4 points about rapid weight loss
"Hefty, rapid weight is linked with poorer long-term outcomes than gradual weight loss"
"The above myth arose in reaction to adverse effects of very low calorie less than 800 calorie per day.
Look Ahead data shows those who lost more wt maintained more wt loss link
Obesity (Silver Spring). 2015 Jul;23(7):1353-6. doi: 10.1002/oby.21112.
The usual concern about losing weight rapidly is that a great deal of muscle is lost as well as fat as this article below shows.
2013 High Protein diet during wt loss may decrease fat free mass loss link to article