The human body, even as a fetus, craves nutrients. Nothing matters to it more than staying alive. It has no higher purpose, and will shut down anything that tries to supersede that, even if it means making you sick in the process.
There is a phenomenon commonly referred to as “starvation mode,” a process where the body becomes hyper-efficient at converting all consumed calories into other, more sustainable forms of fuel. To accomplish this, our body will lower its metabolic rate, leading to a loss of muscle so that the body requires fewer calories to subsist, causing weight loss to slow down. It explains why some of your friends, though otherwise thin, have a protruding stomach. Their bodies believe that they are in a famine situation. That makes it difficult to lose those unwanted pounds.
An ongoing calorie reduced diet (less than 1,200 calories for most people), causes the body to perceive an emergency. It doesn’t understand your reasons for starving it, only that it needs to save every calorie, and will accommodate you by holding on to stored body fat.