Info about the obesity and classification of overweight

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 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. However, studies published in 1998 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.
According to the guidelines, assessment of overweight involves evaluation of three key measures: body mass index (BMI), waist circumference, and a patient's risk factors for diseases and conditions associated with obesity.
Overweight is defined as having a BMI of 25 to 29.9 and obesity as a BMI of 30 and above, which is consistent with the definitions used in many countries. BMI describes body weight relative to height and is strongly correlated with total body fat content in adults. According to the guidelines, a BMI of 30 is about 30 pounds overweight and is equivalent to 221 pounds in a 6' person and to 186 pounds in someone who is 5'6. The BMI numbers apply to both men and women. Some very muscular people may have a high BMI without health risks.
Waist circumference, which is strongly associated with abdominal fat, is another measure of overweight excess abdominal fat and it's an independent predictor of disease risk. A waist circumference of over 40 inches in men and over 35 inches in women signifies increased risk in those who have a BMI of 25 to 34.9. To determine your WC, locate the upper hip bone and place a measuring tape around the abdomen (ensuring that the tape measure is horizontal). The tape measure should be snug but should not cause compressions on the skin.
Both Body Mass Index (BMI) and Waist Circumference (WC) can be useful measures of determining obesity and increased risk for various diseases. According to the National Institutes of Health, a high WC is associated with an increased risk for type 2 diabetes, dyslipidemia, hypertension and cardiovascular disease when BMI is between 25 and 34.9. WC can be useful for those people categorized as normal or overweight in terms of BMI (for example, an athlete with increased muscle mass may have a BMI greater than 25 - making him or her overweight on the BMI scale - but a WC measurement would most likely indicate that he or she is, in fact, not overweight). Changes in WC over time can indicated an increase or decrease in abdominal fat. Increased abdominal fat is associated with an increased risk of heart disease.
Another technique to determine the body fat is the Skinfolds (SKF) measurements that can use the thickness of subcutaneous adipose tissue, to estimate body density; once this is known for an individual, % body fat can be estimated using normative data for their age, gender, race and fitness level. In the medical practice the emphasis is often on the relative fat% indication for specific clients (change in relation to treatment) not on absolute fat%.
Classification of Overweight and Obesity by BMI,
Waist Circumference, and Associated Disease Risks
  Disease Risk* Relative to Normal Weight
and Waist Circumference
Men 102 cm (40 in) or less
Women 88 cm (35 in) or less
Men > 102 cm (40 in)
Women > 88 cm (35 in)
Underweight < 18.5   - -
Normal 18.5 - 24.9   - -
Overweight 25.0 - 29.9   Increased High
Obesity 30.0 - 34.9 I High Very High
  35.0 - 39.9 II Very High Very High
Extreme Obesity 40.0 + III Extremely High Extremely High

*     Disease risk for type 2 diabetes, hypertension, and CVD.
+     Increased waist circumference can also be a marker for increased risk even in persons of normal weight.

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