1 week crash diet

Absolutely! Just think about it in this way. You're happily eating all the junk food you want, munching on your favorite food items day in and day out. You suddenly decide to go on a crash diet and starting eating lettuce and grapefruit daily, three times a day, with lots of water. Do you think you'll be able to sustain it for long? Obviously not! This will only make your weaker and hungrier, and will build a craving for food within your system. You'll immediately stop fasting after a while and start eating food that's worth twice your normal amount - leading to weight gain!

Its just like eating 4-5 meals at one time and then not having it afterwards. Crash diet is never advisable because it has got only temporary results and tendency to regain weight is comparatively high. To know more about crash diet, check this out: http://www.lifemojo.com/lifestyle/why-are-crash-diets-bad-5535090