If you’ve ever tried to lose weight and found the pounds won’t come off easily — or they come right back — you are not alone.

The fact is, when we shed pounds, we trigger mechanisms that make it hard to keep the weight off. There are some factors that are within our control, but many are not. Understanding how this works might make you look more kindly on your body.

We won’t tell you how to lose weight — or whether you even need to. But we will give you five realities about biology.

Biology’s A B*tch: 5 Reasons It’s So Hard To Keep Weight Off

Illustration: Shannon Wright for NPR