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.