The Science of Fat

We may earn a commission from links on this page.

If you're anything like over sixty percent of Americans, you've got a few pounds of fat you could stand to lose. If you saw what a pound of body fat actually looks like, you might be double-motivated. Yet there are a ton of misconceptions about fat—some of which could inform mistakes in our weight-loss endeavors. Let's see if we can't cut through some of the misinformation with a little bit of knowledge.

Welcome to Fitmodo, Gizmodo's gym for your brain and backbone. Don't suffer through life as a sniveling, sickly weakling—brace up and get the blood pumping! Check back every week for the latest in fitness science, workout gear, exercise techniques, and enough vim and vigor to whip you into shape.


What Is Body Fat, Like, Really?

Let's start with a positive spin, shall we? Think of body fat as "potential energy." Calories, which you consume through the food you eat, are fuel. Once these calories make it into your bloodstream, this fuel is burned by your various bodily processes. Yes, your muscles burn them, sure, but so does digestion, breathing, brain function, growing hair, etc. Basic being-alive stuff. Now, sometimes we consume more calories than our bodies are presently prepared to burn. When that happens our bodies say, "Oh dang, I don't need all of this energy right now. I'd better save it, in case I need it later." And so the miracle of fat begins.


Your body then takes these free-wheelin' calories and packages them into cells of fat. So, if calories are gasoline, think of fat cells as rubber balloons filled with gasoline. They expand as they collect more fuel, and they shrink when you use some of the fuel. Now, that's a very high-level explanation. An important note is that when this potential energy is stored inside fat cells, it isn't ready to use, as it was when it was coursing through your blood stream. It undergoes a chemical conversion so that it stores the energy more efficiently. It's kinda like a .ZIP file; it makes the energy more compact and storable, but makes the content itself harder to access. When it's time to pull some energy out of the cells, another chemical conversion takes place to turn it back into usable energy.

How Is Fat Burned?

So, when you lose fat, where does it go? Most people don't really know. If you remember the Principle of Mass Conversion from chemistry, you'll know that matter cannot simply appear or disappear—instead, it goes through chemical conversions and changes states. Just like your car's engine turns gasoline into heat and exhaust, your body utilizes a similar process.


The mitochondria (cellular energy centers) in your muscle or liver cells pull some of the fat (stored as triglycerides) from within your fat cells and put it through a metabolic process. This converts the fat into heat, carbon dioxide, water, and ATP (adenosine triphosphate). Let's break those down.

Heat: Heat energy is vitally important for being not-dead. You know how you, being a warm-blooded mammal, keep your body temperature right around 98.6 degrees Fahrenheit pretty much all the time? Yep, it's by burning calories. When you're cold, you burn way more calories to keep yourself warm. And in case you're wondering just how much heat energy is stored within fat, next time you make a pan of bacon, pour the excess fat into a can and put a wick in it. You will be shocked by how long it burns.


ATP: As you may remember from our look at creatine, we need ATP for muscle function. Our primary source of immediate energy is produced when we break a phosphate molecule off the ATP, and it makes a little explosion of available power in your muscles. Then it becomes ADP, and it can't be used again until it picks up another phosphate molecule. Krebs Cycle, baby. Basically, it carries fuel to your muscles.


Carbon Dioxide: Whenever you burn anything (see heat, above), it gives off carbon dioxide. It's true with gasoline, and it's true with body fat. The carbon dioxide will travel through your bloodstream until it returns to your lungs to be exhaled out.

Water: Fat typically feels kinda wet to the touch, right? That's because there's some water in it. You'll pee it out.


So that's where the weight actually goes when you lose it.

The Scary Stuff About Fat Cells

Here's one of the big misconceptions: When you lose weight, you don't actually lose fat cells. Nope, none of 'em. The average human body contains between 10 billion and 30 billion fat cells, and they are yours forever. Oh, but guess what? If you gain a lot of weight, you can grow more fat cells (obese people may have as many as 100 billion), and again, these cannot be lost (the one exception being liposuction, which actually removes the physical cells). So how the hell do you lose weight?


Remember how we said fat cells were like balloons? When you lose weight, you are letting some of the stuff out of the inflated balloons, thus shrinking the fat cells. You can shrink them all the way down until they're practically empty, but they will always be there—waiting to be refilled, haunting your chubby nightmares.

More bad news: Fat loves to hang out with more fat. Because fat and muscle are basically enemies (we'll get to that in a minute), your fat cells are trying to erode your muscle cells. Worse, while most fat resides under your skin, the more dangerous fat actually accumulates around your internal organs (this is why belly fat is more medically problematic than fat in other areas). This fat, called visceral fat, is metabolically active, and it secretes biochemicals that increase your risk of heart-attack, stroke, liver failure, diabetes, and high blood pressure. Additionally, visceral fat inhibits a very important hormone called adiponectin, which regulates metabolism in your body. In other words, the more visceral fat you take on, the slower your metabolism will be, and so the more easily you take on more fat. It's a tough cycle to break.


Burn Baby Burn

Because body fat is basically just stored calories, the widely-known broad-stroke method for losing weight is essentially this: Make sure you are burning more calories than you are taking in. Do that and your body will begin to pull those calories out of your fat reserves. There's a significant amount of nuance, but for the most part, that's true. But how exactly are those calories burned?


If you've ever done a programmed workout on a treadmill or stationary bike, you've probably seen phrases like "cardio zone" and "fat-burning zone." We'll get to those in a minute, but for now, all you need to know that direct exercise is the smallest slice of the pie when it comes to fat-burning.

There's a terrific article that breaks this down in a lot of detail, but here's the gist. There are three categories of processes responsible for you metabolic burn. 60 to 70 percent of the calories you burn in a given day are burnt just by being alive. That's with no movement at all. It's called basal metabolic rate (BMR). Another 10 to 15 percent is accomplished by the simple act of digesting your food, known as digestive metabolism (or thermic effect of food, TEF). As Active points out, that's between 70 and 85 percent—without so much as lifting a finger. That last 15 to 30 percent comes from physical activity, either in the form of working out (exercise-activity thermogenesis, aka EAT) or just shuffling around your apartment (non-exercise-activity thermogenesis, aka NEAT).

What's the take away? Well, if 60 to 70 percent of your caloric burn comes from your resting metabolism, doesn't it make sense to start with the biggest piece of the puzzle? It does! Going back to our fat candle analogy for a moment, if you look at the video above, you'll see that when the wick is short, the fat is burning off very slowly. Around the 1:50 mark, the wick becomes longer, which gives the candle a much, much bigger flame. With the larger flame, the fat starts burning off much faster.


So, how can we turn up the internal flame of your metabolism?

The simplest answer is by adding muscle. Muscle tissue, at rest, burns two to three times more calories than fat tissue does. So while cardio is certainly important for your overall health and stamina, if fat burning is your goal, then focusing a little more on weight-bearing, muscle building exercises will likely yield better results, and faster. Not because it burns more calories while you're working out, but because it turns up your metabolic flame so that it burns more calories all the time.


Next, let's look at eating. Remember, 10 to 15 percent of your metabolic burn comes just from digesting food. If you want to push that higher you can add more lean protein to the mix. Digesting protein burns two to three times as many calories as digesting carbohydrates or fat. Additionally, while any calories consumed (be they from protein, carbohydrate, or fats) can be stored as fat, the body more readily stores fat taken in from fat consumed, rather than carbs or protein consumed. All that said, a balanced diet is extremely important to keeping healthy, and again—if you want to shed fat, keep the calories coming in lower that the calories you burn.

Lastly, there's the exercise component (15 to 30 percent of your metabolism). So, that whole fat-burning zone and cardio zone on your treadmill? Technically, it isn't wrong. When you exercise at a lower intensity, you are burning more calories that are pulled from fat, whereas when you exercise at high intensities, more of the calories you burn come from the more-readily-available carbs that you've recently consumed.


BUT, here's the thing: Remember how more than two-thirds of the calories you burn have nothing to do with exercise? That only happens if you can create a caloric deficit, and you can create a caloric deficit much, much faster by engaging in high-intensity, interval-type exercise. It simply burns far more calories, so you're getting a lot more bang (fat-loss) for your workout buck. To say it another way: While the slower, "fat-burning zone" technically pulls more calories out of the fat while you're doing it, high-intensity stuff will burn more calories over all, which will result in more calories being pulled out of your fat reserves over time, which will shrink them more. That, and high-intensity exercise, builds muscle better—just look at sprinters versus marathoners. And again, more muscle equates to a higher metabolism, and that equates to faster fat burning.

To be honest, this really barely scratches the surface. This article is intended as a high-level overview, and as such, there is a ton that couldn't be included. The articles we linked to include a lot of the more technical stuff, and for those inclined, we recommend diving in. For everybody else, we hope this gave you a little more insight into the junk in your trunk.


Image credit: Shutterstock/Luis Louro