There’s a very scientific test that involves seeing how much a specific food will change the temperature of water when ignited but most companies don’t do this and rely on the fact that it is has been found that there are 4 calories in a gram of protein, 4 calories in a gram of carbs and 9 calories in a gram of fat. Then they just multiply and add those things together and are usually within 10% of the actual value. In the US, the FDA allows for a 20% margin of error in either direction, so.
Edit: if you grabbed a nutrition label and noticed this is slightly off, I think it’s because if something really has like 9.4 grams of carbs, they just write 9 (in the US, I have seen that they include decimals on Italian nutrition labels), but I’m not sure.
2 ways;
1. If you know the ingredients of a product and their nutrimental value then you can simply do math. This is the case for most products
2. A calorimeter, basically you burn the food in a machine with water around, and you see how much the water increased in temperature. The unit calorie is derived from this method, it is the energy needed to heat up 1ml of water by 1 degree Celsius. This is typically done for food where you can’t really know what its made of, like a fruit or vegetable.
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