The expression "thermic effect of food" is used to describe the energy expended by our bodies in order to consume (bite, chew and swallow) and process (digest, transport, metabolize and store) food. We "expend energy" by burning calories.
Processing protein requires the greatest expenditure of energy, with estimates ranging as high as 30%. Dietary fat, on the other hand, is so easily processed and turned into body fat that there is little thermic effect, perhaps only 2 or 3%. The amount of energy required to process carbohydrates falls between that of protein and fat.
As you can see, all calories are not equal. If you eat an equal number of calories of protein, fat and carbohydrates, the calories in the fat are more likely to end up on your waist as fewer of them are burned off by the thermic effect.
And as you learned in Calories in Protein, Fat and Carbohydrates, a gram of fat contains more than twice as many calories as a gram of protein or carbohydrate. So a gram of fat not only gives you more calories, but a smaller percentage of them will be burned off by the thermic effect. A double whammy.
A figure of 10% is generally used to account for the thermic effect of food. This means that if you want to replace 500 calories burned through activity, you need to eat 10% more, or 550 calories. And if you eat 500 calories, 10% of them will be burned off by the thermic effect, leaving only 450.
However, most people are either unaware of the thermic effect of food or choose to ignore it, making weight loss seem just a little easier.
Note that calculations made with the Activity Calculator do not take into account the thermic effect of food. Nor do basic calculations made with the BMR & RMR Calculator. However, if you apply an activity factor to your BMR or RMR, the activity factor does account for the thermic effect of food.
Activity factors are explained in Calculating BMR and RMR under the heading Using the Calculations.