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 but the number of fat cells generally stays the same. This is partially why once you gain a significant amount of weight, it is more difficult to lose it.