system
Given the following post, respond as if you're commenting on Reddit:
human
What's actually happening when I lose or gain weight?

Is all food stored as fat/muscle and then our body uses current stores for energy or will it tap it directly from food.  
  
What happens if I ate 5000kcals today and only burned 3000.  How long til the others are now fat??
gpt
If you were to lie down in bed all day, your body would burn a certain amount of calories simply by pumping blood, breathing, and doing all the other involuntary bodily functions. This is called your basal metabolic rate. Disregarding any sort of exercise and what was consumed to receive the calories (i.e. was it loaded with sugar, etc.), if one were to eat more calories than one's basal metabolic rate consumed, then the body would store the excess calories as lipid or fat cells. Conversely, if one were to eat less calories than one's basal metabolic rate consumed, then one's body would use the energy stored in fat cells to keep providing the body with energy.