system
Given the following post, respond as if you're commenting on Reddit:
human
ELI5: What is the most calorie dense food, and is there a limit to how many calories something can have?

I've been dieting for a while now and I was thinking about calories. Do we know what the food with the highest calorie density in the world is? And is there a limit to how calorie dense something can be? For example, would it be physically possible for something that weighs 1 gram to contain 1,000,000 calories, or is there a physical limit as to how much energy something can provide?
gpt
Pure fat is the highest calorie food that humans can eat. It is about 9 calories (kcals) per gram.

Compared to protein and carbs (4 cal/gram) and alcohol (7 cal/gram).

That is the known limit for caloric density, as fat are purely hydrocarbons, as close to pure chemical energy we know of.