A recent New York Times article looked at former contestants on Season 8 of "The Biggest Loser." Sadly, almost all of the contestants had regained the weight they'd lost on the show. The article concludes that after drastic weight loss, resting metabolism plummets to below average, and hunger and cravings increase. The article is also a testament to the faults in extreme weight loss through drastic measures.
The article doesn't mention how the follow up investigation was funded by the show's producers, nor does it compare the "Losers' " metabolisms to those of others who had lost equivalent amounts of weight, but at a much slower rate. For example, the show's winner, Danny Cahill lost 191 lbs in 7 months, and had regained 104 lbs in the 6 years since. That's an average weight loss of 7lbs/week for 7 months.
I don't recommend losing any more than 2lbs per week because that's the maximum amount of stored body fat one can lose in a week. Any weight lost beyond that is most often muscle or water. Losing muscle leads to even greater drops in metabolism. Strength training and muscle mass play a very important role in weight loss. As far as weight loss is concerned, "Exercising to build muscle (strength training) is like paying off a mortgage." The muscle mass you build is an investment in increasing your metabolism long term.
Research shows that losing weight at a rate of 1 to 2 lbs per week through a combination of diet, strength training, and cardio leads to greatest success in keeping the weight off long term. It's also important that lifestyle change is maintainable and not extreme. The "Biggest Loser" environment was extremely different from any of the participants' home environment's. They couldn't learn how to lose weight while also going to work and interacting with friends and family.