A: When it comes to weight loss and safety, the difference between safe and unsafe doesn't have to do with how much you lose but how you lose it. Losing 1-2 pounds per week is often cited as a safe amount, but little safety data exists to support that number. What makes 3-4 pounds unsafe? Only the way in which you lose it.
If you drink nothing but lemon juice and maple syrup with a dash of cayenne pepper for a week and lose 10 pounds, I would consider that unsafe weight loss. On the other hand, if you complete three intense, metabolic resistance-training sessions, another three interval-training sessions, and you are diligent about eating a low-calorie, low-carbohydrate diet and lose 10 pounds in one week; I wouldn't say that was unsafe. I would say you worked hard and lost the weight the right way. I should note that 11 pounds is the most weight I have ever had a client lose in one week (not counting athletes that were cutting weight). This client worked extremely hard to lose that weight, and he was relatively fit to begin with.
The question of losing 10 or 20 pounds in one week has become very popular, thanks to TV shows like The Biggest Loser. When losing "biggest loser" type numbers, there are three major factors that come into play: weight to lose, time, and experience/fitness level.
1. How much weight do you have to lose? If you weigh 180 pounds and your goal weight is 130 pounds, losing 20 pounds in one week—the right way—teeters on the brink of impossible. If you're more like a Biggest Loser contestant, weighing 380 pounds, then losing 20 pounds in one week is plausible (especially considering the large amount of water weight you would lose during the first week). This follows the general rule that the more weight you have to lose, the easier it will be for you to lose weight.
2. How much time to you have to dedicate to weight loss? Unlike The Biggest Loser, most people don't have all day to dedicate to weight loss. Chances are you have a job, social life, family responsibilities, etc. that make it hard to put in the time required to burn enough calories to lose 20 pounds in one week.
3. What is your current fitness level and how much experience do you have exercising? This is where safety becomes an issue. Starting out on, most contestants have a hard time walking one mile. If you find yourself in this situation, then embarking on an intense resistance-training program will hinder your progress, not help it (and you'll likely injure yourself in the process). Knowing your physical limits and abilities is an important part of any exercise program, especially if you are going to set a very aggressive weight-loss goal. Medical teams are readily available on TV sets to deal with the contestants collapsing and passing out. You should never put yourself in a situation where you are pushing your fitness level to those limits.
All of this aside, the most important questions are why do you need to lose so much weight so fast, and what is going to happen afterwards? I support setting aggressive weight-loss goals, but why give yourself such a short time span? Expand this to one month, and give yourself a chance to live a little during the process. Allow your body to recover and respond to your intensive training and diet program.