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Definition of obesity

Obesity is an excess of body fat that frequently results 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 first increase in size and later in number. One pound of body fat represents 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. However, some recent studies seem to imply that fat cells can be destroyed as a result of certain medications and that a decrease in fat cell number may occur if a lower body weight is maintained for a prolonged period of time.

Each fat cell weighs a very small amount (about 0.4 to 0.6 micrograms but can be as small as 0.1 micrograms to as large as 1.2 micrograms). In other words, it would take about 5 million fat cells to get just 1 ounce of fat. However, the weight of billions and billions of fat cells does add-up. Obviously, it is not practical to count the number of fat cells in a person's body, so science has come up with easier methods to determine if a person is owerweight or obese.

Earlier methods involved tables of desirable weights at various heights that were derived by life insurance companies. These tables were based on illnesses and death rates.

A better method was developed that more closely correlates with body fat and the metabolic complications of obesity. This is the Body Mass Index ("BMI") and it is calculated as follows:

BMI = Weight (in kg.) / Height (in meters) Squared (ht. x ht).

For example, a man who is 5' 10"(1.78 meters) tall and weighs 285 lbs. (~130 kgs.) would have a BMI of 130/(1.78 x 1.78) = ~ 41.

You can use the input form on the next page to automatically calculate your BMI and your resting metabolic rate (the rate at which you expend energy in a resting state in calories per 24 hours).

Body Mass Index and Basal Metabolic Rate Calculator

Height: feet inches
Weight: pounds
Age: years

Results: BMI and Base Metabolic Rate

BMR for Females: 1,393 calories BODY MASS INDEX: 22.40

If female: You are considered normal weight
If male: You considered normal weight
BMR for Males: 1,566 calories

The risk of metabolic complications, such as hypertension and diabetes, is related to both the BMI and the waist circumference as shown in the following tables:

Classification of Obesity
Obesity Class
Disease Risk (Relative to Normal Weight and Waist Circumference)
Men < 40 in (102 cm)
Women < 35 in (88 cm)
Men > 40 in
Women > 35 in
Very High
Very High
Very High
Extreme Obesity
> 40
Extremely High
Extremely High

In the United States, the percent of the population that is overweight (BMI 25 to 29.9) or obese (BMI > 30) has increased dramatically over the last 20 years. For men, 59% are overweight, while 20% of the total male population is obese. Fewer women are overweight (51%) but more are obese (25%).

If you are overweight or obese, it is important to get appropriate medical help to reduce your weight and avoid the long-term complications associated with excess body fat.

Updated: 27 December 2011

Copyright © 1996 -2011 Michael D. Myers, M.D., Inc.
All rights reserved.

Disclaimer Statement

The above information is for general purposes only and should not be construed as definitive or binding medical advice, diagnosis or treatment. Because each person is medically different, individuals should consult their own personal physicians for specific information and/or treatment recommendations.