If you want to lose weight quickly, then for your own sake, you must have a complete understanding of the inherent risks and implications. The reality of the matter is that crash dieting, whether it's through a liquid cleanse or a ketogenic diet, is not something that most people can or even should aspire to undergo for the rest of their lives. As a rule of thumb, the most ideal weight loss campaign will be the kind that you can steadily integrate into your way of life without abruptly shocking your system.
That being said, while the risks of rapid weight loss are real and should be respected, mainstream nutritionists have somewhat sensationalized the danger of crash dieting as a whole. It is true that a heavy calorie deficit can have deadly implications for a person who hasn't invested any time into substantive nutritional research, but if you control for those who log significant hours into learning about the science of fat burning and their own dietary needs, the forecast is far less catastrophic than many have been led to believe.
For your own personal or professional motivations, you might have come to the conclusion that a short-term weight loss campaign is in your best interests. You are not alone. People decide to crash diet every single hour, but only a fraction of them undergo the necessary calculations and preparations to set themselves up for success.
Using the right application of research and common sense, we can formulate a plan for rapid weight loss that isn't paramount to shooting yourself in the stomach. There are many intelligent and effective ways to crash diet, but there are even more ways to do it terribly, and our goal is to avoid each and every one of those terrible methods.
First and foremost, any diet that proposes just one type of food should immediately be erased from your mind. "Only eat citrus fruit" diets, "only eat broccoli" diets, and any diets remotely like them are nothing but ascetic torture at best and slow suicide at worst. The diet that will allow you to reach your goal alive will be not be an easy one, and anyone who tells you otherwise is either terribly misinformed or lying to you for money. You will be undergoing physiological and psychological challenges that only a very small portion of the general population ever experiences of their own free will, and there is no way around that. If you can make peace with the fact that what you will be doing will be exceedingly difficult, then you have taken the first necessary step in the process.
After steeling your resolve and burning every book in your house that suggests only eating one of anything, it's time for the next most important step: understanding the difference between weight loss and fat loss, and understand exactly which of the two it is that you're truly after. When most people refer to weight loss, what they truly mean is fat loss, and knowing how to differentiate one from the other will save you from a world of confusion and pain.
There isn't a single piece of tissue in your body without weight. Your spleen, liver, lungs, intestine and bones are all contributing to your to your total body weight at all times. Only a certain percentage of your weight is actually made up of fat, and it's that percentage that we're going to be primarily concerned with. Professional athletes generally have a body fat percentage between six and ten percent. A professional athlete who weights 180 pounds and has 5% body fat will have, roughly, 9 pounds of fat in their body.
The reason that the distinction between body fat and weight must be made is to save you from not knowing what your future weight loss is actually constituted by. If you lose 20 pounds, it could either be from fat, muscle, or pure water weight. Large meals and bowel movements can make your total weight fluctuate by as much as an entire pound in an instant; because of this, you can't depend only on the scale to let you know how much fat you've lost. You need to calculate your daily calorie maintenance, find your personal fat percentage, and then adhere to a caloric deficit that will steadily burn fat at a specific rate that you can realistically manage.
To put it in numerically, 3500 calories equates to about one pound of fat. To lose one pound of fat a week, your collective calorie deficit over the course of 7 days would have to add up to 3500, i.e., a daily 500-calorie deficit. You can lose weight quickly in accordance to that specific formula, but based on that alone, more than 2 pounds a week is not advisable.