Losing weight without regaining it? The role of fat cells and brown fat

Adipose tissue (AT) is an important organ, firstly quantitatively, since even in a lean person it can account for 15 to 25% of total weight. It has recently been discovered that adipose tissue represents a real organ that secretes hormones and communicates with the brain. So what happens in the body when you gain a lot of weight or lose it dramatically? Why is it so difficult to maintain weight loss? How does adipose tissue work, and what is it used for?