, Muscle tissue is built up as a result of microtraumas experienced by skeletal muscles during physical stress (such as exercise), and is broken down when caloric deficit is too high to maintain lean mass or when the muscles experience a major trauma from which they cannot sufficiently recover (like crush syndrome). While the fuel used to build muscle may sometimes be provided by the breakdown of body fat, that is not the same as converting one to the other. Fat is broken down into energy, and that energy is used to fuel some kind of bodily process.

If someone tells you differently they’re either lying, misinformed, or selling something.

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