The starting point is defining obesity. What is normal weight, overweight and obese and how are they defined and measured?

Obesity is measured by Body Mass Index (BMI), which attempts to make an estimate about body fat based on a person’s weight and height.
The Formula: BMI = Weight (lb) / (Height (in) x Height (in)) x 703

This means: Weight is divided by Height squared; that result is then multiplied by 703.

For example: someone 6 feet tall (72 inches) and weighing 200 pounds will have a BMI of 27.1 based on the formula:

1. First square height.  72*72 = 5,184
2.  Divide by weight by height squared: 200/5,184 = .0385
3. Then multiple by 703. = 27.

For example: someone 5 feet tall (60 inches) weighing 100 pounds will have a BMI of 18.9:

1. Square height 60*60 = 3600
2. Divide weight by height squared: 100/3600 = .027
3. Multiple by 703. = 18.9

The Body Mass Index is divided into categories:
· Underweight BMI is less than 18.5
·   Normal weight is a BMI between 18.5 and 24.9
· Overweight is a BMI between 25 and 29.9
· Obesity is measured as a BMI of 30 or greater
Obesity can be divided into three categories (although there is not agreement on the English descriptors):.
· Moderately obese is a BMI between 30 and 34.9
· Extremely obese is a BMI between 35 and 39.9
· Morbidly obese is measured as a BMI of 40 or above

So someone 6 feet tall weighing 200 pounds would be categorized as overweight, and 5 feet tall and weighing 100 pounds would be categorized as normal weight.

Two questions come to mind. One is whether these dividing points are objective. For example, why is someone 5 feet tall and weighing 100 pounds not underweight? Who decided that cut-point? Or why is someone 6 feet tall and weighing 200 pounds overweight? I do not know the basis for these cut-points.

I can tell you, however,  that at one time  the 6 foot tall person weighing 200 pounds was categorized in the normal weight range. Until 1998, overweight started with a BMI of 27.8, but then the overweight BMI cut-point was changed to conform with the World Health Organization’s definition. The new (and still current) cut-point was lowered from a BMI 27.8 to a BMI 25. Under the old measure, the 6 foot person weighing 200 pounds was categorized as normal weight; after 1998, that person was categorized as overweight. Similarly, someone 5 feet, 4 inches and weighing 155 pounds was considered normal before 1998. Afterward, that person was overweight. According the Washington Post, an estimated 29 million Americans considered normal weight before the change became redefined as overweight afterward (http://www.washingtonpost.com/wp-srv/style/guideposts/fitness/optimal.htm)

The second question is the validity of the measure. Is this height/weight a good indication of excess fat? The problem is that muscle mass and bone density are factors not accounted for in this simple weight/height BMI formula. Athletes with substantial muscle mass might be counted as overweight or moderately obese even though they have very little fat. There is also some concern that men and women are measured by the same formula.

The BMI has the advantage of being easy to calculate and is used extensively around the world. The BMI is comparable whether a country uses pounds or kilograms (the formula is slightly different but the results are comparable). So, despite is flaws, it is the measure of choice. While the BMI is measured in the same way each time, and thus is reliable, the category of overweight has changed. Research that uses the BMI numbers will have no difficulty with reliability, but research that focuses on the category of overweight will likely show an increase after 1998. A trend line of overweight people should show an increase after the definition was changed. Measures of the overweight category will not be comparable before and after 1998: it is an apples to oranges comparison.