Obesity against the background of fasting is a multifactorial plot. First of all, like any diet, it is associated with forced restrictions. And if food outside the fast was one of the main pleasures of life for you, then the loss makes you think only about food all the time. And fasting in the mass consciousness, unlike most diets, imposes restrictions only on the composition of food, but not on its quantity. Therefore, those who fast for the sake of a diet, paradoxically, begin to eat more.
The lack of protein is the second (not psychological, but physiological) reason for overeating.
The third is the lack of vitamins and microelements contained in the usual diet: if you refuse milk, you receive less calcium; giving up fish, you are deficient in vitamin D; left without meat, you feel a lack of essential amino acids. The body rushes about, trying to somehow compensate for this, but falls into its own trap: the brain can analyze and offer only familiar food – from the list that it used before.
If you rarely included vegetable proteins in the menu in your ordinary (non-lean) life, even now the body does not feel the need for them: they are unfamiliar to it. But salvation is in them.