system
Given the following post, respond as if you're commenting on Reddit:
human
ELI5: Nutritional Value

How do companies and chefs determine the fat, carbs, fiber, ect. of their products and dishes?
gpt
You can burn things to determine calorie content, and you can also separate things by weight with a centrifuge (after blending) which can separate, for example, protein from fat. 

After this has been done once (or a few times) however, you can use already-known information about your ingredients to calculate how much of each nutrient ends up in the finished product. For example, 1/4 pound of ground beef (from a certain part of the cow, or certain combination of different parts) cooked a certain way would have a known nutritional value. McDonalds wouldn't need to test each product individually, and could instead use that known value to figure out nutritional content of a double burger, for example.