Experts recommend shedding no more than 2 pounds a week, or your body may lose disease-fighting capabilities (because of a lack of necessary nutrients) and you may become more susceptible to illness, including the dreaded flu virus.
Also, experts say that if you lose weight slowly, it is much more likely to stay off in the long run. So no crash dieting! Lose those pounds slowly!
(from SHAPE magazine, March 2009, Volume 28, #7)