system
Given the following post, respond as if you're commenting on Reddit:
human
ELI5: When you first start a diet and lose "water weight" that first week, what's really going on?

Why would my body decide that because I quit eating so much I suddenly need to retain less water?  Is there some secret water-to-food ratio that my body tries to keep or something?

Is it even water that's getting lost?  I don't understand.

Edit: Marking this as solved.  Thank you all so much for your replies!
gpt
Carbohydrates exert an 'osmotic' pressure on the water. They retain it. If you have alot of carbohydrates in your tissues then you will have this water weight. When you start dieting, your body uses these carbs in place of your normal food, decreasing the amount of water that is withheld.