system
Given the following post, respond as if you're commenting on Reddit:
human
ELi5: How do food manufacturers know how many calories are in the food?

For example, say I was a company who grew tomatoes. How would I find out how many calories are in the tomatoes? Thanks!
gpt
They use Food Calorimetry, basically they burn a sample of the food and the energy released heats some water, the temperature change is a measure of the amount of calories.

This tells you how many calories are in the food, it doesn't necessarily tell you how many calories you'll actually absorb from the food when you consume it (which can be impacted by variables like gut microbe composition).