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What is Body Fat?

Note: The following is the entirety of Chapter 2 from my Stubborn Fat Solution where I describe what human body fat actually is.

Most people think they know all there is to know about body fat; I’m here to tell you that that isn’t the case.  If you’ve read my articles, you may have seen some of this before but I want to make sure everyone is on the same page before I deluge you with the technical bits.

What is Bodyfat?

The more technical term for body fat is adipose tissue, with individual cells being called adipocytes (adipo = fat ; cyte = cell). In humans, the primary type of fat cell is called white adipose tissue, or WAT, so named because of its color (it’s actually sort of a milky yellow). While there is another type of fat, called brown adipose tissue or BAT (which is actually reddish/orangeish), it’s generally been thought that humans didn’t have much BAT and hence it could be ignored.  As I’ll discuss later, this has been brought into question by recent research. I’ll come back to BAT in the next chapter.

In humans, there may be anywhere from XXX to YYY billion fat cells, which range in diameter from 70 to 120 µm (micro-meters, that’s one millionth of a meter). WAT in humans is composed primarily (anywhere from 80 to 95%) of lipid. By lipid, I mean stored triglycerides (TG) which is simply a glycerol molecule bound to three free fatty acid (FFA) chains.

The remaining part of the fat cell is comprised of a little bit of water as well as all of the cellular machinery needed to produce the various enzymes, proteins, and products that fat cells need to do their duty. As it’s turning out, fat cells produce quite a bit of stuff, some good, some bad, that affects your overall metabolism.

For the record, one pound of fat is 454 grams and let’s assume 90% lipid on average. So about 400 or so grams is actual stored TG. When burned by the body, one gram of fat provides 9 calories so 400 grams of fat contains about 3600 calories of stored energy. Now you know where the old axiom of ~3,500 calories to lose a pound of fat comes from.

What is Bodyfat ‘for’?

Other than being considered unattractive in modern society, you may be wondering what role fat cells play in the body. The main role is as an energy storage dump. In fact, up until about 1994, the predominant view of fat cells was that this is all they were: a passive place for the body to store energy for times when there wasn’t enough food available. This turns out to be drastically incorrect but, before we go on, let’s look at that one aspect of fat cells.

In terms of energy storage, fat cells are truly exceptional in their capacity to do so. As mentioned above, a single pound of fat contains roughly 3,500 calories of stored energy. Assuming you could burn 100% fat as fuel (you can’t for reasons that aren’t important now), this is enough energy for a 150-pound person to walk roughly 35 miles or so before using that energy. That’s ONE pound of fat. Even at an extremely low 5 lbs of body fat, which is near the very low-end, you’re looking at ~15,000 calories of stored energy. That’s 150 miles of straight walking or so.

Here are some more numbers to give you a little better perspective. The average American male who may weigh 160 lbs with 15% body fat has 24 pounds of body fat which is about 84,000 calories of stored energy or so. He’ll have a metabolic rate close to 2,400 calories/day. Even with complete starvation, that’d get him through nearly 35 days without any food, assuming he was using 100% fat.

Extremely obese individuals may have upwards of 50-100 lbs of stored fat to the tune of 175,000-350,000 calories of stored energy. That’ll keep them alive for several months at least without emptying their fat stores. Some very obese individuals have been fasted for up to a year without problems.

In contrast, your other major energy source, which is stored carbohydrate in your muscles and liver, only amounts to about 500 grams at the maximum. Each of those grams of carbs gives your body 4 calories so that’s 2000 calories or so as stored glycogen. Not even enough to meet even a single’s days caloric requirements.

So at the low end, fat has a good 7 times as much energy as all of the carbohydrate in your body. At the high end, it can be several hundred times more. Although you don’t really want to use body protein for energy, assuming you used it all, it’d still only provide about 10,000 calories, still far fewer than your body fat. And you’d die long before you could use it all up anyhow.

The reason for the difference in energy storage capacity is water. Carbohydrate (stored in your muscles and liver as glycogen) is accompanied by a good bit of water. For every gram of glycogen stored, you store anywhere from 3-4 grams of water with it. In contrast, triglycerides only require about 1 gram of water for every gram of fat stored.

To store even 10,000 calories of energy as carbohydrate (2,500 grams of carbohydrate) would require 7,500-10,000 grams of water. Your cells would explode. Since it doesn’t have a lot of water associated with it, fat is a very space efficient way to store energy. Relatively speaking anyhow.

On top of that, a gram of fat provides more useable energy than either a gram of carbohydrate or protein. As above, the commonly given values are 9 calories/gram for fat and only 4 calories/gram for carbohydrates and proteins. So on top of being able to store more grams of fat total, each of those grams provides more useable energy to the body. Body fat is truly an ideal storage form of energy.

Obviously, from an evolutionary standpoint, the exceptional energy storage capacity of fat cells were an excellent advantage. Being able to store an effectively unlimited amount of energy in a relatively limited amount of space made it easier to survive through this time period when food wasn’t available. Now it’s just another evolutionary leftover that makes it a bitch to get into shape.

Speaking of evolutionary explanations, you might be wondering why men and women show such different fat patterning and I don’t mean why as in ‘What causes it to occur’ (which I’ll discuss in later chapter).  Rather, what’s the reason for it in terms of ‘Why did men and women evolve this way.’

To be honest, I don’t recall any good explanation for men’s propensity to store fat viscerally and around the abdomen. Perhaps more visceral fat was helpful in mobilizing fuel quickly when men were hunting; maybe they needed more ab fat to protect their organs when they were beating the shit out of each other to try and get a cavewoman’s attention.  One researcher thinks that male abdominal fat patterning has no actual role, and is simply an effect of gaining fat period.

Women, in contrast, are easier to explain.  Carrying their fat typically in their breasts and hips/thighs, this makes a lot of sense.  Hip and thigh fat, as it turns out, exists to provide energy for breast feeding after pregnancy.  Interestingly, when women are lactating, the normally impossibly stubborn hip/thigh fat becomes the easiest to mobilize.  I’ve wondered for a while if this couldn’t be mimicked somehow (e.g. with drugs) but could never come up with any practical way of doing it.

In addition, women’s fat patterning probably also evolved for sexual selection reasons.  Studies have clearly shown that men have a preference for an ideal waist/hip ratio (which suggests fertility and health) in women.  A woman with a narrow waist and curvy hips/thighs is more likely to have this optimal waist/hip ratio than a female who carries more visceral/abdominal fat.  Quite in fact, many of the situations that cause females to accumulate visceral fat (such as polycystic ovary syndrome or PCOS) are associated with decreased fertility.  This indicates how strong of a signal the waist/hip ratio (reinforced by fat on the hips/thighs) is evolutionarily speaking.

One final comment before moving on: I can’t explain why women put fat on the backs of their arms.  I’ve thought about it for years and simply can’t begin to come up with a reason women put fat there. If you know, or think you know, please email me.

But Wait, There’s More

There’s an old (and incorrect) idea that adult humans don’t make new fat cells. That is, and I’ll discuss this more in a bit, you get born with a certain number of fat cells and you may develop more at puberty or during pregnancy but that’s it; your body doesn’t make new fat cells. Everything in that sentence is true except the last statement; even non-pregnant adults can make new fat cells.

Usually this happens when the fat cells you have reach a certain size; that is, they are as full as they can physically be. When this occurs, the actual stretching of the fat cell stimulates the release of factors, such as Angiotensin II, prostacylin and others, which ‘tell’ the body to make new fat cells from something called preadipocytes. Preadipocytes are dormant cells, sort of soldiers who wait for the signals to get called into action. When the right growth factors are released, preadipocytes mature into normal adipocytes. Voila! New fat cells.

Those newly formed fat cells can now be filled with yet more fat and glucose. In fact, a new class of diabetic drugs (called TZD drugs) appear to work by stimulating the production of new fat cells, which gives the blood glucose and fatty acids another place to go. Oh yeah, if the new fat cells get too large, your body will keep making more.

I’d note that, in certain situations, the body doesn’t make more fat cells and this causes a ton of health problems.  I’d also note that this is only an issue for the extremely obese as a general rule of thumb.  There is a weird medical condition (called partial lipodystrophy) where lean adult humans don’t have enough fat cells and they can get health problems too.  You’re not one of these people.

Unfortunately, getting rid of fat cells is nearly (but not completely) impossible. Sure, liposuction is always available but, beyond that, eliminating fat cells is very difficult and only occurs under extremely severe conditions. I’ll talk about fat cell apoptosis (death) later on.

If there is a single reason for athletes not to get too fat in the first place, this is probably it: if your fat cells get too big, your body will make new ones. And it’s nearly impossible to get rid of the new ones. Obviously, if you’re already very fat, there’s little you can do: you’re pretty much stuck with your fat cells short of liposuction or something along those lines. But if you’re a lean athlete looking to gain weight (and realizing that you must gain some body fat to do it effectively), you should keep a lid on that fat gain. You don’t want to stimulate your body to make new fat cells.

Fat Cells Aren’t All Bad

If you asked most people about body fat, they’d say it was bad, they want to get rid of it, etc. Even researchers tend to promote this view, that body fat is a negative. The main focus tends to be on the negative health consequences of excess body fat (i.e. obesity) and there is certainly much truth to this.

Excess adiposity is associated with a number of health consequences, most of them bad.  From insulin resistance to the maintenance of a low-level chronic inflammation, getting and staying fat in the long-term is generally not healthy.

But this is a simplistic and incorrect view. The existence of body fat has important roles in human health and survival beyond just energy although that is one of its primary roles. Basically, while too much body fat is definitely a health risk, too little can be just as problematic.

One of the roles of body fat is as a physical cushion for your internal organs. The physical nature of body fat allows it to dissipate force more effectively than muscle. Athletes involved in combative sports (football, etc.) may benefit from carrying a little extra body fat, to protect their internal organs when someone rams into them. Of course, in modern society, most people don’t really have to worry about such things.

Fat cells also act as insulation, keeping folks warm when it gets cold. Of course, most modern people can put on more clothes or turn up the heat, something that our ancestors couldn’t do.

A less well-recognized aspect of fat cells are their critical role in immune and inflammatory responses. Pre-adipocytes, which I mentioned above, act like macrophages, cells which are critical for a proper immune system response.

Of course, this certainly doesn’t provide an excuse or reason for carrying excess body fat. But people who diet to extremely lean levels frequently report getting sick more, feeling more inflammation. While this certainly can’t be completely attributed to the loss of body fat, that may be part of the puzzle.  As I noted above, the opposite also holds true: the obese mount a chronic low-level inflammatory response that causes other problems.

As the final important role of body fat for this section, I want to talk about the role of body fat stores in glucose tolerance. For optimal health, the body needs to maintain blood glucose between fairly narrow limits. Anyone familiar with diabetes (either Type I or Type II) knows that a big part of the health consequences of those diseases has to do with the chronic high blood glucose (hyperglycemia) that occurs.

For reference, normal blood glucose is 80-120 ng/dl or so. Type I diabetics can run blood glucose in the 300-400’s or higher, Type II diabetics in the high 180’s or worse. Simply put, running chronically high blood glucose causes a lot of damage to the body.

Body fat is one of the places, along with muscle and liver, that your body stores glucose. Studies of rats, or humans, who have no body fat (the lipodystrophy I mentioned above) show chronic high blood glucose, just like diabetics.

This is because muscle tissue has a limit to how much glucose it can store as glycogen. Diabetics can’t get blood glucose into the cells because they are insulin deficient (Type I) or insulin resistant (Type II) ; rats and humans without any body fat don’t have any fat to store the glucose in. So the mechanism is different but the end result is the same.

Of course, having no body fat is rare and generally causes death at a very early age. So, if you’re reading this book, you don’t have it. My point is simply that all of the functions of body fat aren’t inherently negative. Fat exists for a reason, even if most of us are carrying more of it than we need or want.

But Wait, There’s EVEN More

Hopefully you get the idea that your body fat is an amazingly efficient place to store energy, in addition to having other roles mentioned above. Not only does it store incredible amounts of energy as it is, it can increase its energy storage if needed by making new fat cells. It’d be great, and I could end this chapter, if that’s all there was to it. As usual, it’s not and things are much more complicated.

Since the mid-90’s (1994 to be specific), the image of the fat cell as nothing more than a passive player in the body has gone the way of the dodo. On top of its major role in energy storage, your fat cells do so much more in terms modulating your overall metabolism. Body fat is turning out to be an endocrine organ in its own right.

On top of its obvious role in disposing of dietary fat, fat cells have been shown to play roles in overall glucose metabolism, blood pressure, appetite, fuel utilization, and hormone production to name just a few.

And with each week, it seems as if yet another role for the humble fat cell in modulating human metabolism is found.  I’m not going to get into massive detail in this book (beyond what’s applicable to the stubborn fat issue) but want to make you aware of some of what’s going on in fat cells.

On top of storing and releasing triglycerides, your fat cells also produce hormones themselves. A partial list of the compounds released from your fat cells would include leptin, which is involved in appetite, hormone levels, fat burning and muscle loss, not to mention dozens of other systems. These include:

  1. Angiotensin II, which is involved in blood pressure regulation and even controls blood flow to the fat cell itself
  2. Tumor necrosis factor-alpha, which has many varied functions including fat burning, immune functioning, and cell death
  3. IGF-1, which is an anabolic hormone
  4. Inflammatory cytokines like Interleukin-6, which is involved in immune function among other things
  5. Various prostaglandins, nitric oxide, acylation stimulation protein, resistin, adiponectin and I could probably list a half dozen more if I wanted to.

Fat cells are also one of the major sites of hormone metabolism. Testosterone is converted to estrogen (via the enzyme aromatase) in fat cells in both men and women. In fact, most of the estrogen in males (and in post-menopausal women) comes from the conversion of testosterone in fat cells.

Carrying more body fat for males means not only more estrogen, but also less testosterone (yet another reason for athletes to avoid getting too fat). Athletes who use anabolic steroids (or even prohormones) without using an anti-aromatase will get more conversion to estrogen if they are carrying more body fat. The metabolism of other hormones such as DHEA and androstenedione also occurs in fat cells.

Cortisol is also metabolized in fat cells, via an enzyme 11-beta-steroid dehydrogenase (11-beta-HSD) and there is some indication that differences in the activity of this enzyme may be related to how the body handles/produces cortisol and obesity.

And that’s just a quick look at some of the things that your fat cells are doing in your body. With each week, researchers seem to turn up more. As a quick tangent, the discovery that fat cells were more than a passive place to store energy has revolutionized the study of obesity. So instead of dozens of studies indicating why fiber is important for weight loss, there are now hundreds/thousands of studies dealing with all of the myriad hormones that are released from fat cells, and how they affect metabolism.

Although many of these factors can’t be controlled at this point, their discovery has opened up an entirely new area of possibilities for manipulating body composition and body fat levels. At the very least, we know more about why our bodies are reacting a certain way. Knowing the why gives us at least some possibility of knowing how to do something about it.

The main point for you to get from this chapter is that fat cells are far more than just a passive storage site for excess energy. Yes, they are amazingly well adapted to their role in energy storage but they do far more.  Basically, this is just background for the next chapter.  So turn the page already.

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6 thoughts on “What is Body Fat?

  1. Awesome article / extract from the book Lyle. Very intriguing to say the least.

    I just love the way you combine sound scientific data in an enjoyable, easy to read format. Keep up the great work.

    Reading your articles has inspired me to take my own knowledge to the next level & help others.


  2. Ditto on the above comment .
    What I’m looking for is information on what else is in a fat cell. What about vitamins, minerals etc. are they also stored in a fat cell? Is a fat cell the perfect body food?
    I’m on a 40 day water fast and being told I’ll kill myself by not getting vitamins and minerals etc.
    Would like to know if that’s true.
    I’ve got over 100 lb. to lose.
    Bill B

  3. Thanks for this article! I have been puzzled about the number 3500 cal/lb for body fat. This clears it up perfectly!

  4. Thought could have some interest

    [WORDPRESS HASHCASH] The poster sent us ‘1762401312 which is not a hashcash value.

  5. Great article! Would’ve been nice to add more info on brown fat, as Aingeru indicated.

    Apparently it’s not only cold that activates brown fats and make them burn more of the white fats for energy and heat production … prolonged stress and burns seem to have similar effects:

    Would be nice to read an article on the connection between stress and white / brown fat.

  6. This was written in 2008 (you might note that it is a CHAPTER from the book I published about that time) before the big resurgence in interest in this type of fat. And while I am good at what I do, I am not prescient/able to predict the future.

    Also I talked about it extensively in the SFS Patch 1.1. Also, in humans, it’s more accurately beige/brite fat not brown adipose tissue per se (also indicating that this article is very old befor the distinction was made).

    I also talked about BAT in last week’s site article.

    Stress wouldn’t be the cause so much as the increase in fight or flight hormones but chronic stress isn’t going to be a good way to make BAT. Cold exposure, potentially exercise, I provide a pharmaceutical approach to it in the SFS patch. There is still question as to the quantitative contribution of this type of fat in the first place; see last week’s article.

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