The weight loss industry is lousy with outrageous and untested claims, but probably none more egregious than this one. Beyond being outright professed everywhere from infomercials to the supermarket, this claim underlies much of the misinformation floating around regarding nutrition and metabolism. As we'll see, the idea that you'll lose weight as you gorge yourself with Twinkies (or whatever whatever you want constitutes) violates a physical concept so basic to the universe that you probably didn't even know that it has a name.

"Ridiculous!", weight loss gurus everywhere might bellow. (After which their boggarts morph from grossly obese versions of themselves into happily bouncing beach balls.) Surely their special system for losing weight must do something to circumvent your body's fat-gaining ways. Perhaps those five minutes of exercise a day boost your metabolism, or that special pill melts fat away as you sleep, or only eating whole wheat before noon modifies your body's intake of fat the rest of the day. Or, perhaps not.

The Balance

The concept of the balance permeates all of the physical sciences. It is often put forth as the idea that conserved quantities are neither created nor destroyed, such as with energy in the first law of thermodynamics. However, the general idea is not restricted to conserved quantities, because we can readily account for the creation or destruction of the quantity in question, if such processes are possible.

To build balances (also known as continuity equations), you imagine a little stationary cube floating in space. You then imagine all the ways that the amount of stuff in that cube can change, and add or subtract these as appropriate. Stuff can move into the cube, stuff can move out of the cube, stuff can appear in the cube, or stuff can disappear in the cube. That's all there is, and thus you have the general balance:

Change = In - Out + Generation - Consumption

This equation can apply to any extensive (size-dependant) property of the system. In my field (chemical engineering), we usually stick to mass, energy, and momentum. Sometimes we get crazy with the Cheese Whiz and do balances on angular momentum or electrical charge, but that gets messy. And, if Wikipedia is to be believed, you can even do balances on probability waves when you're working with quantum mechanics. Since thinking about that makes my brain feel all squishy, we'll just move on.

A Balance on Weight ... There's a Pun Here Somewhere

The next step is to specify the quantity you're doing the balance on, and cross out the terms in the equation that don't apply to your situation. For weight loss, we'll be doing a calorie balance on the human body. This is a bit strange to think about, because people are worried about their weight rather than their excess calorie load. The trick is that one pound of body fat is roughly 3500 extra calories; that's 9 calories per gram of fat, with a slight markdown because your body doesn't just store fat as bulk lard or oil. I'll be putting weight change on the left hand side of our balance and calories on the right, so bear this conversion detail in mind.

Let's go through the rest of the equation for weight loss. "In" will be eating, since that is how we take in calories. "Out" will be pooping, since that's how calories move out of your body. "Generation" will be zero, because humans aren't plants and thus we don't make calories inside our bodies. And "Consumption" will be burning calories.

Weight Change = Eat - Poop - Burn

That's all, folks! If you Eat more calories, you're going to gain weight. If you Burn more calories, you're going to lose weight. (It's not trivial to change the number of calories you excrete.) Fancy diet schemes all rely upon obfuscation of this simple equation. You cannot get around this balance without violating the most basic laws of the physical world, regardless of what any infomercial or magazine ad might claim.

Instead of Sweating the Details ...

I'm sure both professional and self-proclaimed amateur experts on nutrition would gag while reading that last paragraph. (Disclaimer: that was not a bulimia joke.) The nitty-gritty details of weight loss are certainly more complex than what I've outlined above, both in general and in special dieting situations.

One example of complication that comes to mind is simple vs. complex carbohydrates. Eating an equivalent caloric amount of these would technically lead to a different weight change. Your body cannot absorb complex carbohydrates directly, and must rely on bacteria in your digestive track to break them down to sugars. Thus, some of the eaten calories are used up by your gut bacteria, and furthermore the likelihood that you'll just excrete these calories before you absorb them increases with the residence time in your gut.

The obvious example from the extreme dieting end of things is the Atkins diet. Briefly, your brain needs sugar to function because it cannot directly burn fat. If you restrict your carbohydrate intake to a sufficient degree for a long enough time, your brain starts running out of sugar. Your body panics at this point, and begins making ketone bodies from fat. Ketone bodies are close enough to sugar that your brain can use it for energy, but they're also fairly strong acids. Your body then beings excreting the ketone bodies in urine to combat this acidification, effectively flushing calories down the drain.

Despite these complications, the calorie balance remains valid. I do not deny that certain dietary activities can have an impact on the various terms in the balance equation. The types of calories you Eat impact what you Poop and Burn. Exercise both Burns calories directly, and might also give you the fitness and energy and motivation to later jog up the stairs rather than take the elevator (increasing your latent Burn rate, or quote-unquote boosting your metabolism). Taking a drug like Orlistat (also known as Xenical, alli, and tetrahydrolipstatin) blocks fat uptake in your intestine, thereby increasing your Poop rate. There are countless such complexities, and their details probably won't be fully understood by nutrition science anytime in the near future.

... Try Sweating at the Gym Instead!

While these complications affect the calorie balance, I argue that they do not dominate it. If you Eat ten thousand carbohydrate calories tomorrow, you are going to gain weight. There are certainly complications: if you eat it all at once, you'll probably Poop a large number of the calories out; if you eat simple sugars, you'll probably just gain it all as weight; if you eat pasta, you'll probably naturally Burn some of it in your gut before converting it to fat. However, none of these complications change the fact that you're going to pay the piper for wolfing all those carbs down, and the piper's going to charge your waistline nearly the same rate regardless of the details.

The weight-loss pseudoscientists thrive on these details. I would categorize this as a type of mystery mongering. The precise details of weight loss are extremely complicated and not fully understood, which gives these guys a huge gray area to hide in. "Eat whatever you want, because my pill decreases calorie uptake! Eat whatever you want, because my exercise plan boosts your metabolism! Eat whatever you want, because my proportion of carbs-to-protein intake increases fat breakdown!"

There is nothing mysterious about the overall weight change equation (the calorie balance). If you eat less food and/or get more exercise than you have in the past, you will lose weight (or at least gain less weight, if you don't tilt the balance enough). The details and complications are there, but they are not the dominant effects. This is called a first approximation, and those tend to be pretty darn good at getting the overall trends. And, that's what the average American is looking for: an overall weight trend that goes downup. instead of

Two points before I wrap this up. Most importantly, talk to your doctor if you're going to try an extreme diet. The ketoacidosis I described above can be an extremely dangerous situation; you are literally mere days from your organs failing due to energy depravation, over-acidification, and/or dehydration. The second point is that weight loss should be a slow process. Most people slowly gain weight over the course of years or decades, so don't be discouraged that you don't drop ten pounds in a month. Make a permanent change to the right hand side of the energy balance (i.e. eat less and exercise more), and be patient while the left hand side responds.