Obesity is an excess of body fat, frequently resulting in a significant impairment of health.
Obesity results when the size or number of fat cells in a person's body increases.
A normal-sized person has between 30 and 35 billion fat cells. When a person gains weight, these fat cells increase in size at first and later in number. When a person starts losing weight, the cells decrease in size. However, no matter how much weight an individual loses, the number of fat cells stays the same. This is partially why once you gain a significant amount of weight, it is more difficult to lose.