Calorie (and kilocalorie)
A unit of measurement for the energy in food. The everyday "calorie" is actually a kilocalorie (kcal), the energy needed to raise 1 kg of water by 1°C.
Technically, a calorie (cal) is the energy needed to heat 1 g of water by 1°C. But in nutrition, when we say “calorie”, we are actually talking about the kilocalorie (kcal = 1,000 cal). A food “with 200 calories” contains 200 kcal. The kilojoule is also used (1 kcal = 4.18 kJ).
The original angle (not all calories are equal): the dogma “a calorie is a calorie” is true in pure thermodynamics, but misleading in physiology. Three reasons:
- The thermic effect differs: 100 kcal of protein “costs” ~25 kcal to digest (leaving ~75), against ~8 kcal for 100 kcal of carbohydrate and ~2 for fat. Protein calories are therefore “less caloric” net.
- The hormonal impact and fullness differ: 100 kcal of fizzy drink (liquid sugar) triggers an insulin spike, does not fill you up, and favours storage. 100 kcal of eggs keeps you full for longer and supports muscle. Same energy, opposite metabolic effects.
- Absorption varies: the calories in nuts or high-fibre foods are partly not absorbed (lost in the stools). The figures shown (the Atwater system) are approximations.
A balanced caveat: this does NOT mean that “calories don’t count”. Energy balance remains the central determinant of weight. But the quality of calories modulates fullness, hormones and body composition, and so, indirectly, how much you eat and where the energy goes. Counting calories can help, but focusing solely on them while ignoring quality is a mistake.