Body Mass Index (BMI)

The Body Mass Index (BMI) is a mathematical formula used to help determine whether an individual is overweight. Although BMI does not provide a direct measurement of body fat, research shows that BMI levels correlate well with results from other, more precise body fat assessments, such as underwater weighing and computerized tomography (a medical imaging method).

A Brief History of BMI

Developed by a group of scientists in the 1980s, the Body Mass Index quickly became an international standard for weight measurement. Prior to that point, doctors used a simple table, which provided a range of body weights for each inch of height, to determine a person's ideal weight.

The BMI formula improved upon this system by taking into consideration a person's body composition, rather than relying solely upon his or her weight. People can easily determine their Body Mass Index by using the BMI calculator.

BMI Fun Fact

In 1998, the National Institute of Health lowered the BMI level for overweight individuals in order to meet international guidelines. Changing the overweight threshold for BMI from 28.7 to 25 moved more than 30 million Americans out of the healthy weight group and into the "overweight" category.

Using the BMI Chart

After you determine your Body Mass Index using the BMI calculator, you can find out where you fall within the basic BMI chart.

For individuals over 20 years old, BMI is interpreted using standard weight categories that do not differ by age or sex. For children and teens, however, BMI interpretation varies according to both gender and age.

Pros of BMI

Here are a few reasons why BMI still remains a popular tool for many people seeking a quick, accurate measure of their body fat levels:

• Because BMI is calculated using only an individual's weight and height, it is a free tool that provides some evaluation of a person's health.
• Unlike many other forms of body fat assessment, BMI requires no trained personnel or special equipment.
• BMI is a valuable tool for comparison. This simple formula provides a way for individuals to compare their own weight status against others in the general population, helping them to understand their weight within a larger context.

Cons of BMI as a Measurement of Obesity

Although BMI provides an accurate estimate of individual body fat levels, the formula is not without its flaws. Some of the shortcomings of this system include the following:

• Lack of distinction between fat and muscle mass: BMI values do not distinguish between muscle mass and body fat. Because muscle weighs more than fat, a body builder might have an extremely high BMI, while actually possessing very little excess body fat. The BMI formula tends to overestimate obesity levels for individuals with extremely developed musculature.
• Inaccurate for the elderly: BMI may tend to underestimate percentages of body fat in elderly adults who have lost muscle mass with age. Although their weight may not have changed considerably, the loss of muscle mass may mean that more of their body weight is composed of fatty tissue. BMI results are not sensitive to these discrepancies.
• Lacks ethnic dimensions: BMI does not take into account variations by ethnic population. For instance, because of differences in build, Asian individuals may have a higher propensity for health problems at a lower BMI than most Caucasians.
• Lacks gender distinctions: BMI does not account for variations by gender. At the same BMI, women generally have more body fat then men.

How to Use BMI

Due to the margin of error and the fact that BMI does not directly measure body fat, the formula is most effective when used in conjunction with other evaluations of physical health.

The National Institute of Health advises that doctors use at least three different modes of measurement when determining a patient's overall level of physical health and fitness. These three measurements include:

• the Body Mass Index
• the Waist Circumference test, which measures abdominal fat. Men should have waistlines that measure no greater than 40 inches. Women, no greater than 34.6 inches, and post-menopausal women, no more than 43.3 inches
• screening for other risk factors associated with obesity, such as high blood sugar, high blood pressure and high levels of LDL (or "bad" cholesterol)

Body Mass Index is considered a screening tool, not a diagnostic method. To obtain the most accurate evaluation of your physical condition, BMI should be used in conjunction with other assessment tools and tests. Individuals with a family history of obesity, or obesity-related diseases, should always consult a medical professional for more precise diagnostic testing.