Everything in the universe obeys the laws of physics, including your body. The Law of Conservation of energy says that energy can neither be created nor destroyed; rather, it transforms from one form to another. This is an absolute law.

That means the energy (calories) in food you eat cannot simply disappear. Every calorie has to be accounted for.

If food is not used for energy, dissipated as heat, or expelled in urine or feces, it will be stored in the body in some way (fat, muscle, or glycogen) -- meaning you gain weight.

For that reason, the only way you can lose weight is through a "calorie deficit" -- eating fewer calories than your body uses -- which will then require your body to retrieve stored energy (e.g., fat). Over time, this calorie deficit causes weight loss.

Likewise, if you are gaining fat, the only reason that is happening is by consuming excess calories.

No specific food or dieting method can magically alter the laws of physics.

The Energy Balance Equation

The equation can be stated as follows:

Change in Body Stores = Energy In - Energy Out

Body Stores: Any tissue available for breakdown; so, water is not included
Energy In: Calories ingested (energy absorbed from the food you eat)
Energy Out: Calories burned (energy expenditure)

A "positive energy balance" is when energy intake exceeds energy expenditure; when this occurs, you gain weight as your body stores calories.

A "negative energy balance" is when energy intake is less than energy expenditure; when this occurs, you lose weight as your body breaks down and burns stored calories.

This is often summed up as "Calories In, Calories Out", or to lose weight you need to "Eat Less and Move More". It's not quite that simple, though, as you'll soon see.

The Energy Balance Equation is Always Correct

This formula has been tested for decades by scientists, and it has always held up to scrutiny.

Sometimes the numbers don't seem to add up, and when that happens, people get understandably frustrated. They'll say "See, Calories In Calories Out doesn't work!". Or "Eat Less, Move More doesn't work!" But it's not because the Energy Balance Equation is wrong. Our bodies always obey the laws of physics. Something has to happen to every consumed calorie; it can't just disappear.

The only reason the formula wouldn't "add up" is if you aren't accounting for all the variables. There are a number of variables to consider, and each one can change and affect the fate of a calorie.

Here is an expanded equation that explains the most important variables:

Change in Body Stores = Energy In (Absorbed Food) - Energy Out (RMR + TEF + PE + NEAT)

Don't worry -- we'll explain what all those letters mean below, and it's pretty simple to understand.

Energy In

Not all food you eat is actually digested and absorbed by the body.

Some foods are simply harder to digest, and if they aren't digested, they don't count as energy-in because it is expelled in the feces.

On average our bodies absorb roughly 80 - 95% of the food we consume; or stated the other way, roughly 5 - 20% of what we eat isn't absorbed at all.

Here are some rough figures for absorption rates of various nutrients:

Fats: 95%
Animal proteins: 90-95%
Vegetable proteins: 80-85%
Carbs: Varies drastically depending on the form (processed, unprocessed) and the fiber content

However, this can vary quite a bit.

In general:

We absorb MORE energy (calories) from highly processed carbs and fats, and from foods that are cooked, chopped, soaked or blended.
We absorb LESS energy (calories) from whole foods, especially foods containing fiber.

Several studies have been done with whole nuts. One study showed that 38% of the fat in peanuts was excreted in the stool, rather than absorbed; in contrast, almost 100% of fat in peanut butter was absorbed. Another study on walnuts showed that only 79% of the calories were absorbed. Another study showed that 95% of calories in pistachios was absorbed (showing once again that absorption rates varies a lot from food to food).

High-fiber diets tend to reduce the amount of calories absorbed, because soluble fiber binds with small amounts of protein and fat in the stomach and carries it out in the feces.

Energy Out (Total Energy Expenditure or TEE)

This side of the equation refers to what happens to calories that are digested and absorbed (basically all food that is not expelled in feces).

This includes several factors:

RMR (Resting Metabolic Rate)
Thermic effect of food (TEF)
Physical Activity, which consists of:
1. Formal Physical Exercise (PE)
2. Non-exercise activity thermogenesis (NEAT)

Resting Metabolic Rate (RMR)

This is the largest contributor to TEE, which is why we've explained it separately. Basically, you burn most of your calories by simply being alive.

Thermic Effect of Food (TEF)

Digesting and absorbing food requires energy. Certain types of food require more energy than others. As your body works to process the food, heat energy is released. If you've ever felt warmer after eating a large meal, that's TEF. This process is also known as Specific Dynamic Action (SDA) or Dietary Induced Thermogenesis (DIT). But we'll refer to it as TEF.

On average, about 10% of the energy (calories) in food you digest is burned during processing.

For example, when you eat protein, it has to be broken down by the liver into amino acids -- and that requires energy. When those amino acids are used to create other proteins, that also requires energy.

Here are the TEF values of various food types:

Protein: 20-30% loss
Carbs: 5-6% loss
Fats: 2-3% loss

As you can see, protein has the highest TEF value, so that's one (of many) reasons why protein is just a great diet food.

Also, you can also see that trying to boost TEF by exchanging carbs for fats, or vice versa, won't make a significant difference.

Physical Activity

This should be obvious -- when you exercise you burn calories. The technical term is the "Thermic Effect of Activity". There are two main types: formal physical exercise, and Non-Exercise Activity Thermogenesis (NEAT).

Formal Physical Exercise (PE)

Here, the vast majority of your calories are burned during the exercise itself. However, your metabolism is in fact a little higher for some period of time after exercise (called "excess post-exercise oxygen consumption" or EPOC -- much like how your car's engine stays warm even after you shut it off).

The amount of calories burned during exercise varies tremendously. When you are trotting on a treadmill, you're burning around 300 - 600 calories per hour. An elite athlete might be burning 2-4 times as much.

Non-Exercise Activity Thermogenesis (NEAT)

This refers to all physical activity outside of formal/purposeful physical exercise. It's sometimes called Spontaneous Physical Activity (SPA). In most people this is the largest contributor to Physical Activity.

NEAT includes all the movement you do naturally. You often don't really think about it; it just happens. For example, fidgeting, staying upright, walking across the room, walking to your car, sitting and standing, and so forth ... that's NEAT. It is strongly affected by changes in caloric intake: that is, if you increase calories, your body has extra energy and your body causes you to be more active. When you reduce calories, the opposite tends to happen: you reduce activity.

You would be surprised how much energy is expended through NEAT. This type of movement can account for 200 to 900 calories per day! One study reported that some people can dissipate as much as 69% of excess calories as heat via NEAT movement. Those less able to activate NEAT store a higher proportion of the excess calories as fat.

NEAT is considered genetic. Some people simply expend more energy spontaneously without thinking about it; such as sudden urge to run instead of walk; they just can't seem to sit still. When such people overeat, their metabolism are capable of burning most or all of it off (we agree: it's not fair).

Jobs that require a lot of movement (walking, lifting, etc) are typically categorized as NEAT, because it's not formal exercise. However because it's not spontaneous, it's possibly more like formal exercise. You can burn through a lot of calories when you are doing physically demanding tasks all day long. It doesn't matter how it's categorized, though; it's all added up as Energy Out.

Example Calculation

This is a very simple example, just to illustrate the concepts.

Say you eat 2000 calories one day.
If 90% is digested and absorbed, that leaves 1800 calories.
If TEF is 12%, that leaves 1600 calories.
If your RMR is 1400 calories per day, that leaves 200 calories.

Those extra 200 calories could be burned through PE or NEAT -- otherwise they're deposited in your body in various storage areas for future energy use.

It's worth pointing out that if this was a real-life scenario, we would be making a lot of assumptions that could be wrong. First, we're assuming that this person really did eat 2000 calories; studies show most people simply cannot accurately judge how much they eat. Furthermore, food labels can be up to 25% inaccurate. Also, the type of food, and your true RMR could swing the numbers by hundreds of calories. So, that 200 calories could end up as 500 calories or higher, or even ZERO or negative calories (a deficit). Real numbers aren't easy to measure accurately unless you are in a controlled metabolic lab.

However, the point of this example is to help you understand what parts of the formula affect your goal to lose weight or maintain, so you understand what you can change, where calculations can go wrong, what matters, and what doesn't.