There are others here way more knowledgable than me, but, here's my experience. I have lost a good deal of weight fasting, but I am still in the obese category.

At first, you will lose a lot of water, and then your metabolism will slow down. You may not lose a lb per day, it may be a lb every two days or even a gain now and then. The body is a mysterious thing. I'm currently plateauing myself. Two days in a row at the same weight (even a very minuscule gain of .2 lb - water retention/redistribution).

So, it's possible you won't even get that low, but pretty close, and once you begin to eat normally and rehydrated again, some weight will return.

Anyway good luck, and I'm not a doctor or an expert or anything, just a civilian. Consult your doctor, get as much advice as you can. There's a lot people here who know way more than me.

Good luck!

You could maybe lose 30 on a 21 day fast. I went from 258.8 to 227.8 this morning day 22.

I would see how it goes but likely would do a 14 day maybe. once you push yourself too low on bodyfat you are into actual starvation.And your body will start eating important things like muscle, etc. Course if you knew bodyfat you would have a much better idea but for your height sure you should likely be 100 or 110. My x was around your height and once he went under 100 she looked like a walking skeleton.

But its your fast and you have to make the choices. I would think you would get good benefits at 14 days and they if you take some time refeeding you will likely still lose a bit more doing that.

While 21, 30 ,40 sounds great on paper and heavy people like me can do it pretty easily for thinner people not the best idea.
Example men with 15% bodyfat lived for about 60 days on water fasting. This was hunger strike studies from think prison. Less bodyfat less time to death.And women have higher what they call essential fat.

So again think 7 to 14 is likey a good time for you.
Or you could maybe do two 7 day fasts with some time in between.
If you are not fasting for weight loss then not much reason to push it too far.
Also if this is first time hope you don't think you can do much work especially if you have to do physical work on water. You can get very weak especially for first several days and you can get faint and dizzy. Juice is much better for first fast.
Wife saw my results and wanted to go on water like me. But she has to work pretty hard at her job.So told her to go juice and she is doing great. She sleeps a lot but her daily energy is pretty good.

Again its your fast and just giving out some opinions.

Hi Sarah,

Losing 2.8 pounds in a day has everything to do with temporary losses such as water and intestinal contents. If you fasted 21 days starting at 122.8 pounds, you could expect to burn about 7 pounds of fat (roughly 1/3 pound per day) and about 1/2-3/4 total pounds of protein type tissue, the worst of it first including unwanted growths if there were any and other unhealthy tissue. For somebody starting at double this weight, i would expect the losses to be about twice this.

If you lost about 15 pounds of temporary weight in addition to this, it would put you temporarily at 100, but with the return of these temporary losses, most likely you would quickly bounce back to about 115 pounds (within 2-3 weeks of breaking the fast), maybe 110 if some of the water loss was less than temporary.

I wouldn't concern myself with the low point in weight at the end of the fast, but rather where i would expect to be 2-3 weeks later with the temporary losses back in place. This being the case, in terms of weight alone you would have nothing to worry about going 21 days.

In my most recent fast of 36 days, because of the temporary losses i looked ludicrously thin and everybody around me was alarmed and felt i should stop, even though i was confidently emphasizing that this was temporary. 2-3 weeks later with my normal water levels restored, i was at my ideal weight and the same people were calmed and amazed by the transformation.