Fasting can be done using different protocols and should always be used in conjunction with a high antioxidant micronutrient rich diet.
Basically when you eat you eat a lot of vitamin, mineral and antioxidant rich foods.
So the body will be nurtured and replenished, leaving no spaces for deficiencies.
As you can see, fasting is done in a very controlled and balanced way and is used under professional guidance.
A very low calorie diet is a different thing. It is not taking into consideration the vitamin, mineral and antioxidant content of foods, but only their caloric power.
I am not a fan of low calorie diets and never use them.
Low calorie diets were used decades ago and the results are not good, leading to malnutrition.
So always think of nutrition as a nurturing thing and never as a number counting game.
There is a huge misconception that fasting leads to nutritional deficiencies and is based on a low calorie diet.