2) Very Low-calorie Diet (VLCD)
A very low-calorie diet is when you consume less than 1200 calories for women and 1400 calories for a man. Granted this does help in the short term for severely overweight or obese individuals. However, this diet should only be followed under strict medical supervision. Likely to lose 3-4lbs per week on these plans.
Long Term Impact:
VLCD can slow your metabolism, stress the body, and cause electrolyte imbalances. With long term use very low-calorie diets the body will start to eat its own lean muscle tissue to provide you some of the additional calories that it needs to function properly. This leads directly to an overall muscle tissue loss, which in turn reduces your metabolic rate and your body needs fewer calories to function properly. Ultimately your weight loss will slow down because muscle tissue is far more metabolically active than fat and it needs energy (calories) to survive. In addition on VLCD you are not consuming the nutrients you need for the body to function properly, which can result in headaches, constipation, weakness, muscle cramps, lower T3, heart palpitations, and many other effects.
Conclusion:
If you really need to lose weight quickly and want to take the low calories option, make sure to wean off the diet properly. Also consider calorie cycling (mix of low calorie and normal calorie days), which is the is gentler on the body. Low calories diets are also okay to jumpstart a weight loss plan; so if the first week of a weight loss plan you want to stay low to get a jump start.