The Physics of Weight Loss

Today I want to talk about weight loss. Or getting into shape. Or both. This is something I’ve been thinking about lately, and while I’m sure probably everyone reading this has had to lose a few pounds at one point or another. Everyone knows the deal: To get in shape you need to watch what you eat, hit the gym, take up running/jogging/biking, so on and so forth.

Time to hit the gym

But why? Why does any of this stuff work? Everyone knows that running can help you lose a few pounds, but most people don’t actually know why, or more importantly where that weight goes. So it’s time for me to do what I do best: overanalyze things with physics.


We often use these two terms interchangeably, but in fact they are completely different—albeit related—things. Your weight is essentially your mass—how much stuff you’re carrying around with you. It’s how hard you’re pulled toward the center of the earth. Every single part of you—fat, muscles, bones, skin, organs, everything—contributes to your weight.

Your shape, on the other hand, is really about how that weight is arranged. In American culture, you’re generally considered to be in shape if you have low amounts of body fat and at least some muscle. Obviously that’s a broad generalization but we have to define our terms somehow.

When talking about either recreational fitness or overall health and wellbeing, your shape is far more important than your weight. Especially since muscle weighs more than fat. It’s not your weight that reduces your lung capacity or puts you at a higher risk of stroke and heart attack, but the amount and distribution of fat in your body. This is why the Body Mass Index (BMI) which compares only your weight to your height, is worse than useless as a measure of health. Just going by their BMIs, most professional football players would be considered obese.

Just look at this fat slob

But there’s another reason for distinguishing between weight and shape, and that’s because the two of them change somewhat independently of each other. Anyone who’s ever started lifting weights to get into shape knows this. As you replace the fat with muscle your shape will improve while your weight increases. And people who starve themselves will see their weight fall while their shape deteriorate as their bodies pack on stores of fat. There is somewhat of a correlation between the two, but what exactly that is differs from person to person depending on body type and genetics.


Physically speaking, losing weight is incredibly easy. One of the most fundamental rules of our universe is the conservation of mass. That is, mass can neither be spontaneously created nor can it spontaneously disappear. And since weight depends on mass, this gives us an equation for the change in a person’s weight:

The amount by which your weight will change over any given time is exactly the difference between how much weight you added to your body (by eating) and how much weight you removed from your body (through various excretions). Notice that this depends only on the amount of food you eat, not the quality of it. In terms of your weight, a pound of celery is just as bad as a pound of birthday cake. Also notice that this does not depend on how much or how little you exercise.  It’s simply the difference between what’s going into your body and what’s coming out.


This is where it gets complicated, and I’ll admit I’m by no means an expert in biochemistry. But I’m not too sure how many people would be interested in those details anyway so I’ll just summarize. Where your weight is all about the mass in your body, your shape is all about energy.

All of our energy comes from what we eat. It’s stored in the chemical bonds of food and we measure it in calories. In our stomachs we use acid to dissolve the food, storing that energy in glucose molecules. That glucose is then either used for energy immediately or stored in the body as glycogen in fat deposits. As our bodies consume energy throughout the day we dip into these fat reserves, so the key to staying shape, or at least the key to not gaining any fat, is to use more energy than you consume.

What makes this difficult is that it’s very hard to measure exactly how much energy your body is using. We are literally using energy all of the time, even when we’re sleeping. It takes energy just to be alive. We’re constantly spending energy to heat ourselves up, and an incredible twenty percent of all of the energy we use throughout the day goes to powering just our brains. We call the amount of energy it takes to sustain your resting body your Resting Metabolic Rate (RMR, although when I learned it we called it the Basal Metabolic Rate), or more commonly known as your metabolism. Since muscle tissue requires more energy to sustain than fat, the more muscles you have the higher your RMR will be. Any extra energy your body uses due to exercise adds to this metabolic rate to determine your body’s total energy output. If you’re consuming less energy than you’re using, your body will naturally take the difference from its stores of fat, leading to you being in better shape.


I’m sure you’ve heard before that you weigh less in the morning than you do at night, and it’s true. It’s not just because of all the food you ate during the day, either. So a big question is: where does that weight go? It’s tempting to say that it’s due to the energy our bodies use while we sleep, as mentioned earlier, but that’s not right. We’re not stars, so we can’t convert mass into energy. Honestly, I think the real answer is even stranger than that.

We breathe all that extra mass away. That might sound ridiculous, but think about it. When we breathe, we inhale air with a density of around 1.2 grams per liter and exhale carbon dioxide which is considerably heavier with a density of 1.977 grams per liter. It’s not enough to notice a difference with a single breath, but multiplied over the course of an entire night it can add up to over a pound. And if this still seems a bit farfetched, consider that the exact opposite process—inhaling carbon dioxide and exhaling lighter oxygen—is where trees get all their mass.


According to everything I’ve said so far, it would seem that there’s no link between exercise and weight loss. But anyone who’s ever exercised knows that isn’t true. I’ve come back from 10 mile rollerblading trips a full three pounds lighter than when I left. So where is that weight going?

As you can probably guess, a significant amount of this is water weight which we lose through sweat, but not all of it can be explained this way. The process of the body using fat to power itself is called ketosis, and through a lengthy series of interactions the glycogen mentioned earlier is converted into ATP, the form of energy which can be used by our cells. The biggest byproducts of this reaction are water and carbon dioxide. So as your body undergoes ketosis, the fat cells shrink as the glycogen is removed from them and you sweat and breathe out the byproducts, causing you to both lose weight and get into better shape.


You’re probably expecting me to have some final wisdom about the best way to get into shape or lose those pesky pounds, and I really wish I did, but ultimately it boils down to knowing your body and using common sense. One of the strangest things working against us in the battle of the bulge is our sense of hunger. Don’t trust it. Studies suggest that for people in developed countries (with ready access to stocked refrigerators and grocery stores) hunger is more of a social and conditioned response than a physiological one. One thing I’ve found lately is that I was just eating way more than I needed to, even though my eating habits before hadn’t been particularly unhealthy. What I’ve come to realize is that what I considered my resting state before was actually my body’s way of saying it was full. I was essentially filling the tank every time it dropped below three-quarters, and in doing so I was keeping my body from using its stored energy. If you find yourself consistently feeling stuffed after meals that might be why. Try to really pay attention to what your stomach is telling you, and just because it’s growling doesn’t necessarily mean it’s empty.