The fat stored in your adipose tissue is a super energy-rich substance that your body can use in a pinch to fuel your cells. If you can't eat for whatever reason, or you just need a little extra energy to grow or reproduce, your body can turn to your fat, which is why from a survival perspective, having some fat is actually a good thing! Still you'd think that losing weight would be pretty straightforward just eat less than you need, and force your body use up some of its fat, then go back to eating a normal amount when you're the size you want to be. But the body doesn't want to lose its energy buffer no matter how large or small it is, so when you cut calories, it reacts in ways that ultimately make it harder to lose weight. A lot of the push back is driven by changes to hormones. One of the most important is leptin, a hormone secreted by your fat cells. The larger your fat cells are, the more leptin they produce. So when you lose weight, leptin levels drop. Parts of your brain like your hypothalamus interpret less leptin as starvation, and it jumps in and starts telling your body to conserve energy and to eat more to rebuild those reserves. Other organs also use hormones to complain to your brain about the decrease in fuel intake. Your stomach tells your brain it's not getting filled by increasing levels of the hormone ghrelin. At the same time, your pancreas secretes less insulin, which regulates blood sugar and amylin, which signals fullness. So when you cut calories, ghrelin levels rise and insulin and amylin levels plummet, signaling your brain to increase appetite, making you feel ravenous.