Leptin is considered the satiety hormone; the hormone is in our fat cells. It regulates our hunger and has a lot to do with our energy balance. When we lose weight " body fat" we feel less hunger but this Naturally decreases our energy balance. Leptin is one of the Key player in our body's homeostasis. So even when we lose weight our body still thinks it's homeostasis is our starting weight. Therefor your brain is continually fight you get you back to homeostasis. That is why the human yo-yo, is so coming. That is why statics show 95% of people regain the weight within 3 years of losing it. There are some other contributing factors, however leptin is the main player. Most people have no clue how their body work or their anatomy, it is very important to now the mechanic of driving a car before you get behind a wheel. To be a good drive, you must learn to be an offensive and defensive driver. Understanding the hormones and how they interplay with ins