What is obesity?

The definition of obesity varies depending on what one reads, but in general, it is a chronic condition defined by an excess amount body fat. A certain amount of body fat is necessary for storing energy, heat insulation, shock absorption, and other functions. The normal amount of body fat (expressed as percentage of body fat) is between 25-30% in women and 18-23% in men. Women with over 30% body fat and men with over 25% body fat are considered obese.

Defining Overweight and Obesity

Overweight and obesity are both labels for ranges of weight that are greater than what is generally considered healthy for a given height. The terms also identify ranges of weight that have been shown to increase the likelihood of certain diseases and other health problems.

 

Healthy BMI Ranges for Adults and Children

What’s considered a healthy BMI?

 
• For adult men and women, a BMI between 18.5 and 24.9 is considered healthy.
 
• Overweight is defined as a BMI between 25.0 and 29.9; and a BMI of 30 or higher is considered obese.

As in adults, obesity is also a growing problem in children and adolescents. Because children grow at different rates, depending on their age and gender, the definitions of overweight and obesity in children and adolescents differ from those in adults.

 
• In the U.S., for example, the definition is based on standard growth charts developed by the Centers for Disease Control and Prevention. In children and adolescents age 2 to 20 years old, a BMI in the 85th to 94th percentiles for age and gender is considered overweight; a BMI in the 95th percentile or higher is considered obese.

It is important to remember that although BMI correlates with the amount of body fat, BMI does not directly measure body fat. As a result, some people, such as athletes, may have a BMI that identifies them as overweight even though they do not have excess body fat. For more information about BMI, visit Body Mass Index.

Other methods of estimating body fat and body fat distribution include measurements of skinfold thickness and waist circumference, calculation of waist-to-hip circumference ratios, and techniques such as ultrasound, computed tomography, and magnetic resonance imaging (MRI).

Waist Size Matters: Abdominal Obesity

One important category of obesity not captured by BMI is so-called “abdominal obesity” - the extra fat found around the middle that is an important factor in health, even independent of BMI.

 
• The simplest and most often used measure of abdominal obesity is waist size. Guidelines generally define abdominal obesity in women as a waist size 35 inches or higher, and in men as a waist size of 40 inches or higher.

Measuring Body Fat

There are a number of ways to measure body fat. Some are well suited to the doctor’s office, such as calculating a person’s BMI. Other, more complex methods require specialized equipment, such as magnetic resonance imaging or dual energy X-ray absorptiometry machines; while these machines can measure body fat very accurately, they are typically only used for this purpose in research settings.

Globally, there are 1.5 billion adults who are either overweight or obese, a number expected to increase to 3 billion by 2030. The epidemic is reaching catastrophic proportions, and one of the key “if small” steps to bringing it under control is to have a common language to describe the problem.