Let’s say I fast and maintain my lean mass and mostly just lose a few kilos of fat by water fasting for a week or so. At the end of the fast, some of my weight loss will be from water weight and some from the fat I lost. But after I rehydrate, can I assume that the water weight that I will regain will be less than the water weight I had before I started the fast, since I weigh less? Does fat storage have anything to do with water retention?
This is a complicated question and it involves many factors. First, carbs signal the body to retain water and sodium and a reduction in carbs signals the body to reduce water. If you fast and then refeed with mostly just protein and healthy fat, you will find the water staying away. If you refeed with high sugar and carb foods, your water retention will fluctuate.
The first week I did ADF, I lost 7 lbs, and there’s no possible way that was all fat loss. Probably 5 lbs of it was water. I kept losing the water because I kept carbs low, and I was down 17 lbs the first month–again, mostly water.
On another note, you will likely find yourself needing a lot more salt from the fluid loss.
Inflammation would be reduced so you’d expect to look different while rehydrated. If you still had fat to lose while rehydrating and eating carbs I’d say you’d notice the water. If you kept it low carb or ketogenic you won’t retain water.