The body stores glycogen in muscles and other areas. When you start a diet or a fast your body releases those glycogen stores and it does so through releasing water through urine. You also lose electrolytes in the process which is why most people feel weak, nauseated, headaches and cramps.
I don’t know all the sciency stuff but you can check out a book called The Complete Guide to Fasting ans it breaks down a lot of the things that happens to the body when you fast. Most diets are really starvation diets which is similar to fasting in a way, so it still applies.
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