Like kilometre is the measure of distance, a calorie is the measure of energy.
One food calorie (1Kcal) is the energy that will raise the temperature of 1 litre of water by 1 degree C.
The old method of calorie counting was simple and was called the bomb calorimeter method.
Food (after removing its moisture) was kept inside a chamber that was surrounded by an insulated jar filled with a measured amount of water.
Any air from the chamber will be sucked out and pure oxygen would be added; then a fuse would ignite the food sample causing it to burn.
The resulting change in the water temperature multiplied by the amount of water gave the calorie count.
For example, if you burnt 100g of Pringles potato chips, surrounded by 2 litres of water, and the water temperature increased by 30 degrees C, you could say 100g of Pringles chips have 60KCal (2 Litres X 30 degrees C).
This method, however, had a flaw.
Food usually contains fibre that would burn in the calorimeter, but in reality, it is not absorbed into the bloodstream and is thrown out of our body into the toilet.
This system, therefore, was modified using the current Atwater System.
Dr. Atwater, in the 1800s, took a group of people and monitored the core content of their diets.
He also took the core content of their poop & urine (gases were ignored) and subtracted energy lost in them from energy obtained from the diet (as measured using Bomb Calorimeter Method).
Based on multiple readings, he concluded it as a 4-9-4-7 method, i.e., proteins produce 4Kcal/gram, fats produce 9 Kcal/gram, carbohydrates produce 4Kcal/gram, and alcohol produces 7Kcal/gram.
Using the 4-9-4-7 method, you can also calculate calories in your diet.
This model, too, is far from perfect because two foods with the same number of calories on the label can have a different number of calories in reality as absorption of food can vary with each person and the type of food.
These deviations, however, have been considered insignificant, but the search for a better system is still on.
