A calorie is the energy content in food, but if we’re getting technical, a calorie is what it takes to raise the temperature of one liter of water by one degree.
Calories are determined by using calorimeters, which are like mini incinerators. They involve a chamber surrounded by water, where freeze-dried powdered food is placed and incinerated. The temperature of the water is then measured, and this is how we determine the energy content of food in kilocalorie form.
Also, this is why it is said that “all foods aren’t created equal.” After all, one handful of lettuce will burn up quickly, leading to a minimal rise in water temperature, while the same portion size of almonds will be much denser, thus leading to higher temperatures. One cup of almonds will have around 530 calories, while one cup of lettuce will have maybe just five.
However, our body doesn’t “incinerate” calories; it digests them. This makes calorie counting far from a perfect science because, not only do you not know exactly how many calories you’re burning each day, but you can’t be sure of the calorie content in food.
The FDA allows labels to be off by as much as 20% in either direction, and most nutritional information comes from databases and not actual calorie measurements. This doesn’t mean calories are insignificant—far from it—but trying to measure things down to the calorie will be next to impossible.