Calorie
A calorie on a food label is really a kilocalorie: the energy needed to raise one kilogram of water by one degree Celsius, and the unit every nutrition figure on this site is given in.
The word is used loosely everywhere, including here. When a label says a biscuit has 50 calories, it means 50 kilocalories — 50,000 of the physicists' calories. Nobody writes it the strict way, and no confusion results, because food energy is only ever discussed in one unit.
Where the number on a label comes from
Not usually from burning the food. Most values are calculated: the food is analysed for protein, fat, carbohydrate and alcohol, and each is multiplied by its energy factor — roughly 4, 9, 4 and 7 kcal per gram. The sum is the calorie figure. This is the Atwater system, and it is over a century old.
Why it is an approximation
Three reasons worth knowing. The factors are averages across food types, not constants. Not everything you swallow is absorbed — whole nuts, for instance, yield measurably less energy than their analysed content suggests. And labelling regulations allow a tolerance, typically around 20 per cent, between the printed figure and the real one.
What that means for tracking
It means a food log is a consistent estimate, not a measurement. That is enough. If your estimate is wrong by a similar margin every week, the weekly change in your weight still tells you exactly what you need to know.
Sources
- 1.Food energy: methods of analysis and conversion factors — Food and Agriculture Organization of the United Nations
- 2.FoodData Central — USDA Agricultural Research Service
Related reading
- How to read a nutrition label in ten seconds
Which column to look at, what the serving size is hiding, how much tolerance the numbers carry, and the four claims on the front of the pack that mean nothing.
- How accurate are calorie counts, really?
Labels carry a legal tolerance, databases disagree and absorption varies. How wrong calorie counts really are, and why it changes little about tracking.
- How to track calories without giving up in week two
A method for counting calories that survives home cooking, restaurants and a normal week, using the accuracy you need rather than what apps pretend to offer.
- Why two apps give the same roti different calories
Different databases, source types, raw versus cooked states and user-submitted entries. Why calorie apps disagree, and which number you should actually use.