What is 1 kg sustance made of ? If you are asking for carbohydrates it’s 1 gm = 4 calories so 1 kg of carbohydrates is 4000 kcal.
I gm of protein is again same 4 calories so 1 kg. Of protein is 4000 kcal
1 gm of fat is 8 calories do 1 kg of fat is 8000 kcal.
Thus if you want to loose 1 kg of weight consume 8000 calories less in week to loss 1 kg a week from body weight. Gaining will be more then 1 kg if you eat 8000 extra calories . Both example are subject to stagnant and same other parameters.
Most dangerous is 1 gm of alchohol contains 10 calories so 1 kg of alchohol is 10000 kcal .