system
Given the following post, respond as if you're commenting on Reddit:
human
ELI5: How and why does our body function relatively normally if we don't eat healthily?
gpt
The calorie is THE main form of energy, and unhealthy foods tend to have a ton of them. Without calories, you will die. A good analogy is a car. 

Calories are the gas to help keep the car running. If you eat too many calories, you start storing that extra gas in the trunk. It then requires more energy to move the car because it's heavier, and the other parts of the car will wear out faster because of heavier workload.

All the other parts of the car - the pistons, transmission, engine etc are your organs and body parts. They need regular upkeep (nutrients such as vitamins/minerals) to run efficiently. The car may run a long time if you neglect these areas because you have gas, but it won't run as smoothly or as long as you may have liked.

I hope this helps!