Calorie Defined

The official definition of calorie is: A measurement of energy- the amount of energy needed to raise 1 gram of water 1 degree Celsius at standard atmospheric pressure. Calories in food are actually measured in kilocalories, so 1,000 actual calories for every 1 Calorie listed on food packages. Europe uses the actual kilocalories or kilojoules.

During the 1880s, Wilbur O. Atwater decided to determine how much energy different types of foods contained. He decided to treat different foods just like coal and burn them to ash in a furnace and measure how much heat (or calories) each one produced. He gave a numerical value to the calories produced by each food. He measured nine calories per gram from high-fat foods, and about four calories per gram from carbohydrates and proteins. We still use this system, to an extent.

During 2003, a US university team of nutritionists tested two slimming diets with the same number of calories on a group of overweight women. One diet was very low-fat and relatively high in carbohydrate. The other was high in fat, but low in carbohydrate. The low-fat dieters lost 3.9kg (almost 9lb), but the high-fat dieters lost more than twice as much weight at 8.5kg (almost 19lb).
The calories are the same, but the body reacts differently when it uses them. Maybe it is time to rethink how we count calories.
Calories listed on food labels are only an approximation. The US FDA allows food manufacturers to look at their ingredients and determine how many grams of fat, carbohydrates, and protein they contain, and then assume that each gram of protein and carbohydrates gives 4 kilocalories, each gram of fat gives 9, etc. Then they subtract 4 kilocalories for every gram of fiber, and that is the official, government sanctioned calorie measurement.
In addition to the above, different bodies deal with Calories differently. Genetic conditions, illnesses, and other factors can cause foods to be metabolized differently by some people vs. others. A 100 Calorie snack for you, might only be an 80 Calorie snack for someone else.

Bottom line, since Calories are an approximation, ingesting a few hundred more or less on any given day is not going to make much difference on your weight.