Up front: I am biased against extreme diets like water-only fasts. I can see a use case in carefully medically supervised settings, such as for a cancer treatment, and I know that some religious practitioners use them. I’ve never tried them and have never been morbidly obese.
The only truly releveant paper I found that looked relevant was a case study of a woman whose 40-day water fast caused thiamine deficiency, which led to her developing a severe neurological disorder called Wernicke’s encephalopathy (source).
The academic literature on prolonged water-only fasts is extremely limited, particularly for weight-loss purposes in humans. Prolonged water-only fasts have a few more studies in non-human organisms and are sometimes used as anti-cancer treatments.
I would be really careful about basing decisions on reddit anecdotes. Questions I would be wondering about:
What nutritional deficiencies and other potentially serious health risks might result from a sustained water-only fast?
For those who lost weight on a prolonged water fast, did they keep it off long-term?
Of those who try a water fast, how many fail to sustain it?
How much selection bias in favor of success stories when people post their weight-loss experiences in /r/fasting?
How does a failed extreme dieting attempt affect your motivation to try again in the future?
The metis in the dieting world appears to be that crash diets tend to be yo-yo diets, leading either to failure of the diet attempt or to rapid weight regain after the diet ends. My picture of successful dieting tends to look more like what I’m doing:
Eating a lower but sustainable number of calories per day (like 1200 calories for a 200-pound person), mainly by replacing oils/fats, sugars and starches with fruits and non-starch vegetables and by improving the ability to eat limited portions
Exercising, especially resistance training, which has the best evidence for counteracting the decrease in resting metabolic rate that occurs when you start dieting
Gaining intellectual knowledge of nutritional concepts like resting metabolic rate, caloric density, glycemic index, and how the shift from water/glycogen-burning to fat-burning affects your rate of weight loss
Developing a typology of your hunger symptoms, what they seem to mean (i.e. my stomach isn’t full vs. my blood sugar is low), whether they should motivate you to eat, and if so, what type of food and how much. For example, in my case, stomach rumbles and a feeling of emptiness seems to mean my stomach is empty, whereas wooziness, shortness of breath, and irritability seem to mean my blood sugar is low.
Figuring out at what stage of hunger and in what circumstances you do or do not have self-control over your eating, and making sure to avoid circumstances where you lose control and impulse-eat
Adjusting your shopping, meal-planning, and restaurant-ordering behaviors
Getting socially connected: talking about your goals with family and friends, with a doctor or nutritionist, with an exercise coach, and being at least occasionally involved with support gorups
Making it fun and motivating: building up a positive picture of what benefits you expect to reap and a clear-eyed understanding of where your weight and weight-linked physiological traits are now and where you’re headed if you don’t change course
There are amazing reports about the new weight loss drugs like Wegovy, but I have not tried them and they are currently very expensive and hard to get
Please don’t take this as medical advice—it’s just a description of what I’m doing. I’d highly recommend exploring a range of dieting options and considering each on their merits before selecting one, and ideally interfacing with a medical professional to consider your specific situation.