[gateorade]

This is largely incorrect, and also not what the person you're replying too is saying. Calorie numbers in food are just a weighted sum of the grams of carbs, fats, and proteins in the food. Each of those groups are assigned a fixed weight based on an estimate of how many usable calories are in a gram of each.

The commenter above is talking about how we often ignore the adaptability of the body, and that if you suddenly cut caloric intake by say 300 a day, the body will surprisingly quickly adapt by lowering metabolic rate to compensate. This can be done through the simple modulation of hormones and may also involve the shedding of metabolically expensive tissue, like muscle.