I alternate between water fasting and Keto, lost 75 pounds in about 5 months. I took a break during the holidays but I'm getting back on track now.
Water fasts are a great way to jumpstart your body into Ketosis. I give myself a nice cheat day beforehand, eat whatever I want, as much as I want, and then some. I pretty much let myself overindulge. It's nice, because there are some foods that I genuinely miss while on Keto (milk, orange juice, potato salad, deserts, sugary juices and sodas, etc.) Then afterwards, I go on a 5+ day water fast, and usually by the 3rd day I'm back in Ketosis. My longest water fast so far has been 12 days, one day I'm going to try for 14 days.
I've gone from a whopping 400 pounds to 325, still losing. I try to give myself one good cheat day and water fast per month. I try to schedule my cheat days on family events so that I'm not fasting or on a Keto diet and missing out on food (because god forbid if I show up at my mother-in-law's house and don't eat her food, she'll take it like an insult...)
Anybody ever watch this guy? He was kinda inspiration for me to try this (at least the water fasting part), despite how many times he says not to in his video lol
Finally, a small but necessary legal disclaimer, don't try anything in this post without the advice of a doctor. I am not a doctor, and I am not responsible if you screw up something. Water fasting and Keto can both be dangerous and/or lethal in certain cases.