Holland Clinic

There was a television commercial decades ago that showed a heavy
man sitting on a couch, eating pizza, watching an exercise video and
looking very proud of himself because each time the video told him to
"pump", he grabbed and ate another bite of pizza. The point of the
commercial was that weight loss isn't quite so easy.

Still, there is a grain of truth to the notion that eating can burn
calories; it actually does, and how many calories are consumed by
eating depends largely upon the macronutrient composition of the food
consumed. This effect of calorie burning from eating is known variously
as the "Thermic Effect of Food", "Specific Dynamic Action of Food" or
"Diet Induced Thermogenesis". Let's examine this a bit more closely.

Digestion and metabolism are not perfectly energetically efficient so
that some of the energy (calories) in food cannot be utilized to make
body fat under any circumstances, but is instead lost as heat. The
percentage of calories lost as heat varies for each macronutrient
rather dramatically as shown below:

Fat: 2-3% of calories burned off as heat
Carbohydrate: 5-10% of calories burned off as heat
Protein: 25-30% of calories burned off as heat
Alcohol: 20-30% of calories burned off as heat.

As you can see, the highest "thermic effect" is found with protein
where as much as 30% of the calories are lost to heat. This is rather
incredible information. What it means is that not all calories have an
equal ability to cause weight gain and that protein is least likely of all.
To put it another way, protein has always been said to hold 4 calories
of energy per gram (and indeed it does when it is burned in a
bomb-calorimeter), but when it is used by the body, it effectively only
has about 3 calories per gram.

How is this possible?

Well, first let's look at how calories are actually measured in a nutrition
laboratory. Generally calorie measurement is acheived by literally
burning a precisely measured amount of food inside a super-insulated
container using pure or nearly pure oxygen and measuring how much
heat is thus produced. Heat production can be easily calculated by
jacketing the device in copper tubing filled with a known amount of
water and then using a thermometer to see how hot the water gets.

Since a dietary calorie is the amount of heat required to raise the
temperature of one liter of water by one degree centigrade, it's a
simple matter to deduce how many calories were in the
now-burned-up food. This device is called a "bomb calorimeter" and
variations of it are still used in nutrition laboratories today. At any
rate, the point of all this explanation is that we measure food calories
by burning food down into carbon dioxide, water and nitrogen gas.
This tells us the total amount of energy that was contained in the
food, but it doesn't say much about how the human body will actually
use that energy when the food is eaten.

In fact, the goal of digestion and metabolism is not to produce heat at
all, but to use every possible calorie to feed metabolism and movement
and, if after those processes are complete, there is any energy left
over, to use those remaining calories to make body fat. Heat is
produced in this process not deliberately, but rather because it
cannot, by the laws of thermodynamics, be avoided.

A car is an example. We put gas in the tank to make the car move. It
does this by spraying the gasoline into a closed cylinder, mixing it with
oxygen in the air and then using a spark plug to ignite the whole thing
so that the explosion produced pushes a piston that ultimately drives
the car. In this case, some of the energy in the gasoline is used to
move the car, and some of it is lost as heat. This is why the engine
gets hot and why we need a cooling system to keep it from melting.
Now just like the human body, the goal with a car is to use as much
energy as possible for movement and waste as little as we can as
heat. The better we are at this, the better the mileage, but just as
with the human body, there is a limit to how efficient any car can be.
No matter how well engineered the car, some of the energy in the
gasoline that powers it will be lost as heat. No matter how good our
bodies are at capturing calories for useful work and for fat storage,
some of the energy in food is lost as heat.

This was bad news for our ancestors who routinely faced famine, but
good news for people today who are trying to lose weight. The
thermic effect of food can be harnessed to help burn off calories as
heat. In the next section we'll examine ways to do this.