system
Given the following post, respond as if you're commenting on Reddit:
human
ELI5: How do we know the calorie content of food when not everything that burns is digestible?

Like cellulose, it produces a lot of energy when burnt, but gives you zero energy if you eat it because humans cant digest it.
gpt
Calories are just a measurement of energy, like joules. Just because the human body can't digest and use some of those calories doesn't mean they don't exist. Calorie counts on food nutrition labels tell you the total number of calories in that food, but they don't tell you how many of those calories you'll actually digest. For example, the more processed a food is, the more calories our bodies are capable of absorbing (the processing did some of the digestion work for us). The health of your digestive system also plays a major role in how many calories your body gets from eating a food. If two people ate the exact same bag of chips, one of those people would probably absorb a few more calories from the chips than the other person, but neither would get as many as are on the label.