Calories, per se, have nothing to do with health or fat/lean.

Calories are a unit of heat measurement, and was invented back during the steam engine days to figure out how much wood or coal to put into the engine to generate the power needed.

A "calorie" is the amount of heat needed to raise the temperature of water.

That has nothing to do with the biochemistry of the human body.

What matters is the macronutrient content, since the body processes (via digestion, absorption, and assimilation) the protein, fat, and carbohydrate taken in.

Each of these macros is processed differently and has a different effect on the hormones of the body.

There was a man who did several self-experiments where he proved that eating a set number of "calories" would have different effects on lean mass vs. bodyfat, if the types of food were different, even thought the amount of total "calories" was the same.

He ate different diets of 5,000 calories per day, which the mainstream advice says would make him fat. Eating a meat-based diet, he did not get fat. Eating a vegan diet, he did. Eating a junk food diet, he piled on the fat. These were all the same 5,000 calories/day for 21 days.

He did another diet of 3,500 calories. Meat-based, he lost bodyfat. Plant-based, he got a little fat.

So, the amount of food ("calories") does have some importance, but it is the amount of WHAT foods that matters most.

Fasting is the total absence of food, and can have a dramatic effect on overall heath and function of the body -- especially good for healing the sick.