Its basic knowledge that if you consume less calories than you burn then you should lose weight. Unfortunately, people have taken this information and created extreme diets that cause more harm than good, here’s why......
After several days of under-eating the body switches to ‘energy conservation mode’ Which slows your metabolism down (how quickly your body converts food into useable energy).
When the carbohydrate stores run low, protein and fat become and primarily energy source, before you get excited... This is a bad thing because, red blood cell production is largely effected as they run off glucose that we retain from carbs themselves.
The brain also functions from carbs but will adapt itself to power off ketone bodies derived from fat if you under consume carbs.
Therefore, to meet basic energy requirements your body ramps up the breakdown of MUSCLE and ORGAN TISSUE before FAT which is ironic because most people start dieting to lose fat and all you’re actually doing is slowing down your body’s ability to do that by underrating, over-training and neglecting essential macronutrients.