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 first increase in size and later in number. One pound of body fat represent about 3500 calories.

When a person starts losing weight, the cells decrease in size but the number of fat cells generally stays the same. This is part of the reason that once you gain a significant amount of weight, it is more difficult to lose it.

Each fat cell weighs a very small amount (about 0.4 to 0.6 micrograms), the weight of these billions and billions of fat cells add up significantly. It’s not very practical to count and measure the number of fat cells in a person's body, so science has come up with easier methods to determine if a person is obese.

Most common is a weight scale that you may have in your own bathroom but less common is BMI or Body Mass Index. This is the calculation to find yours.

What causes one person to gain weight while another person maintains their weight?

Obesity is a serious illness that has a load of medical complications. It is relatively rare for doctors to treat obesity itself because it’s a difficult long-term process to treat effectively. However, treatment for the complications is done at enormous cost.

Regardless of your age, you gain weight when the amount of calories coming in equals more than the amount of calories burned. But obesity is not that simple. Weight gain is affected by a variety of factors in addition to caloric intake.

Metabolism. A fast metabolism burns calories quicker than a slow metabolism. Exercise is the main factor that affects metabolism. Although rare, medical conditions you have and medications that you take also may cause changes. To some extent, how much you eat, when you eat and when you skip meals also may affect your metabolism.

High fat foods are obesity promoting in animals and humans. In the last 20 years, the ready availability of high fat foods (ex: fast foods) combined with the decreased calorie requirements because of decreased physical activity is felt to be the major factor in the sharp rise in the more obese people.

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