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 as shown below:
The thermic effect of food (TEF) is the energy "cost" (in calories) associated with eating, digesting, processing, storing and using its nutrients. To put it more simply, eating and digestion burn calories. Those calories are the thermic effect of of food.
Many people have heard the claim that "it takes more calories to digest celery than you get from eating it. It's hard to know if the claim is true but it illustrates TEF well. Another way of stating the celery claim is this: "the thermic effect of celery exceeds its calorie content".
What Causes the Thermic Effect of Food?
The thermic effect of food is result of the fact that no process in the universe is perfectly efficient and in order to get calories from food and use or store them, we have to burn a few calories along the way. We burn them by the muscular action of chewing and swallowing and peristalsis in the gut. We burn them by making and secreting enzymes and acids in the stomach and gut. We burn them by transporting nutrients across the wall of the intestine and through the blood stream. We burn more still in the chemical and metabolic processing of those nutrients in the liver and elsewhere. In short, a calorie on the plate is not a calorie on the body but is instead some fraction of a calorie. The difference is caused by the thermic effect of food.
This is useful and interesting information. For example, the high thermic effect of protein is one of the reasons that high protein diets tend to be helpful for weight loss.
Macronutrient composition is not the only thing that determine a food's thermic effect. It is also affected by the amount of mechanical work required to eat the food.
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.