For many years people have said that extreme diets may cause the metabolism to slow down more than they should with more gentle dieting methods.
The University Of Chile found that when overweight women went on extreme weight loss diets with calories just 20 calories per kg of bodyweight reduced resting energy expenditure by 168 calories per day, a 10.6% on average change.
The resting energy expenditure was higher with women with a higher muscle mass, showing muscle does indeed help increase calories burned daily.
They concluded that body adapts to calorie reduction by reducing energy expenditure, when forced on an extreme weight loss diet.
Taken from Nutricion Hospitilara 2015.